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experiments_with_alternate_currents_of_high_potential_and_high_frequency_by_nikola_tesla [2018/04/21 03:32] (current)
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 +====== Experiments with Alternate Currents of High Potential and High Frequency ======
 +by Nikola Tesla
 +[[:​my_inventions_by_nikola_tesla|My Inventions By Nikola Tesla]]
 +<div align="​center"><​img src="​images/​actitle.gif"​ alt="​Title Page"
 +width="​470"​ border="​1"></​div>​
 +<​h2>​NIKOLA TESLA.</​h2>​
 +<h2>A LECTURE</​h2>​
 +<div align="​center"><​i>​With a Portrait and Biographical Sketch</​i><​br>​
 +<i>of the Author</​i>​.<​br>​
 +NEW YORK:<​br>​
 +<​p>&​nbsp;</​p><​!-- Page 2 -->
 +<​p>&​nbsp;</​p><​!-- Page 3 -->
 +<!-- The following image was obtained from another source. -->
 +<div align="​center">​
 +<img src="​images/​actesla.gif"​ alt="​Portrait of Nikola Tesla" width="​280"​ height="​459"​ border="​0">​
 +<​h2>​Biographical Sketch of Nikola Tesla.</​h2>​
 +<​p>​While a large portion of the European family has been surging westward
 +during the last three or four hundred years, settling the vast
 +continents of America, another, but smaller, portion has been doing
 +frontier work in the Old World, protecting the rear by beating back
 +the &​quot;​unspeakable Turk&​quot;​ and reclaiming gradually the fair lands that
 +endure the curse of Mohammedan rule. For a long time the Slav
 +people&​mdash;​who,​ after the battle of Kosovopjolje,​ in which the Turks
 +defeated the Servians, retired to the confines of the present
 +Montenegro, Dalmatia, Herzegovina and Bosnia, and &​quot;​Borderland&​quot;​ of
 +Austria&​mdash;​knew what it was to deal, as our Western pioneers did, with
 +foes ceaselessly fretting against their frontier; and the races of
 +these countries, through their strenuous struggle against the armies
 +of the Crescent, have developed notable qualities of bravery and
 +sagacity, while maintaining a patriotism and independence unsurpassed
 +in any other nation.</​p>​
 +<p>It was in this interesting border region, and from among these valiant
 +Eastern folk, that Nikola Tesla was born in the year 1857, and the
 +fact that he, to-day, finds himself in America and one of our foremost
 +electricians,​ is striking evidence of the extraordinary attractiveness
 +alike of electrical pursuits and of the country where electricity
 +enjoys its widest application.
 +<!-- Page 4 -->
 +Mr. Tesla'​s native place was Smiljan,
 +Lika, where his father was an eloquent clergyman of the Greek Church,
 +in which, by the way, his family is still prominently represented. His
 +mother enjoyed great fame throughout the countryside for her skill and
 +originality in needlework, and doubtless transmitted her ingenuity to
 +Nikola; though it naturally took another and more masculine direction.</​p>​
 +<​p>​The boy was early put to his books, and upon his father'​s removal to
 +Gospic he spent four years in the public school, and later, three
 +years in the Real School, as it is called. His escapades were such as
 +most quick witted boys go through, although he varied the programme on
 +one occasion by getting imprisoned in a remote mountain chapel rarely
 +visited for service; and on another occasion by falling headlong into
 +a huge kettle of boiling milk, just drawn from the paternal herds. A
 +third curious episode was that connected with his efforts to fly when,
 +attempting to navigate the air with the aid of an old umbrella, he
 +had, as might be expected, a very bad fall, and was laid up for six weeks.</​p>​
 +<​p>​About this period he began to take delight in arithmetic and physics.
 +One queer notion he had was to work out everything by three or the
 +power of three. He was now sent to an aunt at Cartstatt, Croatia, to
 +finish his studies in what is known as the Higher Real School. It was
 +there that, coming from the rural fastnesses, he saw a steam engine
 +for the first time with a pleasure that he remembers to this day. At
 +Cartstatt he was so diligent as to compress the four years' course into three,
 +and graduated in 1873. Returning home during an epidemic of cholera, he was
 +<!-- Page 5 -->
 +stricken down by the disease and suffered so
 +seriously from the consequences that his studies were interrupted for
 +fully two years. But the time was not wasted, for he had become
 +passionately fond of experimenting,​ and as much as his means and
 +leisure permitted devoted his energies to electrical study and
 +investigation. Up to this period it had been his father'​s intention to
 +make a priest of him, and the idea hung over the young physicist like
 +a very sword of Damocles. Finally he prevailed upon his worthy but
 +reluctant sire to send him to Gratz in Austria to finish his studies
 +at the Polytechnic School, and to prepare for work as professor of
 +mathematics and physics. At Gratz he saw and operated a Gramme machine
 +for the first time, and was so struck with the objections to the use
 +of commutators and brushes that he made up his mind there and then to
 +remedy that defect in dynamo-electric machines. In the second year of
 +his course he abandoned the intention of becoming a teacher and took
 +up the engineering curriculum. After three years of absence he
 +returned home, sadly, to see his father die; but, having resolved to
 +settle down in Austria, and recognizing the value of linguistic
 +acquirements,​ he went to Prague and then to Buda-Pesth with the view
 +of mastering the languages he deemed necessary. Up to this time he had
 +never realized the enormous sacrifices that his parents had made in
 +promoting his education, but he now began to feel the pinch and to
 +grow unfamiliar with the image of Francis Joseph I. There was
 +considerable lag between his dispatches and the corresponding
 +remittance from home; and when the mathematical expression for
 +<!-- Page 6 -->
 +the value of the lag assumed the shape of an eight laid flat on its back,
 +Mr. Tesla became a very fair example of high thinking and plain
 +living, but he made up his mind to the struggle and determined to go
 +through depending solely on his own resources. Not desiring the fame
 +of a faster, he cast about for a livelihood, and through the help of
 +friends he secured a berth as assistant in the engineering department
 +of the government telegraphs. The salary was five dollars a week. This
 +brought him into direct contact with practical electrical work and
 +ideas, but it is needless to say that his means did not admit of much
 +experimenting. By the time he had extracted several hundred thousand
 +square and cube roots for the public benefit, the limitations,​
 +financial and otherwise, of the position had become painfully
 +apparent, and he concluded that the best thing to do was to make a
 +valuable invention. He proceeded at once to make inventions, but their
 +value was visible only to the eye of faith, and they brought no grist
 +to the mill. Just at this time the telephone made its appearance in
 +Hungary, and the success of that great invention determined his
 +career, hopeless as the profession had thus far seemed to him. He
 +associated himself at once with telephonic work, and made various
 +telephonic inventions, including an operative repeater; but it did not
 +take him long to discover that, being so remote from the scenes of
 +electrical activity, he was apt to spend time on aims and results
 +already reached by others, and to lose touch. Longing for new opportunities
 +and anxious for the development of which he felt himself possible, if once
 +he could place himself within the genial and direct influences of the gulf
 +<!-- Page 7 -->
 +streams of electrical thought, he broke away from the ties and traditions of the past,
 +and in 1881 made his way to Paris. Arriving in that city, the ardent young Likan obtained
 +employment as an electrical engineer with one of the largest electric
 +lighting companies. The next year he went to Strasburg to install a
 +plant, and on returning to Paris sought to carry out a number of ideas
 +that had now ripened into inventions. About this time, however, the
 +remarkable progress of America in electrical industry attracted his
 +attention, and once again staking everything on a single throw, he
 +crossed the Atlantic.</​p>​
 +<​p>​Mr. Tesla buckled down to work as soon as he landed on these shores,
 +put his best thought and skill into it, and soon saw openings for his
 +talent. In a short while a proposition was made to him to start his
 +own company, and, accepting the terms, he at once worked up a
 +practical system of arc lighting, as well as a potential method of
 +dynamo regulation, which in one form is now known as the &​quot;​third brush
 +regulation.&​quot;​ He also devised a thermo-magnetic motor and other kindred
 +devices, about which little was published, owing to legal
 +complications. Early in 1887 the Tesla Electric Company of New York
 +was formed, and not long after that Mr. Tesla produced his admirable
 +and epoch-marking motors for multiphase alternating currents, in
 +which, going back to his ideas of long ago, he evolved machines having
 +neither commutator nor brushes. It will be remembered that about the
 +time that Mr. Tesla brought out his motors, and read his thoughtful
 +paper before the American Institute of Electrical Engineers, Professor
 +Ferraris, in Europe, published his discovery of principles
 +<!-- Page 8 -->
 +analogous to those enunciated by Mr. Tesla. There is no doubt, however, that Mr.
 +Tesla was an independent inventor of this rotary field motor, for
 +although anticipated in dates by Ferraris, he could not have known
 +about Ferraris'​ work as it had not been published. Professor Ferraris
 +stated himself, with becoming modesty, that he did not think Tesla
 +could have known of his (Ferraris'​) experiments at that time, and adds
 +that he thinks Tesla was an independent and original inventor of this
 +principle. With such an acknowledgment from Ferraris there can be
 +little doubt about Tesla'​s originality in this matter.</​p>​
 +<​p>​Mr. Tesla'​s work in this field was wonderfully timely, and its worth
 +was promptly appreciated in various quarters. The Tesla patents were
 +acquired by the Westinghouse Electric Company, who undertook to
 +develop his motor and to apply it to work of different kinds. Its use
 +in mining, and its employment in printing, ventilation,​ etc., was
 +described and illustrated in <​i>​The Electrical World</​i>​ some years ago.
 +The immense stimulus that the announcement of Mr. Tesla'​s work gave to
 +the study of alternating current motors would, in itself, be enough to
 +stamp him as a leader.</​p>​
 +<​p>​Mr. Tesla is only 35 years of age. He is tall and spare with a
 +clean-cut, thin, refined face, and eyes that recall all the stories
 +one has read of keenness of vision and phenomenal ability to see
 +through things. He is an omnivorous reader, who never forgets; and he
 +possesses the peculiar facility in languages that enables the least
 +educated native of eastern Europe to talk and write in at least half a
 +dozen tongues. A more congenial companion cannot be desired for the
 +hours when one &​quot;​pours out heart affluence in discursive
 +<!-- Page 9 -->
 +talk,&​quot;​ and when the conversation,​ dealing at first with things near at hand and
 +next to us, reaches out and rises to the greater questions of life, duty and destiny.</​p>​
 +<p>In the year 1890 he severed his connection with the Westinghouse
 +Company, since which time he has devoted himself entirely to the study
 +of alternating currents of high frequencies and very high potentials,
 +with which study he is at present engaged. No comment is necessary on
 +his interesting achievements in this field; the famous London lecture
 +published in this volume is a proof in itself. His first lecture on
 +his researches in this new branch of electricity,​ which he may be said
 +to have created, was delivered before the American Institute of
 +Electrical Engineers on May 20, 1891, and remains one of the most
 +interesting papers read before that society. It will be found
 +reprinted in full in <​i>​The Electrical World</​i>,​ July 11, 1891. Its
 +publication excited such interest abroad that he received numerous
 +requests from English and French electrical engineers and scientists
 +to repeat it in those countries, the result of which has been the
 +interesting lecture published in this volume.</​p>​
 +<​p>​The present lecture presupposes a knowledge of the former, but it may
 +be read and understood by any one even though he has not read the
 +earlier one. It forms a sort of continuation of the latter, and
 +includes chiefly the results of his researches since that time.</​p>​
 +<!-- Page 10 -->
 +<​h3>​WITH </h3>
 +<​h2>​Alternate Currents of High Potential </h2>
 +<​h2>​and High Frequency.</​h2>​
 +<p>I cannot find words to express how deeply I feel the honor of
 +addressing some of the foremost thinkers of the present time, and so
 +many able scientific men, engineers and electricians,​ of the country
 +greatest in scientific achievements.</​p>​
 +<​p>​The results which I have the honor to present before such a gathering
 +I cannot call my own. There are among you not a few who can lay better
 +claim than myself on any feature of merit which this work may contain.
 +I need not mention many names which are world-known&​mdash;​names of those
 +among you who are recognized as the leaders in this enchanting
 +science; but one, at least, I must mention&​mdash;​a name which could not be
 +omitted in a demonstration of this kind. It is a name associated with
 +the most beautiful invention ever made: it is Crookes!</​p>​
 +<​p>​When I was at college, a good time ago, I read, in a translation (for
 +then I was not familiar with your magnificent language), the
 +description of his experiments on radiant matter. I read it only once
 +in my life&​mdash;​that time&​mdash;​yet every
 +<!-- Page 11 -->
 +detail about that charming work I can remember this day. Few are the books,
 +let me say, which can make such an impression upon the mind of a student.</​p>​
 +<​p>​But if, on the present occasion, I mention this name as one of many
 +your institution can boast of, it is because I have more than one
 +reason to do so. For what I have to tell you and to show you this
 +evening concerns, in a large measure, that same vague world which
 +Professor Crookes has so ably explored; and, more than this, when I
 +trace back the mental process which led me to these advances&​mdash;​which
 +even by myself cannot be considered trifling, since they are so
 +appreciated by you&​mdash;​I believe that their real origin, that which
 +started me to work in this direction, and brought me to them, after a
 +long period of constant thought, was that fascinating little book
 +which I read many years ago.</​p>​
 +<​p>​And now that I have made a feeble effort to express my homage and
 +acknowledge my indebtedness to him and others among you, I will make a
 +second effort, which I hope you will not find so feeble as the first,
 +to entertain you.</​p>​
 +<​p>​Give me leave to introduce the subject in a few words.</​p>​
 +<p>A short time ago I had the honor to bring before our American
 +Institute of Electrical Engineers<​a name="​FNanchor_A_1">​
 +</​a><​a href="#​Footnote_A_1"><​sup>​[A]</​sup></​a>​ some results then arrived at by
 +me in a novel line of work. I need not assure you that the many evidences which
 +I have received that English scientific men and engineers were interested
 +<!-- Page 12 -->
 +in this work have been for me a great reward and encouragement. I will not dwell upon
 +the experiments already described, except with the view of completing, or more clearly
 +expressing, some ideas advanced by me before, and also with the view
 +of rendering the study here presented self-contained,​ and my remarks
 +on the subject of this evening'​s lecture consistent.</​p>​
 +<a name="​Footnote_A_1"></​a><​a href="#​FNanchor_A_1">​[A]</​a>​
 +<div class="​fnote"><​p>​ For Mr. Tesla'​s American lecture on this subject see THE
 +ELECTRICAL WORLD of July 11, 1891, and for a report of his French
 +lecture see THE ELECTRICAL WORLD of March 26, 1892.</​p></​div>​
 +<​p>​This investigation,​ then, it goes without saying, deals with
 +alternating currents, and, to be more precise, with alternating
 +currents of high potential and high frequency. Just in how much a very
 +high frequency is essential for the production of the results
 +presented is a question which even with my present experience, would
 +embarrass me to answer. Some of the experiments may be performed with
 +low frequencies;​ but very high frequencies are desirable, not only on
 +account of the many effects secured by their use, but also as a
 +convenient means of obtaining, in the induction apparatus employed,
 +the high potentials, which in their turn are necessary to the
 +demonstration of most of the experiments here contemplated.</​p>​
 +<p>Of the various branches of electrical investigation,​ perhaps the most
 +interesting and immediately the most promising is that dealing with
 +alternating currents. The progress in this branch of applied science
 +has been so great in recent years that it justifies the most sanguine
 +hopes. Hardly have we become familiar with one fact, when novel
 +experiences are met with and new avenues of research are opened. Even
 +at this hour possibilities not dreamed of before are, by the use of these currents,
 +partly realized. As in nature all is ebb and tide, all is wave motion, so it seems
 +<!-- Page 13 -->
 +that; in all branches of industry alternating currents&​mdash;​electric wave
 +motion&​mdash;​will have the sway.</​p>​
 +<​p>​One reason, perhaps, why this branch of science is being so rapidly
 +developed is to be found in the interest which is attached to its
 +experimental study. We wind a simple ring of iron with coils; we
 +establish the connections to the generator, and with wonder and
 +delight we note the effects of strange forces which we bring into
 +play, which allow us to transform, to transmit and direct energy at
 +will. We arrange the circuits properly, and we see the mass of iron
 +and wires behave as though it were endowed with life, spinning a heavy
 +armature, through invisible connections,​ with great speed and
 +power&​mdash;​with the energy possibly conveyed from a great distance. We
 +observe how the energy of an alternating current traversing the wire
 +manifests itself&​mdash;​not so much in the wire as in the surrounding
 +space&​mdash;​in the most surprising manner, taking the forms of heat, light,
 +mechanical energy, and, most surprising of all, even chemical
 +affinity. All these observations fascinate us, and fill us with an
 +intense desire to know more about the nature of these phenomena. Each
 +day we go to our work in the hope of discovering,&​mdash;​in the hope that
 +some one, no matter who, may find a solution of one of the pending
 +great problems,&​mdash;​and each succeeding day we return to our task with
 +renewed ardor; and even if we <​i>​are</​i>​ unsuccessful,​ our work has not
 +been in vain, for in these strivings, in these efforts, we have found
 +hours of untold pleasure, and we have directed our energies to the
 +benefit of mankind.</​p>​
 +<p>We may take&​mdash;​at random, if you choose&​mdash;​any of the
 +many experiments which may be performed with alternating
 +<!-- Page 14 -->
 +currents; a few of which only, and by no means the most striking, form the subject of this
 +evening'​s demonstration:​ they are all equally interesting,​ equally inciting to thought.</​p>​
 +<​p>​Here is a simple glass tube from which the air has been partially
 +exhausted. I take hold of it; I bring my body in contact with a wire
 +conveying alternating currents of high potential, and the tube in my
 +hand is brilliantly lighted. In whatever position I may put it,
 +wherever I may move it in space, as far as I can reach, its soft,
 +pleasing light persists with undiminished brightness.</​p>​
 +<​p>​Here is an exhausted bulb suspended from a single wire. Standing on an
 +insulated support. I grasp it, and a platinum button mounted in it is
 +brought to vivid incandescence.</​p>​
 +<​p>​Here,​ attached to a leading wire, is another bulb, which, as I touch
 +its metallic socket, is filled with magnificent colors of
 +phosphorescent light.</​p>​
 +<​p>​Here still another, which by my fingers'​ touch casts a shadow&​mdash;​the
 +Crookes shadow, of the stem inside of it.</​p>​
 +<​p>​Here,​ again, insulated as I stand on this platform, I bring my body in
 +contact with one of the terminals of the secondary of this induction
 +coil&​mdash;​with the end of a wire many miles long&​mdash;​and you see streams of
 +light break forth from its distant end, which is set in violent
 +<​p>​Here,​ once more, I attach these two plates of wire gauze to the
 +terminals of the coil. I set them a distance apart, and I set the coil
 +to work. You may see a small spark pass between the plates. I insert a
 +thick plate of one of the best dielectrics between them, and instead of rendering
 +altogether impossible, as we are used to expect, I <​i>​aid</​i>​ the passage
 +<!-- Page 15 -->
 +of the discharge, which, as I insert the plate, merely changes in appearance
 +and assumes the form of luminous streams.</​p>​
 +<p>Is there, I ask, can there be, a more interesting study than that of
 +alternating currents?</​p>​
 +<p>In all these investigations,​ in all these experiments,​ which are so
 +very, very interesting,​ for many years past&​mdash;​ever since the greatest
 +experimenter who lectured in this hall discovered its principle&​mdash;​we
 +have had a steady companion, an appliance familiar to every one, a
 +plaything once, a thing of momentous importance now&​mdash;​the induction
 +coil. There is no dearer appliance to the electrician. From the ablest
 +among you, I dare say, down to the inexperienced student, to your
 +lecturer, we all have passed many delightful hours in experimenting
 +with the induction coil. We have watched its play, and thought and
 +pondered over the beautiful phenomena which it disclosed to our
 +ravished eyes. So well known is this apparatus, so familiar are these
 +phenomena to every one, that my courage nearly fails me when I think
 +that I have ventured to address so able an audience, that I have
 +ventured to entertain you with that same old subject. Here in reality
 +is the same apparatus, and here are the same phenomena, only the
 +apparatus is operated somewhat differently,​ the phenomena are
 +presented in a different aspect. Some of the results we find as
 +expected, others surprise us, but all captivate our attention, for in
 +scientific investigation each novel result achieved may be the centre
 +of a new departure, each novel fact learned may lead to important
 +developments. </p>
 +<!-- Page 16 -->
 +<​p>​Usually in operating an induction coil we have set up a
 +vibration of moderate frequency in the primary, either by means of an
 +interrupter or break, or by the use of an alternator. Earlier English
 +investigators,​ to mention only Spottiswoode and J.E.H. Gordon, have
 +used a rapid break in connection with the coil. Our knowledge and
 +experience of to-day enables us to see clearly why these coils under
 +the conditions of the tests did not disclose any remarkable
 +phenomena, and why able experimenters failed to perceive many of the
 +curious effects which have since been observed.</​p>​
 +<p>In the experiments such as performed this evening, we operate the coil
 +either from a specially constructed alternator capable of giving many
 +thousands of reversals of current per second, or, by disruptively
 +discharging a condenser through the primary, we set up a vibration in
 +the secondary circuit of a frequency of many hundred thousand or
 +millions per second, if we so desire; and in using either of these
 +means we enter a field as yet unexplored.</​p>​
 +<p>It is impossible to pursue an investigation in any novel line without
 +finally making some interesting observation or learning some useful
 +fact. That this statement is applicable to the subject of this lecture
 +the many curious and unexpected phenomena which we observe afford a
 +convincing proof. By way of illustration,​ take for instance the most
 +obvious phenomena, those of the discharge of the induction coil.</​p>​
 +<​p>​Here is a coil which is operated by currents vibrating with extreme rapidity,
 +obtained by disruptively discharging a Leyden jar. It would not surprise a student were
 +<!-- Page 17 -->
 +the lecturer to say that the secondary of this coil consists of a small length of
 +comparatively stout wire; it would not surprise him were the lecturer to state that,
 +in spite of this, the coil is capable of giving any potential which the best
 +insulation of the turns is able to withstand: but although he may be
 +prepared, and even be indifferent as to the anticipated result, yet
 +the aspect of the discharge of the coil will surprise and interest
 +him. Every one is familiar with the discharge of an ordinary coil; it
 +need not be reproduced here. But, by way of contrast, here is a form
 +of discharge of a coil, the primary current of which is vibrating
 +several hundred thousand times per second. The discharge of an
 +ordinary coil appears as a simple line or band of light. The discharge
 +of this coil appears in the form of powerful brushes and luminous
 +streams issuing from all points of the two straight wires attached to
 +the terminals of the secondary. (Fig. 1.)</​p>​
 +<div align="​center"><​img src="​images/​acfig01.gif"​ width="​492"​ height="​599"​ border="​0"​
 +<​p>​Now compare this phenomenon which you have just witnessed with the
 +discharge of a Holtz or Wimshurst machine&​mdash;​that other interesting
 +appliance so dear to the experimenter. What a difference there is
 +between these phenomena! And yet, had I made the necessary
 +arrangements&​mdash;​which could have been made easily, were it not that they
 +would interfere with other experiments&​mdash;​I could have produced with
 +this coil sparks which, had I the coil hidden from your view and only
 +two knobs exposed, even the keenest observer among you would find it
 +difficult, if not impossible, to distinguish from those of an
 +influence or friction machine. This may be done in many ways&​mdash;​for
 +instance, by operating the induction coil which charges the condenser
 +<!-- Page 18 -->
 +from an alternating-current machine of very low frequency, and
 +preferably adjusting the discharge circuit so that there are no
 +oscillations set up in it. We then obtain in the secondary circuit, if
 +the knobs are of the required size and properly set, a more or less rapid <br>
 +succession of sparks of great intensity and small quantity, which possess
 +<!-- Page 19 -->
 +the same brilliancy, and are accompanied by the same sharp crackling sound,
 +as those obtained from a friction or influence machine.</​p>​
 +<img src="​images/​acfig02.gif"​ width="​178"​ height="​663"​ border="​0"​ align="​left"​ hspace="​10"​
 +Another way is to pass through two primary circuits, having a common
 +secondary, two currents of a slightly different period, which produce
 +in the secondary circuit sparks occurring at comparatively long
 +intervals. But, even with the means at hand this evening, I may
 +succeed in imitating the spark of a Holtz machine. For this purpose I
 +establish between the terminals of the coil which charges the
 +condenser a long, unsteady arc, which is periodically interrupted by
 +the upward current of air produced by it. To increase the current of
 +air I place on each side of the arc, and close to it, a large plate of
 +mica. The condenser charged from this coil discharges into the primary
 +circuit of a second coil through a small air gap, which is necessary
 +to produce a sudden rush of current through the primary. The scheme of
 +connections in the present experiment is indicated in Fig. 2.</​p>​
 +<​p><​i>​G</​i>​ is an ordinarily constructed alternator, supplying the primary <​i>​P</​i>​
 +of an induction coil, the secondary <​i>​S</​i>​ of which
 +<!-- Page 20 -->
 +charges the condensers or jars <​i>​C&​nbsp;​C</​i>​. The terminals of the secondary
 +are connected to the inside coatings of the jars, the outer coatings being connected
 +to the ends of the primary <​i>​p&​nbsp;​p</​i>​ of a second induction coil. This
 +primary <​i>​p&​nbsp;​p</​i>​ has a small air gap <​i>​a&​nbsp;​b</​i>​.</​p>​
 +<​p>​The secondary <​i>​s</​i>​ of this coil is provided with knobs or spheres <​i>​K&​nbsp;​K</​i>​
 +of the proper size and set at a distance suitable for the experiment.</​p>​
 +<p>A long arc is established between the terminals <​i>​A&​nbsp;​B</​i>​ of the first
 +induction coil. <​i>​M&​nbsp;​M</​i>​ are the mica plates.</​p>​
 +<​p>​Each time the arc is broken between <​i>​A</​i>​ and <​i>​B</​i>​ the jars are quickly
 +charged and discharged through the primary <​i>​p&​nbsp;​p</​i>,​ producing a snapping
 +spark between the knobs <​i>​K&​nbsp;​K</​i>​. Upon the arc forming between <​i>​A</​i>​ and <​i>​B</​i>​
 +the potential falls, and the jars cannot be charged to such high
 +potential as to break through the air gap <​i>​a&​nbsp;​b</​i>​ until the arc is again
 +broken by the draught.</​p>​
 +<p>In this manner sudden impulses, at long intervals, are produced in the
 +primary <​i>​p&​nbsp;​p</​i>,​ which in the secondary <​i>​s</​i>​ give a corresponding number
 +of impulses of great intensity. If the secondary knobs or spheres,
 +<​i>​K&​nbsp;​K</​i>,​ are of the proper size, the sparks show much resemblance to
 +those of a Holtz machine.</​p>​
 +<​p>​But these two effects, which to the eye appear so very different, are
 +only two of the many discharge phenomena. We only need to change the
 +conditions of the test, and again we make other observations of
 +<​p>​When,​ instead of operating the induction coil as in the last two experiments,​
 +we operate it from a high frequency alternator, as in the next experiment, a systematic study
 +<!-- Page 21 -->
 +of the phenomena is rendered much more easy. In such case, in varying the strength
 +and frequency of the currents through the primary, we may observe five distinct forms
 +of discharge, which I have described in my former paper on the subject
 +<a name="​FNanchor_A_2"></​a><​a href="#​Footnote_A_2"><​sup>​[A]</​sup></​a>​
 +before the American Institute of Electrical Engineers, May 20, 1891.</​p>​
 +<a name="​Footnote_A_2"></​a><​a href="#​FNanchor_A_2">​[A]</​a><​div class="​fnote">​
 +<p> See THE ELECTRICAL WORLD, July 11, 1891.</​p></​div>​
 +<p>It would take too much time, and it would lead us too far from the
 +subject presented this evening, to reproduce all these forms, but it
 +seems to me desirable to show you one of them. It is a brush
 +discharge, which is interesting in more than one respect. Viewed from
 +a near position it resembles much a jet of gas escaping under great
 +pressure. We know that the phenomenon is due to the agitation of the
 +molecules near the terminal, and we anticipate that some heat must be
 +developed by the impact of the molecules against the terminal or
 +against each other. Indeed, we find that the brush is hot, and only a
 +little thought leads us to the conclusion that, could we but reach
 +sufficiently high frequencies,​ we could produce a brush which would
 +give intense light and heat, and which would resemble in every
 +particular an ordinary flame, save, perhaps, that both phenomena might
 +not be due to the same agent&​mdash;​save,​ perhaps, that chemical affinity
 +might not be <​i>​electrical</​i>​ in its nature.</​p>​
 +<p>As the production of heat and light is here due to the impact of the
 +molecules, or atoms of air, or something else besides, and, as we can augment
 +the energy simply by raising the potential, we might, even with frequencies obtained
 +<!-- Page 22 -->
 +from a dynamo machine, intensify the action to such a degree as to bring
 +the terminal to melting heat. But with such low frequencies we would have to deal
 +always with something of the nature of an electric current. If I approach a conducting
 +object to the brush, a thin little spark passes, yet, even with the
 +frequencies used this evening, the tendency to spark is not very
 +great. So, for instance, if I hold a metallic sphere at some distance
 +above the terminal you may see the whole space between the terminal
 +and sphere illuminated by the streams without the spark passing; and
 +with the much higher frequencies obtainable by the disruptive
 +discharge of a condenser, were it not for the sudden impulses, which
 +are comparatively few in number, sparking would not occur even at very
 +small distances. However, with incomparably higher frequencies,​ which
 +we may yet find means to produce efficiently,​ and provided that
 +electric impulses of such high frequencies could be transmitted
 +through a conductor, the electrical characteristics of the brush
 +discharge would completely vanish&​mdash;​no spark would pass, no shock would
 +be felt&​mdash;​yet we would still have to deal with an <​i>​electric</​i>​
 +phenomenon, but in the broad, modern interpretation of the word. In my
 +first paper before referred to I have pointed out the curious
 +properties of the brush, and described the best manner of producing
 +it, but I have thought it worth while to endeavor to express myself
 +more clearly in regard to this phenomenon, because of its absorbing
 +<​p>​When a coil is operated with currents of very high frequency,
 +beautiful brush effects may be produced, even if the coil be of
 +comparatively small dimensions. The experimenter
 +<!-- Page 23 -->
 +may vary them in many ways, and, if it were nothing else, they afford a pleasing sight.
 +What adds to their interest is that they may be produced with one
 +single terminal as well as with two&​mdash;​in fact, often better with one
 +than with two.</​p>​
 +<​p>​But of all the discharge phenomena observed, the most pleasing to the
 +eye, and the most instructive,​ are those observed with a coil which is
 +operated by means of the disruptive discharge of a condenser. The
 +power of the brushes, the abundance of the sparks, when the conditions
 +are patiently adjusted, is often amazing. With even a very small coil,
 +if it be so well insulated as to stand a difference of potential of
 +several thousand volts per turn, the sparks may be so abundant that
 +the whole coil may appear a complete mass of fire.</​p>​
 +<​p>​Curiously enough the sparks, when the terminals of the coil are set at
 +a considerable distance, seem to dart in every possible direction as
 +though the terminals were perfectly independent of each other. As the
 +sparks would soon destroy the insulation it is necessary to prevent
 +them. This is best done by immersing the coil in a good liquid
 +insulator, such as boiled-out oil. Immersion in a liquid may be
 +considered almost an absolute necessity for the continued and
 +successful working of such a coil.</​p>​
 +<p>It is of course out of the question, in an experimental lecture, with
 +only a few minutes at disposal for the performance of each experiment,
 +to show these discharge phenomena to advantage, as to produce each
 +phenomenon at its best a very careful adjustment is required. But even
 +if imperfectly produced, as they are likely to be this evening,
 +<!-- Page 24 -->
 +they are sufficiently striking to interest an intelligent audience.</​p>​
 +<​p>​Before showing some of these curious effects I must, for the sake of
 +completeness,​ give a short description of the coil and other apparatus
 +used in the experiments with the disruptive discharge this evening.</​p>​
 +<div align="​center"><​img src="​images/​acfig03.gif"​ width="​476"​ height="​575"​ border="​0"​
 +alt="​FIG. 3.&​mdash;​DISRUPTIVE DISCHARGE COIL."></​div>​
 +<p>It is contained in a box <​i>​B</​i>​ (Fig. 3) of thick boards of hard wood,
 +covered on the outside with zinc sheet <​i>​Z</​i>,​ which is
 +<!-- Page 25 -->
 +carefully soldered all around. It might be advisable, in a strictly scientific
 +investigation,​ when accuracy is of great importance, to do away with
 +the metal cover, as it might introduce many errors, principally on
 +account of its complex action upon the coil, as a condenser of very
 +small capacity and as an electrostatic and electromagnetic screen.
 +When the coil is used for such experiments as are here contemplated,​
 +the employment of the metal cover offers some practical advantages,
 +but these are not of sufficient importance to be dwelt upon.</​p>​
 +<​p>​The coil should be placed symmetrically to the metal cover, and the
 +space between should, of course, not be too small, certainly not less
 +than, say, five centimetres,​ but much more if possible; especially the
 +two sides of the zinc box, which are at right angles to the axis of
 +the coil, should be sufficiently remote from the latter, as otherwise
 +they might impair its action and be a source of loss.</​p>​
 +<​p>​The coil consists of two spools of hard rubber <​i>​R&​nbsp;​R</​i>,​ held apart at a
 +distance of 10 centimetres by bolts <​i>​c</​i>​ and nuts <​i>​n</​i>,​ likewise of hard
 +rubber. Each spool comprises a tube <​i>​T</​i>​ of approximately 8 centimetres
 +inside diameter, and 3 millimetres thick, upon which are screwed two
 +flanges <​i>​F&​nbsp;​F</​i>,​ 24 centimetres square, the space between the flanges
 +being about 3 centimetres. The secondary, <​i>​S&​nbsp;​S</​i>,​ of the best gutta
 +percha-covered wire, has 26 layers, 10 turns in each, giving for each
 +half a total of 260 turns. The two halves are wound oppositely and
 +connected in series, the connection between both being made over the
 +primary. This disposition,​ besides being convenient, has the advantage
 +that when the coil is well balanced&​mdash;​that is, when both of
 +<!-- Page 26 -->
 +its terminals <​i>​T</​i><​sub>​1</​sub>&​nbsp;<​i>​T</​i><​sub>​1</​sub>​ are connected
 +to bodies or devices of equal capacity&​mdash;​there is not much danger of
 +breaking through to the primary, and the insulation between the primary and
 +the secondary need not be thick. In using the coil it is advisable to attach to
 +<​i>​both</​i>​ terminals devices of nearly equal capacity, as, when the capacity of the
 +terminals is not equal, sparks will be apt to pass to the primary. To
 +avoid this, the middle point of the secondary may be connected to the
 +primary, but this is not always practicable.</​p>​
 +<​p>​The primary <​i>​P&​nbsp;​P</​i>​ is wound in two parts, and oppositely, upon a wooden
 +spool <​i>​W</​i>,​ and the four ends are led out of the oil through hard
 +rubber tubes <​i>​t&​nbsp;​t</​i>​. The ends of the secondary <​i>​T</​i><​sub>​1</​sub>&​nbsp;<​i>​T</​i><​sub>​1</​sub>​
 +are also led out of the oil through rubber tubes <​i>​t</​i><​sub>​1</​sub>&​nbsp;<​i>​t</​i><​sub>​1</​sub>​
 +of great thickness. The primary and secondary layers are insulated by cotton cloth, the
 +thickness of the insulation, of course, bearing some proportion to the
 +difference of potential between the turns of the different layers.
 +Each half of the primary has four layers, 24 turns in each, this
 +giving a total of 96 turns. When both the parts are connected in
 +series, this gives a ratio of conversion of about 1:2.7, and with the
 +primaries in multiple, 1:5.4; but in operating with very rapidly
 +alternating currents this ratio does not convey even an approximate
 +idea of the ratio of the E.M.Fs. in the primary and secondary
 +circuits. The coil is held in position in the oil on wooden supports,
 +there being about 5 centimetres thickness of oil all round. Where the
 +oil is not specially needed, the space is filled with pieces of wood,
 +and for this purpose principally the wooden box <​i>​B</​i>​ surrounding the
 +whole is used. </p>
 +<!-- Page 27 -->
 +<​p>​The construction here shown is, of course, not the
 +best on general principles, but I believe it is a good and convenient
 +one for the production of effects in which an excessive potential and
 +a very small current are needed.</​p>​
 +<p>In connection with the coil I use either the ordinary form of
 +discharger or a modified form. In the former I have introduced two
 +changes which secure some advantages, and which are obvious. If they
 +are mentioned, it is only in the hope that some experimenter may find
 +them of use.</​p>​
 +<div align="​center"><​img src="​images/​acfig04.gif"​ width="​692"​ height="​367"​ border="​0"​
 +<​p>​One of the changes is that the adjustable knobs <​i>​A</​i>​ and <​i>​B</​i>​ (Fig. 4),
 +of the discharger are held in jaws of brass, <​i>​J&​nbsp;​J</​i>,​ by spring pressure,
 +this allowing of turning them successively into different positions,
 +and so doing away with the tedious process of frequent polishing up.</​p>​
 +<​p>​The other change consists in the employment of a strong electromagnet
 +<​i>​N&​nbsp;​S</​i>,​ which is placed with its axis at right angles to the line
 +joining the knobs <​i>​A</​i>​ and <​i>​B</​i>,​ and produces a strong magnetic field
 +between them. The pole pieces of
 +<!-- Page 28 -->the magnet are movable and properly
 +formed so as to protrude between the brass knobs, in order to make the
 +field as intense as possible; but to prevent the discharge from
 +jumping to the magnet the pole pieces are protected by a layer of
 +mica, <​i>​M&​nbsp;​M</​i>,​ of sufficient thickness.
 +and <​i>​s</​i><​sub>​2</​sub>&​nbsp;<​i>​s</​i><​sub>​2</​sub>​ are
 +screws for fastening the wires. On each side one of the screws is for
 +large and the other for small wires. <​i>​L&​nbsp;​L</​i>​ are screws for fixing in
 +position the rods <​i>​R&​nbsp;​R</​i>,​ which support the knobs.</​p>​
 +<p>In another arrangement with the magnet I take the discharge between
 +the rounded pole pieces themselves, which in such case are insulated
 +and preferably provided with polished brass caps.</​p>​
 +<​p>​The employment of an intense magnetic field is of advantage
 +principally when the induction coil or transformer which charges the
 +condenser is operated by currents of very low frequency. In such a
 +case the number of the fundamental discharges between the knobs may be
 +so small as to render the currents produced in the secondary
 +unsuitable for many experiments. The intense magnetic field then
 +serves to blow out the arc between the knobs as soon as it is formed,
 +and the fundamental discharges occur in quicker succession.</​p>​
 +<​p>​Instead of the magnet, a draught or blast of air may be employed with
 +some advantage. In this case the arc is preferably established between
 +the knobs <​i>​A&​nbsp;​B</​i>,​ in Fig. 2 (the knobs <​i>​a&​nbsp;​b</​i>​ being generally joined, or
 +entirely done away with), as in this disposition the arc is long and
 +unsteady, and is easily affected by the draught.</​p>​
 +<div align="​center"><​img src="​images/​acfig05.gif"​ width="​588"​ height="​210"​ border="​0"​
 +<​p>​When a magnet is employed to break the arc, it is
 +<!-- Page 29 -->
 +better to choose the connection indicated diagrammatically in Fig. 5, as in this case
 +the currents forming the arc are much more powerful, and the magnetic
 +field exercises a greater influence. The use of the magnet permits,
 +however, of the arc being replaced by a vacuum tube, but I have
 +encountered great difficulties in working with an exhausted tube.</​p>​
 +<div align="​center"><​img src="​images/​acfig06.gif"​ width="​564"​ height="​226"​ border="​0"​
 +alt="​FIG. 6.&​mdash;​DISCHARGER WITH MULTIPLE GAPS."></​div>​
 +<​p>​The other form of discharger used in these and similar experiments is
 +indicated in Figs. 6 and 7. It consists of a number of brass pieces
 +<​i>​c&​nbsp;​c</​i>​ (Fig. 6), each of which comprises a spherical middle portion <​i>​m</​i>​
 +with an extension <​i>​e</​i>​ below&​mdash;​which is merely used to fasten the piece
 +in a lathe when polishing up the discharging surface&​mdash;​and a column
 +above, which consists of a knurled flange <​i>​f</​i>​ surmounted by a threaded
 +stem <​i>​l</​i>​ carrying a nut <​i>​n</​i>,​ by means of which a
 +<!-- Page 30 -->
 +wire is fastened to the column. The flange <​i>​f</​i>​ conveniently serves for holding
 +the brass piece when fastening the wire, and also for turning it in any position
 +when it becomes necessary to present a fresh discharging surface. Two
 +stout strips of hard rubber <​i>​R&​nbsp;​R</​i>,​ with planed grooves <​i>​g&​nbsp;​g</​i>​ (Fig. 7)
 +to fit the middle portion of the pieces <​i>​c&​nbsp;​c</​i>,​ serve to clamp the latter
 +and hold them firmly in position by means of two bolts <​i>​C&​nbsp;​C</​i>​ (of which
 +only one is shown) passing through the ends of the strips.</​p>​
 +<div align="​center"><​img src="​images/​acfig07.gif"​ width="​557"​ height="​373"​ border="​0"​
 +alt="​FIG. 7.&​mdash;​DISCHARGER WITH MULTIPLE GAPS."></​div>​
 +<p>In the use of this kind of discharger I have found three principal
 +advantages over the ordinary form. First, the dielectric strength of a
 +given total width of air space is greater when a great many small air
 +gaps are used instead of one, which permits of working with a smaller
 +length of air gap, and that means smaller loss and less deterioration of the metal;
 +secondly by reason of splitting the arc up into smaller arcs, the polished surfaces
 +are made to last much longer; and, thirdly, the apparatus affords some
 +<!-- Page 31 -->
 +gauge in the experiments. I usually set the pieces by putting between them
 +sheets of uniform thickness at a certain very small distance which is known from the
 +experiments of Sir William Thomson to require a certain electromotive
 +force to be bridged by the spark.</​p>​
 +<p>It should, of course, be remembered that the sparking distance is much
 +diminished as the frequency is increased. By taking any number of
 +spaces the experimenter has a rough idea of the electromotive force,
 +and he finds it easier to repeat an experiment, as he has not the
 +trouble of setting the knobs again and again. With this kind of
 +discharger I have been able to maintain an oscillating motion without
 +any spark being visible with the naked eye between the knobs, and they
 +would not show a very appreciable rise in temperature. This form of
 +discharge also lends itself to many arrangements of condensers and
 +circuits which are often very convenient and time-saving. I have used
 +it preferably in a disposition similar to that indicated in Fig. 2,
 +when the currents forming the arc are small.</​p>​
 +<p>I may here mention that I have also used dischargers with single or
 +multiple air gaps, in which the discharge surfaces were rotated with
 +great speed. No particular advantage was, however, gained by this
 +method, except in cases where the currents from the condenser were
 +large and the keeping cool of the surfaces was necessary, and in cases
 +when, the discharge not being oscillating of itself, the arc as soon
 +as established was broken by the air current, thus starting the vibration
 +at intervals in rapid succession. I have also used mechanical interrupters
 +in many ways. To avoid the difficulties with frictional contacts, the preferred
 +<!-- Page 32 -->
 +plan adopted was to establish the arc and rotate through it at great speed a
 +rim of mica provided with many holes and fastened to a steel plate. It is understood,
 +of course, that the employment of a magnet, air current, or other interrupter,​
 +produces no effect worth noticing, unless the self-induction,​ capacity
 +and resistance are so related that there are oscillations set up upon
 +each interruption.</​p>​
 +<p>I will now endeavor to show you some of the most note-worthy of these
 +discharge phenomena.</​p>​
 +<p>I have stretched across the room two ordinary cotton covered wires,
 +each about 7 metres in length. They are supported on insulating cords
 +at a distance of about 30 centimetres. I attach now to each of the
 +terminals of the coil one of the wires and set the coil in action.
 +Upon turning the lights off in the room you see the wires strongly
 +illuminated by the streams issuing abundantly from their whole surface
 +in spite of the cotton covering, which may even be very thick. When
 +the experiment is performed under good conditions, the light from the
 +wires is sufficiently intense to allow distinguishing the objects in a
 +room. To produce the best result it is, of course, necessary to adjust
 +carefully the capacity of the jars, the arc between the knobs and the
 +length of the wires. My experience is that calculation of the length
 +of the wires leads, in such case, to no result whatever. The
 +experimenter will do best to take the wires at the start very long,
 +and then adjust by cutting off first long pieces, and then smaller and
 +smaller ones as he approaches the right length.</​p>​
 +<p>A convenient way is to use an oil condenser of very small capacity,
 +consisting of two small adjustable metal
 +<!-- Page 33 -->
 +plates, in connection with this and similar experiments. In such case I take wires
 +rather short and set at the beginning the condenser plates at maximum distance.
 +If the streams for the wires increase by approach of the plates, the
 +length of the wires is about right; if they diminish the wires are too
 +long for that frequency and potential. When a condenser is used in
 +connection with experiments with such a coil, it should be an oil
 +condenser by all means, as in using an air condenser considerable
 +energy might be wasted. The wires leading to the plates in the oil
 +should be very thin, heavily coated with some insulating compound, and
 +provided with a conducting covering&​mdash;​this preferably extending under
 +the surface of the oil. The conducting cover should not be too near
 +the terminals, or ends, of the wire, as a spark would be apt to jump
 +from the wire to it. The conducting coating is used to diminish the
 +air losses, in virtue of its action as an electrostatic screen. As to
 +the size of the vessel containing the oil, and the size of the plates,
 +the experimenter gains at once an idea from a rough trial. The size of
 +the plates <i>in oil</​i>​ is, however, calculable, as the dielectric losses are very small.</​p>​
 +<p>In the preceding experiment it is of considerable interest to know
 +what relation the quantity of the light emitted bears to the frequency
 +and potential of the electric impulses. My opinion is that the heat as
 +well as light effects produced should be proportionate,​ under
 +otherwise equal conditions of test, to the product of frequency and
 +square of potential, but the experimental verification of the law,
 +whatever it may be, would be exceedingly difficult. One
 +<!-- Page 34 -->
 +thing is certain, at any rate, and that is, that in augmenting the potential
 +and frequency we rapidly intensify the streams; and, though it may be
 +very sanguine, it is surely not altogether hopeless to expect that we
 +may succeed in producing a practical illuminant on these lines. We
 +would then be simply using burners or flames, in which there would be
 +no chemical process, no consumption of material, but merely a transfer
 +of energy, and which would, in all probability emit more light and
 +less heat than ordinary flames.</​p>​
 +<​p>​The luminous intensity of the streams is, of course, considerably
 +<!-- Page 35 -->
 +increased when they are focused upon a small surface. This may be
 +shown by the following experiment:</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig08.gif"​ width="​554"​ height="​514"​ border="​0"​
 +<p>I attach to one of the terminals of the coil a wire <​i>​w</​i>​ (Fig. 8), bent
 +in a circle of about 30 centimetres in diameter, and to the other
 +terminal I fasten a small brass sphere <​i>​s</​i>,​ the surface of the wire
 +being preferably equal to the surface of the sphere, and the centre of
 +the latter being in a line at right angles to the plane of the wire
 +circle and passing through its centre. When the discharge is
 +established under proper conditions, a luminous hollow cone is formed,
 +and in the dark one-half of the brass sphere is strongly illuminated,​
 +as shown in the cut.</​p>​
 +<p>By some artifice or other, it is easy to concentrate the streams upon
 +small surfaces and to produce very strong light effects. Two thin
 +wires may thus be rendered intensely luminous.</​p>​
 +<p>In order to intensify the streams the wires should be very thin and
 +short; but as in this case their capacity would be generally too small
 +for the coil&​mdash;​at least, for such a one as the present&​mdash;​it is necessary
 +to augment the capacity to the required value, while, at the same
 +time, the surface of the wires remains very small. This may be done in
 +many ways.</​p>​
 +<​p>​Here,​ for instance, I have two plates, <​i>​R&​nbsp;​R</​i>,​ of hard rubber (Fig. 9),
 +upon which I have glued two very thin wires <​i>​w&​nbsp;​w</​i>,​ so as to form a
 +name. The wires may be bare or covered with the best insulation&​mdash;​it is
 +immaterial for the success of the experiment. Well insulated wires, if anything, are preferable.
 +On the back of each plate, indicated by the shaded portion, is a tinfoil coating
 +<!-- Page 36 -->
 +<​i>​t&​nbsp;​t</​i>​. The plates are placed in line at a sufficient distance to prevent a
 +spark passing from one to the other wire. The two tinfoil coatings I have joined by a
 +conductor <​i>​C</​i>,​ and the two wires I presently connect to the terminals of the
 +coil. It is now easy, by varying the strength and frequency of the
 +currents through the primary, to find a point at which, the capacity
 +of the system is best suited to the conditions, and the wires become
 +so strongly luminous that, when the light in the room is turned off
 +the name formed by them appears in brilliant letters.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig09.gif"​ width="​557"​ height="​528"​ border="​0"​
 +<p>It is perhaps preferable to perform this experiment with a coil
 +operated from an alternator of high frequency, as
 +<!-- Page 37 -->
 +then, owing to the harmonic rise and fall, the streams are very uniform, though
 +they are less abundant then when produced with such a coil as the present. This
 +experiment, however, may be performed with low frequencies,​ but much
 +less satisfactorily.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig10.gif"​ width="​325"​ height="​559"​ border="​0"​
 +alt="​FIG. 10.&​mdash;​LUMINOUS DISCS.">​
 +<​p>​When two wires, attached to the terminals of the coil, are set at the
 +proper distance, the streams between them may be so intense as to
 +produce a continuous luminous sheet. To show this phenomenon I have here
 +two circles, <​i>​C</​i>​ and <​i>​c</​i>​ (Fig. 10), of rather stout wire, one being about
 +<!-- Page 38 -->
 +80 centimetres and the other 30 centimetres in diameter.
 +To each of the terminals of the coil I attach one of the circles. The
 +supporting wires are so bent that the circles may be placed in the
 +same plane, coinciding as nearly as possible. When the light in the
 +room is turned off and the coil set to work, you see the whole space
 +between the wires uniformly filled with streams, forming a luminous
 +disc, which could be seen from a considerable distance, such is the
 +intensity of the streams. The outer circle could have been much larger
 +than the present one; in fact, with this coil I have used much larger
 +circles, and I have been able to produce a strongly luminous sheet,
 +covering an area of more than one square metre, which is a remarkable
 +effect with this very small coil. To avoid uncertainty,​ the circle has
 +been taken smaller, and the area is now about 0.43 square metre.</​p>​
 +<​p>​The frequency of the vibration, and the quickness of succession of the
 +sparks between the knobs, affect to a marked degree the appearance of
 +the streams. When the frequency is very low, the air gives way in more
 +or less the same manner, as by a steady difference of potential, and
 +the streams consist of distinct threads, generally mingled with thin
 +sparks, which probably correspond to the successive discharges
 +occurring between the knobs. But when the frequency is extremely high,
 +and the arc of the discharge produces a very <​i>​loud</​i>​ but <​i>​smooth</​i>​
 +sound&​mdash;​showing both that oscillation takes place and that the sparks
 +succeed each other with great rapidity&​mdash;​then the luminous streams
 +formed are perfectly uniform. To reach this result very small coils
 +and jars of small capacity should be used. I
 +<!-- Page 39 -->
 +take two tubes of thick Bohemian glass, about 5 centimetres in diameter and
 +20 centimetres long. In each of the tubes I slip a primary of very thick copper wire.
 +On the top of each tube I wind a secondary of much thinner
 +gutta-percha covered wire. The two secondaries I connect in series,
 +the primaries preferably in multiple arc. The tubes are then placed in
 +a large glass vessel, at a distance of 10 to 15 centimetres from each
 +other, on insulating supports, and the vessel is filled with boiled
 +out oil, the oil reaching about an inch above the tubes. The free ends
 +of the secondary are lifted out of the oil and placed parallel to each
 +other at a distance of about 10 centimetres. The ends which are
 +scraped should be dipped in the oil. Two four-pint jars joined in
 +series may be used to discharge through the primary. When the
 +necessary adjustments in the length and distance of the wires above
 +the oil and in the arc of discharge are made, a luminous sheet is
 +produced between the wires which is perfectly smooth and textureless,​
 +like the ordinary discharge through a moderately exhausted tube.</​p>​
 +<p>I have purposely dwelt upon this apparently insignificant experiment.
 +In trials of this kind the experimenter arrives at the startling
 +conclusion that, to pass ordinary luminous discharges through gases,
 +no particular degree of exhaustion is needed, but that the gas may be
 +at ordinary or even greater pressure. To accomplish this, a very high
 +frequency is essential; a high potential is likewise required, but this is a
 +merely incidental necessity. These experiments teach us that, in endeavoring
 +to discover novel methods of producing light by the agitation of atoms, or
 +<!-- Page 40 -->
 +molecules, of a gas, we need not limit our research to the vacuum tube, but may
 +look forward quite seriously to the possibility of obtaining the light effects without
 +the use of any vessel whatever, with air at ordinary pressure.</​p>​
 +<​p>​Such discharges of very high frequency, which render luminous the air
 +at ordinary pressures, we have probably often occasion to witness in
 +Nature. I have no doubt that if, as many believe, the aurora borealis
 +is produced by sudden cosmic disturbances,​ such as eruptions at the
 +sun's surface, which set the electrostatic charge of the earth in an
 +extremely rapid vibration, the red glow observed is not confined to
 +the upper rarefied strata of the air, but the discharge traverses, by
 +reason of its very high frequency, also the dense atmosphere in the
 +form of a <​i>​glow</​i>,​ such as we ordinarily produce in a slightly
 +exhausted tube. If the frequency were very low, or even more so, if
 +the charge were not at all vibrating, the dense air would break down
 +as in a lightning discharge. Indications of such breaking down of the
 +lower dense strata of the air have been repeatedly observed at the
 +occurrence of this marvelous phenomenon; but if it does occur, it can
 +only be attributed to the fundamental disturbances,​ which are few in
 +number, for the vibration produced by them would be far too rapid to
 +allow a disruptive break. It is the original and irregular impulses
 +which affect the instruments;​ the superimposed vibrations probably
 +pass unnoticed.</​p>​
 +<​p>​When an ordinary low frequency discharge is passed through moderately
 +rarefied air, the air assumes a purplish hue. If by some means or other
 +we increase the intensity of the molecular, or atomic, vibration, the gas changes to
 +<!-- Page 41 -->
 +a white color. A similar change occurs at ordinary pressures with electric impulses
 +of very high frequency. If the molecules of the air around a wire are moderately agitated,
 +the brush formed is reddish or violet; if the vibration is rendered
 +sufficiently intense, the streams become white. We may accomplish this
 +in various ways. In the experiment before shown with the two wires
 +across the room, I have endeavored to secure the result by pushing to
 +a high value both the frequency and potential: in the experiment with
 +the thin wires glued on the rubber plate I have concentrated the
 +action upon a very small surface&​mdash;​in other words, I have worked with a
 +great electric density.</​p>​
 +<p>A most curious form of discharge is observed with such a coil when the
 +frequency and potential are pushed to the extreme limit. To perform
 +the experiment, every part of the coil should be heavily insulated,
 +and only two small spheres&​mdash;​or,​ better still, two sharp-edged metal
 +discs (<​i>​d&​nbsp;​d</​i>,​ Fig. 11) of no more than a few centimetres in
 +diameter&​mdash;​should be exposed to the air. The coil here used is immersed
 +in oil, and the ends of the secondary reaching out of the oil are
 +covered with an air-tight cover of hard rubber of great thickness. All
 +cracks, if there are any, should be carefully stopped up, so that the
 +brush discharge cannot form anywhere except on the small spheres or
 +plates which are exposed to the air. In this case, since there are no
 +large plates or other bodies of capacity attached to the terminals,
 +the coil is capable of an extremely rapid vibration. The potential may
 +be raised by increasing, as far as the experimenter judges proper, the
 +rate of change of the primary current. With a coil not widely
 +<!-- Page 42 -->
 +differing from the present, it is best to connect the two primaries
 +in multiple arc; but if the secondary should have a much greater
 +number of turns the primaries should preferably be used in series, as
 +otherwise the vibration might be too fast for the secondary. It occurs
 +under these conditions that misty white streams break forth from the
 +edges of the discs and spread out phantom-like into space. </p>
 +<div align="​center">​
 +<img src="​images/​acfig11.gif"​ width="​548"​ height="​535"​ border="​0"​
 +alt="​FIG. 11.&​mdash;​PHANTOM STREAMS.">​
 +<​p>​With this coil, when fairly well produced, they are about 25 to 30 centimetres
 +long. When the hand is held against them no sensation is produced, and
 +a spark, causing a shock, jumps from the terminal only upon the hand
 +being brought much nearer. If the oscillation of the primary
 +<!-- Page 43 -->
 +current is rendered intermittent by some means or other, there is a
 +corresponding throbbing of the streams, and now the hand or other
 +conducting object may be brought in still greater proximity to the
 +terminal without a spark being caused to jump.</​p>​
 +<​p>​Among the many beautiful phenomena which may be produced with such a
 +coil I have here selected only those which appear to possess some
 +features of novelty, and lead us to some conclusions of interest. One
 +will not find it at all difficult to produce in the laboratory, by
 +means of it, many other phenomena which appeal to the eye even more
 +than these here shown, but present no particular feature of novelty.</​p>​
 +<​p>​Early experimenters describe the display of sparks produced by an
 +ordinary large induction coil upon an insulating plate separating the
 +terminals. Quite recently Siemens performed some experiments in which
 +fine effects were obtained, which were seen by many with interest. No
 +doubt large coils, even if operated with currents of low frequencies,​
 +are capable of producing beautiful effects. But the largest coil ever
 +made could not, by far, equal the magnificent display of streams and
 +sparks obtained from such a disruptive discharge coil when properly
 +adjusted. To give an idea, a coil such as the present one will cover
 +easily a plate of 1 metre in diameter completely with the streams. The
 +best way to perform such experiments is to take a very thin rubber or
 +a glass plate and glue on one side of it a narrow ring of tinfoil of
 +very large diameter, and on the other a circular washer, the centre of the
 +latter coinciding with that of the ring, and the surfaces of both being preferably
 +<!-- Page 44 -->
 +equal, so as to keep the coil well balanced. The washer and ring should be
 +connected to the terminals by heavily insulated thin wires. It is easy in observing
 +the effect of the capacity to produce a sheet of uniform streams, or a fine network
 +of thin silvery threads, or a mass of loud brilliant sparks, which
 +completely cover the plate.</​p>​
 +<​p>​Since I have advanced the idea of the conversion by means of the
 +disruptive discharge, in my paper before the American Institute of
 +Electrical Engineers at the beginning of the past year, the interest
 +excited in it has been considerable. It affords us a means for
 +producing any potentials by the aid of inexpensive coils operated from
 +ordinary systems of distribution,​ and&​mdash;​what is perhaps more
 +appreciated&​mdash;​it enables us to convert currents of any frequency into
 +currents of any other lower or higher frequency. But its chief value
 +will perhaps be found in the help which it will afford us in the
 +investigations of the phenomena of phosphorescence,​ which a disruptive
 +discharge coil is capable of exciting in innumerable cases where
 +ordinary coils, even the largest, would utterly fail.</​p>​
 +<​p>​Considering its probable uses for many practical purposes, and its
 +possible introduction into laboratories for scientific research, a few
 +additional remarks as to the construction of such a coil will perhaps
 +not be found superfluous.</​p>​
 +<p>It is, of course, absolutely necessary to employ in such a coil wires
 +provided with the best insulation.</​p>​
 +<​p>​Good coils may be produced by employing wires covered with several
 +layers of cotton, boiling the coil a long time in pure wax, and
 +cooling under moderate pressure. The advantage
 +<!-- Page 45 -->
 +of such a coil is that it can be easily handled, but it cannot probably give
 +as satisfactory results as a coil immersed in pure oil. Besides, it seems that
 +the presence of a large body of wax affects the coil disadvantageously,​
 +whereas this does not seem to be the case with oil. Perhaps it is
 +because the dielectric losses in the liquid are smaller.</​p>​
 +<p>I have tried at first silk and cotton covered wires with oil
 +immersion, but I have been gradually led to use gutta-percha covered
 +wires, which proved most satisfactory. Gutta-percha insulation adds,
 +of course, to the capacity of the coil, and this, especially if the
 +coil be large, is a great disadvantage when extreme frequencies are
 +desired; but on the other hand, gutta-percha will withstand much more
 +than an equal thickness of oil, and this advantage should be secured
 +at any price. Once the coil has been immersed, it should never be
 +taken out of the oil for more than a few hours, else the gutta-percha
 +will crack up and the coil will not be worth half as much as before.
 +Gutta-percha is probably slowly attacked by the oil, but after an
 +immersion of eight to nine months I have found no ill effects.</​p>​
 +<p>I have obtained in commerce two kinds of gutta-percha wire: in one the
 +insulation sticks tightly to the metal, in the other it does not.
 +Unless a special method is followed to expel all air, it is much safer
 +to use the first kind. I wind the coil within an oil tank so that all
 +interstices are filled up with the oil. Between the layers I use cloth
 +boiled out thoroughly in oil, calculating the thickness according to
 +the difference of potential between the turns. There seems not to be a
 +very great difference whatever kind of oil is used; I use paraffine or
 +linseed oil.</​p>​
 +<!-- Page 46 -->
 +<p>To exclude more perfectly the air, an excellent way to proceed, and
 +easily practicable with small coils, is the following: Construct a box
 +of hard wood of very thick boards which have been for a long time
 +boiled in oil. The boards should be so joined as to safely withstand
 +the external air pressure. The coil being placed and fastened in
 +position within the box, the latter is closed with a strong lid, and
 +covered with closely fitting metal sheets, the joints of which are
 +soldered very carefully. On the top two small holes are drilled,
 +passing through the metal sheet and the wood, and in these holes two
 +small glass tubes are inserted and the joints made air-tight. One of
 +the tubes is connected to a vacuum pump, and the other with a vessel
 +containing a sufficient quantity of boiled-out oil. The latter tube
 +has a very small hole at the bottom, and is provided with a stopcock.
 +When a fairly good vacuum has been obtained, the stopcock is opened
 +and the oil slowly fed in. Proceeding in this manner, it is impossible
 +that any big bubbles, which are the principal danger, should remain
 +between the turns. The air is most completely excluded, probably
 +better than by boiling out, which, however, when gutta-percha coated
 +wires are used, is not practicable.</​p>​
 +<​p>​For the primaries I use ordinary line wire with a thick cotton
 +coating. Strands of very thin insulated wires properly interlaced
 +would, of course, be the best to employ for the primaries, but they
 +are not to be had.</​p>​
 +<p>In an experimental coil the size of the wires is not of great importance.
 +In the coil here used the primary is No. 12 and the secondary No. 24 Brown &amp;
 +Sharpe gauge wire; but the sections may be varied considerably. It would only
 +<!-- Page 47 -->
 +imply different adjustments;​ the results aimed at would not be materially affected.</​p>​
 +<p>I have dwelt at some length upon the various forms of brush discharge
 +because, in studying them, we not only observe phenomena which please
 +our eye, but also afford us food for thought, and lead us to
 +conclusions of practical importance. In the use of alternating
 +currents of very high tension, too much precaution cannot be taken to
 +prevent the brush discharge. In a main conveying such currents, in an
 +induction coil or transformer,​ or in a condenser, the brush discharge
 +is a source of great danger to the insulation. In a condenser
 +especially the gaseous matter must be most carefully expelled, for in
 +it the charged surfaces are near each other, and if the potentials are
 +high, just as sure as a weight will fall if let go, so the insulation
 +will give way if a single gaseous bubble of some size be present,
 +whereas, if all gaseous matter were carefully excluded, the condenser
 +would safely withstand a much higher difference of potential. A main
 +conveying alternating currents of very high tension may be injured
 +merely by a blow hole or small crack in the insulation, the more so as
 +a blowhole is apt to contain gas at low pressure; and as it appears
 +almost impossible to completely obviate such little imperfections,​ I
 +am led to believe that in our future distribution of electrical energy
 +by currents of very high tension liquid insulation will be used. The
 +cost is a great drawback, but if we employ an oil as an insulator the
 +distribution of electrical energy with something like 100,000 volts,
 +and even more, become, at least with higher frequencies,​ so easy that
 +they could be hardly called engineering
 +<!-- Page 48 -->
 +feats. With oil insulation and alternate current motors transmissions of power
 +can be effected with safety and upon an industrial basis at distances of
 +as much as a thousand miles.</​p>​
 +<p>A peculiar property of oils, and liquid insulation in general, when
 +subjected to rapidly changing electric stresses, is to disperse any
 +gaseous bubbles which may be present, and diffuse them through its
 +mass, generally long before any injurious break can occur. This
 +feature may be easily observed with an ordinary induction coil by
 +taking the primary out, plugging up the end of the tube upon which the
 +secondary is wound, and filling it with some fairly transparent
 +insulator, such as paraffine oil. A primary of a diameter something
 +like six millimetres smaller than the inside of the tube may be
 +inserted in the oil. When the coil is set to work one may see, looking
 +from the top through the oil, many luminous points&​mdash;​air bubbles which
 +are caught by inserting the primary, and which are rendered luminous
 +in consequence of the violent bombardment. The occluded air, by its
 +impact against the oil, heats it; the oil begins to circulate,
 +carrying some of the air along with it, until the bubbles are
 +dispersed and the luminous points disappear. In this manner, unless
 +large bubbles are occluded in such way that circulation is rendered
 +impossible, a damaging break is averted, the only effect being a
 +moderate warming up of the oil. If, instead of the liquid, a solid
 +insulation, no matter how thick, were used, a breaking through and
 +injury of the apparatus would be inevitable.</​p>​
 +<​p>​The exclusion of gaseous matter from any apparatus
 +<!-- Page 49 -->
 +in which the dielectric is subjected to more or less rapidly changing
 +electric forces is, however, not only desirable in order to avoid a possible
 +injury of the apparatus, but also on account of economy. In a
 +condenser, for instance, as long as only a solid or only a liquid
 +dielectric is used, the loss is small; but if a gas under ordinary or
 +small pressure be present the loss may be very great. Whatever the
 +nature of the force acting in the dielectric may be, it seems that in
 +a solid or liquid the molecular displacement produced by the force is
 +small; hence the product of force and displacement is insignificant,​
 +unless the force be very great; but in a gas the displacement,​ and
 +therefore this product, is considerable;​ the molecules are free to
 +move, they reach high speeds, and the energy of their impact is lost
 +in heat or otherwise. If the gas be strongly compressed, the
 +displacement due to the force is made smaller, and the losses are
 +<p>In most of the succeeding experiments I prefer, chiefly on account of
 +the regular and positive action, to employ the alternator before
 +referred to. This is one of the several machines constructed by me for
 +the purposes of these investigations. It has 384 pole projections,​ and
 +is capable of giving currents of a frequency of about 10,000 per
 +second. This machine has been illustrated and briefly described in my
 +first paper before the American Institute of Electrical Engineers, May
 +20, 1891, to which I have already referred. A more detailed
 +description,​ sufficient to enable any engineer to build a similar
 +machine, will be found in several electrical journals of that period.</​p>​
 +<​p>​The induction coils operated from the machine are rather
 +<!-- Page 50 -->
 +small, containing from 5,000 to 15,000 turns in the secondary. They are immersed
 +in boiled-out linseed oil, contained in wooden boxes covered with zinc sheet.</​p>​
 +<p>I have found it advantageous to reverse the usual position of the
 +wires, and to wind, in these coils, the primaries on the top; this
 +allowing the use of a much bigger primary, which, of course, reduces
 +the danger of overheating and increases the output of the coil. I make
 +the primary on each side at least one centimetre shorter than the
 +secondary, to prevent the breaking through on the ends, which would
 +surely occur unless the insulation on the top of the secondary be very
 +thick, and this, of course, would be disadvantageous.</​p>​
 +<​p>​When the primary is made movable, which is necessary in some
 +experiments,​ and many times convenient for the purposes of adjustment,
 +I cover the secondary with wax, and turn it off in a lathe to a
 +diameter slightly smaller than the inside of the primary coil. The
 +latter I provide with a handle reaching out of the oil, which serves
 +to shift it in any position along the secondary.</​p>​
 +<p>I will now venture to make, in regard to the general manipulation of
 +induction coils, a few observations bearing upon points which have not
 +been fully appreciated in earlier experiments with such coils, and are
 +even now often overlooked.</​p>​
 +<​p>​The secondary of the coil possesses usually such a high self-induction
 +that the current through the wire is inappreciable,​ and may be so even
 +when the terminals are joined by a conductor of small resistance. If
 +capacity is added to the terminals, the self-induction is counteracted,​
 +<!-- Page 51 -->
 +and a stronger current is made to flow through the secondary,
 +though its terminals are insulated from each other. To one
 +entirely unacquainted with the properties of alternating currents
 +nothing will look more puzzling. This feature was illustrated in the
 +experiment performed at the beginning with the top plates of wire
 +gauze attached to the terminals and the rubber plate. When the plates
 +of wire gauze were close together, and a small arc passed between
 +them, the arc <​i>​prevented</​i>​ a strong current from passing through the
 +secondary, because it did away with the capacity on the terminals;
 +when the rubber plate was inserted between, the capacity of the
 +condenser formed counteracted the self-induction of the secondary, a
 +stronger current passed now, the coil performed more work, and the
 +discharge was by far more powerful.</​p>​
 +<​p>​The first thing, then, in operating the induction coil is to combine
 +capacity with the secondary to overcome the self-induction. If the
 +frequencies and potentials are very high gaseous matter should be
 +carefully kept away from the charged surfaces. If Leyden jars are
 +used, they should be immersed in oil, as otherwise considerable
 +dissipation may occur if the jars are greatly strained. When high
 +frequencies are used, it is of equal importance to combine a condenser
 +with the primary. One may use a condenser connected to the ends of the
 +primary or to the terminals of the alternator, but the latter is not to be
 +recommended,​ as the machine might be injured. The best way is undoubtedly
 +to use the condenser in series with the primary and with the alternator, and to
 +adjust its capacity so as to annul the self-induction of both the latter. The condenser
 +<!-- Page 52 -->
 +should be adjustable by very small steps, and for a finer adjustment a small
 +oil condenser with movable plates may be used conveniently.</​p>​
 +<p>I think it best at this juncture to bring before you a phenomenon,
 +observed by me some time ago, which to the purely scientific
 +investigator may perhaps appear more interesting than any of the
 +results which I have the privilege to present to you this evening.</​p>​
 +<p>It may be quite properly ranked among the brush phenomena&​mdash;​in fact, it
 +is a brush, formed at, or near, a single terminal in high vacuum.</​p>​
 +<p>In bulbs provided with a conducting terminal, though it be of
 +aluminium, the brush has but an ephemeral existence, and cannot,
 +unfortunately,​ be indefinitely preserved in its most sensitive state,
 +even in a bulb devoid of any conducting electrode. In studying the
 +phenomenon, by all means a bulb having no leading-in wire should be
 +used. I have found it best to use bulbs constructed as indicated in
 +Figs. 12 and 13.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig12_13.gif"​ width="​518"​ height="​578"​ border="​0"​
 +<p>In Fig. 12 the bulb comprises an incandescent lamp globe <​i>​L</​i>,​ in the
 +neck of which is sealed a barometer tube <​i>​b</​i>,​ the end of which is
 +blown out to form a small sphere <​i>​s</​i>​. This sphere should be sealed as
 +closely as possible in the centre of the large globe. Before sealing,
 +a thin tube <​i>​t</​i>,​ of aluminium sheet, may be slipped in the barometer
 +tube, but it is not important to employ it.</​p>​
 +<​p>​The small hollow sphere <​i>​s</​i>​ is filled with some conducting powder, and
 +a wire <​i>​w</​i>​ is cemented in the neck for the purpose of connecting the
 +conducting powder with the generator.</​p>​
 +<!-- Page 53 -->
 +<​p>​The construction shown in Fig. 13 was chosen in order to remove from
 +the brush any conducting body which might possibly affect it. The bulb consists
 +in this case of a lamp globe <​i>​L</​i>,​ which has a neck <​i>​n</​i>,​ provided with
 +a tube <​i>​b</​i>​ and small sphere <​i>​s</​i>,​ sealed to it, so that two entirely independent
 +compartments are formed, as indicated in the drawing. When the bulb is in use, the
 +neck <​i>​n</​i>​ is provided with a tinfoil coating, which is connected to the generator and acts
 +<!-- Page 54 -->
 +inductively upon the moderately rarefied and highly conducting gas inclosed in the neck.
 +From there the current passes through the tube <​i>​b</​i>​ into the small sphere <​i>​s</​i>​ to
 +act by induction upon the gas contained in the globe <​i>​L</​i>​.</​p>​
 +<p>It is of advantage to make the tube <​i>​t</​i>​ very thick, the hole through
 +it very small, and to blow the sphere <​i>​s</​i>​ very thin. It is of the
 +greatest importance that the sphere <​i>​s</​i>​ be placed in the centre of the
 +globe <​i>​L</​i>​.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig14.gif"​ width="​578"​ height="​425"​ border="​0"​
 +<​p>​Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
 +Fig. 14 shows the brush as it first appears in a bulb provided with a
 +conducting terminal; but, as in such a bulb it very soon
 +disappears&​mdash;​often after a few minutes&​mdash;​I will confine myself to the
 +description of the phenomenon as seen in a bulb without conducting
 +electrode. It is observed under the following conditions:</​p>​
 +<​p>​When the globe <​i>​L</​i>​ (Figs. 12 and 13) is exhausted to a
 +<!-- Page 55 -->
 +very high degree, generally the bulb is not excited upon connecting the wire
 +<​i>​w</​i>​ (Fig. 12) or the tinfoil coating of the bulb (Fig. 13) to the terminal
 +of the induction coil. To excite it, it is usually sufficient to grasp
 +the globe <​i>​L</​i>​ with the hand. An intense phosphorescence then spreads
 +at first over the globe, but soon gives place to a white, misty light.
 +Shortly afterward one may notice that the luminosity is unevenly
 +distributed in the globe, and after passing the current
 +<!-- Page 56 -->
 +for some time the bulb appears as in Fig. 15. From this stage the
 +phenomenon will gradually pass to that indicated in Fig. 16, after
 +some minutes, hours, days or weeks, according as the bulb is worked.
 +Warming the bulb or increasing the potential hastens the transit.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig15_16.gif"​ width="​518"​ height="​552"​ border="​0"​
 +<​p>​When the brush assumes the form indicated in Fig. 16, it maybe brought
 +to a state of extreme sensitiveness to electrostatic and magnetic
 +influence. The bulb hanging straight down from a wire, and all objects
 +being remote from it, the approach of the observer at a few paces from
 +the bulb will cause the brush to fly to the opposite side, and if he
 +walks around the bulb it will always keep on the opposite side. It may
 +begin to spin around the terminal long before it reaches that
 +sensitive stage. When it begins to turn around principally,​ but also
 +before, it is affected by a magnet, and at a certain stage it is
 +susceptible to magnetic influence to an astonishing degree. A small
 +permanent magnet, with its poles at a distance of no more than two
 +centimetres,​ will affect it visibly at a distance of two metres,
 +slowing down or accelerating the rotation according to how it is held
 +relatively to the brush. I think I have observed that at the stage
 +when it is most sensitive to magnetic, it is not most sensitive to
 +electrostatic,​ influence. My explanation is, that the electrostatic
 +attraction between the brush and the glass of the bulb, which retards
 +the rotation, grows much quicker than the magnetic influence when the
 +intensity of the stream is increased.</​p>​
 +<​p>​When the bulb hangs with the globe <​i>​L</​i>​ down, the rotation
 +is always clockwise. In the southern hemisphere it would occur
 +in the opposite direction and on the equator
 +<!-- Page 57 -->
 +the brush should not turn at all. The rotation may be reversed by
 +a magnet kept at some distance. The brush rotates best, seemingly,
 +when it is at right angles to the lines of force of the earth.
 +It very likely rotates, when at its maximum speed, in synchronism
 +with the alternations,​ say 10,000 times a second. The rotation can
 +be slowed down or accelerated by the approach or receding
 +of the observer, or any conducting body, but it cannot be reversed by
 +putting the bulb in any position. When it is in the state of the
 +highest sensitiveness and the potential or frequency be varied the
 +sensitiveness is rapidly diminished. Changing either of these but
 +little will generally stop the rotation. The sensitiveness is likewise
 +affected by the variations of temperature. To attain great
 +sensitiveness it is necessary to have the small sphere <​i>​s</​i>​ in the
 +centre of the globe <​i>​L</​i>,​ as otherwise the electrostatic action of the
 +glass of the globe will tend to stop the rotation. The sphere <​i>​s</​i>​
 +should be small and of uniform thickness; any dissymmetry of course
 +has the effect to diminish the sensitiveness.</​p>​
 +<​p>​The fact that the brush rotates in a definite direction in a permanent
 +magnetic field seems to show that in alternating currents of very high
 +frequency the positive and negative impulses are not equal, but that
 +one always preponderates over the other.</​p>​
 +<p>Of course, this rotation in one direction may be due to the action of
 +two elements of the same current upon each other, or to the action of
 +the field produced by one of the elements upon the other, as in a
 +series motor, without necessarily one impulse being stronger than the
 +other. The fact that the brush turns, as far as I could observe, in any
 +<!-- Page 58 -->
 +position, would speak for this view. In such case it would turn
 +at any point of the earth'​s surface. But, on the other hand, it is
 +then hard to explain why a permanent magnet should reverse the
 +rotation, and one must assume the preponderance of impulses of one
 +<p>As to the causes of the formation of the brush or stream, I think it
 +is due to the electrostatic action of the globe and the dissymmetry of
 +the parts. If the small bulb <​i>​s</​i>​ and the globe <​i>​L</​i>​ were perfect
 +concentric spheres, and the glass throughout of the same thickness and
 +quality, I think the brush would not form, as the tendency to pass
 +would be equal on all sides. That the formation of the stream is due
 +to an irregularity is apparent from the fact that it has the tendency
 +to remain in one position, and rotation occurs most generally only
 +when it is brought out of this position by electrostatic or magnetic
 +influence. When in an extremely sensitive state it rests in one
 +position, most curious experiments may be performed with it. For
 +instance, the experimenter may, by selecting a proper position,
 +approach the hand at a certain considerable distance to the bulb, and
 +he may cause the brush to pass off by merely stiffening the muscles of
 +the arm. When it begins to rotate slowly, and the hands are held at a
 +proper distance, it is impossible to make even the slightest motion
 +without producing a visible effect upon the brush. A metal plate
 +connected to the other terminal of the coil affects it at a great
 +distance, slowing down the rotation often to one turn a second.</​p>​
 +<p>I am firmly convinced that such a brush, when we learn how to
 +produce it properly, will prove a valuable aid in the investigation
 +of the nature of the forces acting in an electrostatic
 +<!-- Page 59 -->
 +or magnetic field. If there is any motion which is measurable going on
 +in the space, such a brush ought to reveal it. It is, so to speak, a beam
 +of light, frictionless,​ devoid of inertia.</​p>​
 +<p>I think that it may find practical applications in telegraphy. With
 +such a brush it would be possible to send dispatches across the
 +Atlantic, for instance, with any speed, since its sensitiveness may be
 +so great that the slightest changes will affect it. If it were
 +possible to make the stream more intense and very narrow, its
 +deflections could be easily photographed.</​p>​
 +<p>I have been interested to find whether there is a rotation of the
 +stream itself, or whether there is simply a stress traveling around in
 +the bulb. For this purpose I mounted a light mica fan so that its
 +vanes were in the path of the brush. If the stream itself was rotating
 +the fan would be spun around. I could produce no distinct rotation of
 +the fan, although I tried the experiment repeatedly; but as the fan
 +exerted a noticeable influence on the stream, and the apparent
 +rotation of the latter was, in this case, never quite satisfactory,​
 +the experiment did not appear to be conclusive.</​p>​
 +<p>I have been unable to produce the phenomenon with the disruptive
 +discharge coil, although every other of these phenomena can be well
 +produced by it&​mdash;​many,​ in fact, much better than with coils operated
 +from an alternator.</​p>​
 +<p>It may be possible to produce the brush by impulses of one direction,
 +or even by a steady potential, in which case it would be still more
 +sensitive to magnetic influence.</​p>​
 +<p>In operating an induction coil with rapidly alternating currents,
 +we realize with astonishment,​ for the first time,
 +<!-- Page 60 -->
 +the great importance of the relation of capacity, self-induction and frequency
 +as regards the general result. The effects of capacity are the most striking,
 +for in these experiments,​ since the self-induction and frequency both are
 +high, the critical capacity is very small, and need be but slightly
 +varied to produce a very considerable change. The experimenter may
 +bring his body in contact with the terminals of the secondary of the
 +coil, or attach to one or both terminals insulated bodies of very
 +small bulk, such as bulbs, and he may produce a considerable rise or
 +fall of potential, and greatly affect the flow of the current through
 +the primary. In the experiment before shown, in which a brush appears
 +at a wire attached to one terminal, and the wire is vibrated when the
 +experimenter brings his insulated body in contact with the other
 +terminal of the coil, the sudden rise of potential was made evident.</​p>​
 +<p>I may show you the behavior of the coil in another manner which
 +possesses a feature of some interest. I have here a little light fan
 +of aluminium sheet, fastened to a needle and arranged to rotate freely
 +in a metal piece screwed to one of the terminals of the coil. When the
 +coil is set to work, the molecules of the air are rhythmically
 +attracted and repelled. As the force with which they are repelled is
 +greater than that with which they are attracted, it results that there
 +is a repulsion exerted on the surfaces of the fan. If the fan were
 +made simply of a metal sheet, the repulsion would be equal on the
 +opposite sides, and would produce no effect. But if one of the
 +opposing surfaces is screened, or if, generally speaking, the bombardment
 +on this side is weakened in some way or other, there remains the repulsion
 +<!-- Page 61 -->
 +exerted upon the other, and the fan is set in rotation. The
 +screening is best effected by fastening upon one of the opposing sides
 +of the fan insulated conducting coatings, or, if the fan is made in
 +the shape of an ordinary propeller screw, by fastening on one side,
 +and close to it, an insulated metal plate. The static screen may,
 +however, be omitted, and simply a thickness of insulating material
 +fastened to one of the sides of the fan.</​p>​
 +<p>To show the behavior of the coil, the fan may be placed upon the
 +terminal and it will readily rotate when the coil is operated by
 +currents of very high frequency. With a steady potential, of course,
 +and even with alternating currents of very low frequency, it would not
 +turn, because of the very slow exchange of air and, consequently,​
 +smaller bombardment;​ but in the latter case it might turn if the
 +potential were excessive. With a pin wheel, quite the opposite rule
 +holds good; it rotates best with a steady potential, and the effort is
 +the smaller the higher the frequency. Now, it is very easy to adjust
 +the conditions so that the potential is normally not sufficient to
 +turn the fan, but that by connecting the other terminal of the coil
 +with an insulated body it rises to a much greater value, so as to
 +rotate the fan, and it is likewise possible to stop the rotation by
 +connecting to the terminal a body of different size, thereby
 +diminishing the potential.</​p>​
 +<​p>​Instead of using the fan in this experiment, we may use the &​quot;​electric&​quot;​
 +radiometer with similar effect. But in this case it will be found that
 +the vanes will rotate only at high exhaustion or at ordinary pressures;
 +they will not rotate at moderate pressures, when the air is highly conducting.
 +<!-- Page 62 -->
 +This curious observation was made conjointly by
 +Professor Crookes and myself. I attribute the result to the high
 +conductivity of the air, the molecules of which then do not act as
 +independent carriers of electric charges, but act all together as a
 +single conducting body. In such case, of course, if there is any
 +repulsion at all of the molecules from the vanes, it must be very
 +small. It is possible, however, that the result is in part due to the
 +fact that the greater part of the discharge passes from the leading-in
 +wire through the highly conducting gas, instead of passing off from
 +the conducting vanes.</​p>​
 +<p>In trying the preceding experiment with the electric radiometer the
 +potential should not exceed a certain limit, as then the electrostatic
 +attraction between the vanes and the glass of the bulb may be so great
 +as to stop the rotation.</​p>​
 +<p>A most curious feature of alternate currents of high frequencies and
 +potentials is that they enable us to perform many experiments by the
 +use of one wire only. In many respects this feature is of great
 +<p>In a type of alternate current motor invented by me some years ago I
 +produced rotation by inducing, by means of a single alternating
 +current passed through a motor circuit, in the mass or other circuits
 +of the motor, secondary currents, which, jointly with the primary or
 +inducing current, created a moving field of force. A simple but crude form
 +of such a motor is obtained by winding upon an iron core a primary, and
 +close to it a secondary coil, joining the ends of the latter and placing a
 +freely movable metal disc within the influence of the field produced by both. The
 +<!-- Page 63 -->
 +iron core is employed for obvious reasons, but it is not essential to the operation.
 +To improve the motor, the iron core is made to encircle the armature. Again to
 +improve, the secondary coil is made to overlap partly the primary, so
 +that it cannot free itself from a strong inductive action of the
 +latter, repel its lines as it may. Once more to improve, the proper
 +difference of phase is obtained between the primary and secondary
 +currents by a condenser, self-induction,​ resistance or equivalent
 +<p>I had discovered, however, that rotation is produced by means of a
 +single coil and core; my explanation of the phenomenon, and leading
 +thought in trying the experiment, being that there must be a true time
 +lag in the magnetization of the core. I remember the pleasure I had
 +when, in the writings of Professor Ayrton, which came later to my
 +hand, I found the idea of the time lag advocated. Whether there is a
 +true time lag, or whether the retardation is due to eddy currents
 +circulating in minute paths, must remain an open question, but the
 +fact is that a coil wound upon an iron core and traversed by an
 +alternating current creates a moving field of force, capable of
 +setting an armature in rotation. It is of some interest, in
 +conjunction with the historical Arago experiment, to mention that in
 +lag or phase motors I have produced rotation in the opposite direction
 +to the moving field, which means that in that experiment the magnet
 +may not rotate, or may even rotate in the opposite direction to the
 +moving disc. Here, then, is a motor (diagrammatically illustrated in
 +Fig. 17), comprising a coil and iron core, and a freely movable copper
 +disc in proximity to the latter.</​p>​
 +<!-- Page 64 -->
 +<div align="​center">​
 +<img src="​images/​acfig17.gif"​ width="​556"​ height="​569"​ border="​0"​
 +alt="​FIG. 17.&​mdash;​SINGLE WIRE AND &​quot;​NO-WIRE&​quot;​ MOTOR.">​
 +<p>To demonstrate a novel and interesting feature, I have, for a reason
 +which I will explain, selected this type of motor. When the ends of
 +the coil are connected to the terminals of an alternator the disc is
 +set in rotation. But it is not this experiment, now well known, which
 +I desire to perform. What I wish to show you is that this motor
 +rotates with <​i>​one single</​i>​ connection between it and the generator;
 +that is to say, one terminal of the motor is connected to one terminal
 +of the generator&​mdash;​in this case the secondary of a high-tension
 +induction coil&​mdash;​the other terminals of
 +<!-- Page 65 -->
 +motor and generator being insulated in space. To produce rotation it is
 +generally (but not absolutely) necessary to connect the free end of the motor coil
 +to an insulated body of some size. The experimenter'​s body is more than
 +sufficient. If he touches the free terminal with an object held in the
 +hand, a current passes through the coil and the copper disc is set in
 +rotation. If an exhausted tube is put in series with the coil, the
 +tube lights brilliantly,​ showing the passage of a strong current.
 +Instead of the experimenter'​s body, a small metal sheet suspended on a
 +cord may be used with the same result. In this case the plate acts as
 +a condenser in series with the coil. It counteracts the self-induction
 +of the latter and allows a strong current to pass. In such a
 +combination,​ the greater the self-induction of the coil the smaller
 +need be the plate, and this means that a lower frequency, or
 +eventually a lower potential, is required to operate the motor. A
 +single coil wound upon a core has a high self-induction;​ for this
 +reason principally,​ this type of motor was chosen to perform the
 +experiment. Were a secondary closed coil wound upon the core, it would
 +tend to diminish the self-induction,​ and then it would be necessary to
 +employ a much higher frequency and potential. Neither would be
 +advisable, for a higher potential would endanger the insulation of the
 +small primary coil, and a higher frequency would result in a
 +materially diminished torque.</​p>​
 +<p>It should be remarked that when such a motor with a closed
 +secondary is used, it is not at all easy to obtain rotation with excessive
 +frequencies,​ as the secondary cuts off almost completely the lines of
 +the primary&​mdash;​and this, of course,
 +<!-- Page 66 -->
 +the more, the higher the frequency&​mdash;​and allows the passage of but
 +a minute current. In such a case, unless the secondary is closed through
 +a condenser, it is almost essential, in order to produce rotation, to make the
 +primary and secondary coils overlap each other more or less.</​p>​
 +<​p>​But there is an additional feature of interest about this motor,
 +namely, it is not necessary to have even a single connection between
 +the motor and generator, except, perhaps, through the ground: for not
 +only is an insulated plate capable of giving off energy into space,
 +but it is likewise capable of deriving it from an alternating
 +electrostatic field, though in the latter case the available energy is
 +much smaller. In this instance one of the motor terminals is connected
 +to the insulated plate or body located within the alternating
 +electrostatic field, and the other terminal preferably to the ground.</​p>​
 +<p>It is quite possible, however, that such &​quot;​no-wire&​quot;​ motors, as they
 +might be called, could be operated by conduction through the rarefied
 +air at considerable distances. Alternate currents, especially of high
 +frequencies,​ pass with astonishing freedom through even slightly
 +rarefied gases. The upper strata of the air are rarefied. To reach a
 +number of miles out into space requires the overcoming of difficulties
 +of a merely mechanical nature. There is no doubt that with the
 +enormous potentials obtainable by the use of high frequencies and oil
 +insulation luminous discharges might be passed through many miles of
 +rarefied air, and that, by thus directing the energy of many
 +hundreds or thousands of horse-power,​ motors or lamps might be
 +operated at considerable distances from stationary sources. But such
 +<!-- Page 67 -->
 +schemes are mentioned merely as possibilities. We shall have no need
 +to transmit power in this way. We shall have no need to <​i>​transmit</​i>​
 +power at all. Ere many generations pass, our machinery will be driven
 +by a power obtainable at any point of the universe. This idea is not
 +novel. Men have been led to it long ago by instinct or reason. It has
 +been expressed in many ways, and in many places, in the history of old
 +and new. We find it in the delightful myth of Antheus, who derives
 +power from the earth; we find it among the subtile speculations of one
 +of your splendid mathematicians,​ and in many hints and statements of
 +thinkers of the present time. Throughout space there is energy. Is
 +this energy static or kinetic? If static our hopes are in vain; if
 +kinetic&​mdash;​and this we know it is, for certain&​mdash;​then it is a mere
 +question of time when men will succeed in attaching their machinery to
 +the very wheelwork of nature. Of all, living or dead, Crookes came
 +nearest to doing it. His radiometer will turn in the light of day and
 +in the darkness of the night; it will turn everywhere where there is
 +heat, and heat is everywhere. But, unfortunately,​ this beautiful
 +little machine, while it goes down to posterity as the most
 +interesting,​ must likewise be put on record as the most inefficient
 +machine ever invented!</​p>​
 +<​p>​The preceding experiment is only one of many equally interesting
 +experiments which may be performed by the use of only one wire with
 +alternate currents of high potential and frequency. We may connect an
 +insulated line to a source of such currents, we may pass an
 +inappreciable current over the line, and on any point of the same we are
 +<!-- Page 68 -->
 +able to obtain a heavy current, capable of fusing a thick copper
 +wire. Or we may, by the help of some artifice, decompose a solution in
 +any electrolytic cell by connecting only one pole of the cell to the
 +line or source of energy. Or we may, by attaching to the line, or only
 +bringing into its vicinity, light up an incandescent lamp, an
 +exhausted tube, or a phosphorescent bulb.</​p>​
 +<​p>​However impracticable this plan of working may appear in many cases,
 +it certainly seems practicable,​ and even recommendable,​ in the
 +production of light. A perfected lamp would require but little energy,
 +and if wires were used at all we ought to be able to supply that
 +energy without a return wire.</​p>​
 +<p>It is now a fact that a body may be rendered incandescent or
 +phosphorescent by bringing it either in single contact or merely in
 +the vicinity of a source of electric impulses of the proper character,
 +and that in this manner a quantity of light sufficient to afford a
 +practical illuminant may be produced. It is, therefore, to say the
 +least, worth while to attempt to determine the best conditions and to
 +invent the best appliances for attaining this object.</​p>​
 +<​p>​Some experiences have already been gained in this direction, and I
 +will dwell on them briefly, in the hope that they might prove useful.</​p>​
 +<​p>​The heating of a conducting body inclosed in a bulb, and connected to
 +a source of rapidly alternating electric impulses, is dependent on so
 +many things of a different nature, that it would be difficult to give
 +a generally applicable rule under which the maximum heating occurs. As
 +regards the size of the vessel, I have lately found that at ordinary
 +<!-- Page 69 -->
 +or only slightly differing atmospheric pressures, when air is a good
 +insulator, and hence practically the same amount of energy by a
 +certain potential and frequency is given off from the body, whether
 +the bulb be small or large, the body is brought to a higher
 +temperature if inclosed in a small bulb, because of the better
 +confinement of heat in this case.</​p>​
 +<p>At lower pressures, when air becomes more or less conducting, or if
 +the air be sufficiently warmed as to become conducting, the body is
 +rendered more intensely incandescent in a large bulb, obviously
 +because, under otherwise equal conditions of test, more energy may be
 +given off from the body when the bulb is large.</​p>​
 +<p>At very high degrees of exhaustion, when the matter in the bulb
 +becomes &​quot;​radiant,&​quot;​ a large bulb has still an advantage, but a
 +comparatively slight one, over the small bulb.</​p>​
 +<​p>​Finally,​ at excessively high degrees of exhaustion, which cannot be
 +reached except by the employment of special means, there seems to be,
 +beyond a certain and rather small size of vessel, no perceptible
 +difference in the heating.</​p>​
 +<​p>​These observations were the result of a number of experiments,​ of
 +which one, showing the effect of the size of the bulb at a high degree
 +of exhaustion, may be described and shown here, as it presents a
 +feature of interest. Three spherical bulbs of 2 inches, 3 inches and 4
 +inches diameter were taken, and in the centre of each was mounted an
 +equal length of an ordinary incandescent lamp filament of uniform thickness.
 +In each bulb the piece of filament was fastened to the leading-in wire of platinum, contained
 +<!-- Page 70 -->
 +in a glass stem sealed in the bulb; care being taken, of course, to make everything
 +as nearly alike as possible. On each glass stem in the inside of the bulb was
 +slipped a highly polished tube made of aluminium sheet, which fitted the stem
 +and was held on it by spring pressure. The function of this aluminium
 +tube will be explained subsequently. In each bulb an equal length of
 +filament protruded above the metal tube. It is sufficient to say now
 +that under these conditions equal lengths of filament of the same
 +thickness&​mdash;​in other words, bodies of equal bulk&​mdash;​were brought to
 +incandescence. The three bulbs were sealed to a glass tube, which was
 +connected to a Sprengel pump. When a high vacuum had been reached, the
 +glass tube carrying the bulbs was sealed off. A current was then
 +turned on successively on each bulb, and it was found that the
 +filaments came to about the same brightness, and, if anything, the
 +smallest bulb, which was placed midway between the two larger ones,
 +may have been slightly brighter. This result was expected, for when
 +either of the bulbs was connected to the coil the luminosity spread
 +through the other two, hence the three bulbs constituted really one
 +vessel. When all the three bulbs were connected in multiple arc to the
 +coil, in the largest of them the filament glowed brightest, in the
 +next smaller it was a little less bright, and in the smallest it only
 +came to redness. The bulbs were then sealed off and separately tried.
 +The brightness of the filaments was now such as would have been
 +expected on the supposition that the energy given off was proportionate
 +to the surface of the bulb, this surface in each case representing
 +<!-- Page 71 -->
 +one of the coatings of a condenser. Accordingly,​ time was less difference between
 +the largest and the middle sized than between the latter and the smallest bulb.</​p>​
 +<p>An interesting observation was made in this experiment. The three
 +bulbs were suspended from a straight bare wire connected to a terminal
 +of the coil, the largest bulb being placed at the end of the wire, at
 +some distance from it the smallest bulb, and an equal distance from
 +the latter the middle-sized one. The carbons glowed then in both the
 +larger bulbs about as expected, but the smallest did not get its share
 +by far. This observation led me to exchange the position of the bulbs,
 +and I then observed that whichever of the bulbs was in the middle it
 +was by far less bright than it was in any other position. This
 +mystifying result was, of course, found to be due to the electrostatic
 +action between the bulbs. When they were placed at a considerable
 +distance, or when they were attached to the corners of an equilateral
 +triangle of copper wire, they glowed about in the order determined by
 +their surfaces.</​p>​
 +<p>As to the shape of the vessel, it is also of some importance,
 +especially at high degrees of exhaustion. Of all the possible
 +constructions,​ it seems that a spherical globe with the refractory
 +body mounted in its centre is the best to employ. In experience it has
 +been demonstrated that in such a globe a refractory body of a given
 +bulk is more easily brought to incandescence than when otherwise
 +shaped bulbs are used. There is also an advantage in giving to the
 +incandescent body the shape of a sphere, for self-evident reasons. In
 +any case the body should be mounted in the centre, where the atoms
 +rebounding from the glass collide.
 +<!-- Page 72 -->
 +This object is best attained in the spherical bulb; but it is also attained in a
 +cylindrical vessel with one or two straight filaments coinciding with its axis,
 +and possibly also in parabolical or spherical bulbs with the refractory
 +body or bodies placed in the focus or foci of the same; though the
 +latter is not probable, as the electrified atoms should in all cases
 +rebound normally from the surface they strike, unless the speed were
 +excessive, in which case they <​i>​would</​i>​ probably follow the general law
 +of reflection. No matter what shape the vessel may have, if the
 +exhaustion be low, a filament mounted in the globe is brought to the
 +same degree of incandescence in all parts; but if the exhaustion be
 +high and the bulb be spherical or pear-shaped,​ as usual, focal points
 +form and the filament is heated to a higher degree at or near such
 +<p>To illustrate the effect, I have here two small bulbs which are alike,
 +only one is exhausted to a low and the other to a very high degree.
 +When connected to the coil, the filament in the former glows uniformly
 +throughout all its length; whereas in the latter, that portion of the
 +filament which is in the centre of the bulb glows far more intensely
 +than the rest. A curious point is that the phenomenon occurs even if
 +two filaments are mounted in a bulb, each being connected to one
 +terminal of the coil, and, what is still more curious, if they be very
 +near together, provided the vacuum be very high. I noted in
 +experiments with such bulbs that the filaments would give way usually
 +at a certain point, and in the first trials I attributed it to a
 +defect in the carbon. But when the phenomenon occurred many times in
 +succession I recognized its real cause.</​p>​
 +<!-- Page 73 -->
 +<p>In order to bring a refractory body inclosed in a bulb to
 +incandescence,​ it is desirable, on account of economy, that all the
 +energy supplied to the bulb from the source should reach without loss
 +the body to be heated; from there, and from nowhere else, it should be
 +radiated. It is, of course, out of the question to reach this
 +theoretical result, but it is possible by a proper construction of the
 +illuminating device to approximate it more or less.</​p>​
 +<​p>​For many reasons, the refractory body is placed in the centre of the
 +bulb, and it is usually supported on a glass stem containing the
 +leading-in wire. As the potential of this wire is alternated, the
 +rarefied gas surrounding the stem is acted upon inductively,​ and the
 +glass stem is violently bombarded and heated. In this manner by far
 +the greater portion of the energy supplied to the bulb&​mdash;​especially
 +when exceedingly high frequencies are used&​mdash;​may be lost for the
 +purpose contemplated. To obviate this loss, or at least to reduce it
 +to a minimum, I usually screen the rarefied gas surrounding the stem
 +from the inductive action of the leading-in wire by providing the stem
 +with a tube or coating of conducting material. It seems beyond doubt
 +that the best among metals to employ for this purpose is aluminium, on
 +account of its many remarkable properties. Its only fault is that it
 +is easily fusible, and, therefore, its distance from the incandescing
 +body should be properly estimated. Usually, a thin tube, of a diameter
 +somewhat smaller than that of the glass stem, is made of the finest
 +aluminium sheet, and slipped on the stem. The tube is conveniently
 +prepared by wrapping around a rod fastened in a lathe a piece of aluminium
 +<!-- Page 74 -->
 +sheet of the proper size, grasping the sheet firmly with
 +clean chamois leather or blotting paper, and spinning the rod very
 +fast. The sheet is wound tightly around the rod, and a highly polished
 +tube of one or three layers of the sheet is obtained. When slipped on
 +the stem, the pressure is generally sufficient to prevent it from
 +slipping off, but, for safety, the lower edge of the sheet may be
 +turned inside. The upper inside corner of the sheet&​mdash;​that is, the one
 +which is nearest to the refractory incandescent body&​mdash;​should be cut
 +out diagonally, as it often happens that, in consequence of the
 +intense heat, this corner turns toward the inside and comes very near
 +to, or in contact with, the wire, or filament, supporting the
 +refractory body. The greater part of the energy supplied to the bulb
 +is then used up in heating the metal tube, and the bulb is rendered
 +useless for the purpose. The aluminium sheet should project above the
 +glass stem more or less&​mdash;​one inch or so&​mdash;​or else, if the glass be too
 +close to the incandescing body, it may be strongly heated and become
 +more or less conducting, whereupon it may be ruptured, or may, by its
 +conductivity,​ establish a good electrical connection between the metal
 +tube and the leading-in wire, in which case, again, most of the energy
 +will be lost in heating the former. Perhaps the best way is to make the
 +top of the glass tube, for about an inch, of a much smaller diameter.
 +To still further reduce the danger arising from the heating of the glass stem,
 +and also with the view of preventing an electrical connection between the
 +metal tube and the electrode, I preferably wrap the stem with several layers of
 +thin mica, which extends at least as far as the metal tube. In
 +<!-- Page 75 -->
 +some bulbs I have also used an outside insulating cover.</​p>​
 +<​p>​The preceding remarks are only made to aid the experimenter in the
 +first trials, for the difficulties which he encounters he may soon
 +find means to overcome in his own way.</​p>​
 +<p>To illustrate the effect of the screen, and the advantage of using it,
 +I have here two bulbs of the same size, with their stems, leading-in
 +wires and incandescent lamp filaments tied to the latter, as nearly
 +alike as possible. The stem of one bulb is provided with an aluminium
 +tube, the stem of the other has none. Originally the two bulbs were
 +joined by a tube which was connected to a Sprengel pump. When a high
 +vacuum had been reached, first the connecting tube, and then the
 +bulbs, were sealed off; they are therefore of the same degree of
 +exhaustion. When they are separately connected to the coil giving a
 +certain potential, the carbon filament in the bulb provided with the
 +aluminium screen is rendered highly incandescent,​ while the filament
 +in the other bulb may, with the same potential, not even come to
 +redness, although in reality the latter bulb takes generally more
 +energy than the former. When they are both connected together to the
 +terminal, the difference is even more apparent, showing the importance
 +of the screening. The metal tube placed on the stem containing the
 +leading-in wire performs really two distinct functions: First: it acts
 +more or less as an electrostatic screen, thus economizing the energy
 +supplied to the bulb; and, second, to whatever extent it may fail to
 +act electrostatically,​ it acts mechanically,​
 +<!-- Page 76 -->
 +preventing the bombardment,​ and consequently intense heating and possible
 +deterioration of the slender support of the refractory incandescent
 +body, or of the glass stem containing the leading-in wire. I say
 +<​i>​slender</​i>​ support, for it is evident that in order to confine the heat
 +more completely to the incandescing body its support should be very
 +thin, so as to carry away the smallest possible amount of heat by
 +conduction. Of all the supports used I have found an ordinary
 +incandescent lamp filament to be the best, principally because among
 +conductors it can withstand the highest degrees of heat.</​p>​
 +<​p>​The effectiveness of the metal tube as an electrostatic screen depends
 +largely on the degree of exhaustion.</​p>​
 +<p>At excessively high degrees of exhaustion&​mdash;​which are reached by using
 +great care and special means in connection with the Sprengel
 +pump&​mdash;​when the matter in the globe is in the ultra-radiant state, it
 +acts most perfectly. The shadow of the upper edge of the tube is then
 +sharply defined upon the bulb.</​p>​
 +<p>At a somewhat lower degree of exhaustion, which is about the ordinary
 +&​quot;​non-striking&​quot;​ vacuum, and generally as long as the matter moves
 +predominantly in straight lines, the screen still does well. In
 +elucidation of the preceding remark it is necessary to state that what
 +is a &​quot;​non-striking&​quot;​ vacuum for a coil operated, as ordinarily, by
 +impulses, or currents, of low-frequency,​ is not, by far, so when the
 +coil is operated by currents of very high frequency. In such case the discharge
 +may pass with great freedom through the rarefied gas through which a low-frequency
 +discharge may not pass, even though the potential be much higher. At
 +<!-- Page 77 -->
 +ordinary atmospheric pressures just the reverse rule holds good: the higher
 +the frequency, the less the spark discharge is able to jump between the terminals,
 +especially if they are knobs or spheres of some size.</​p>​
 +<​p>​Finally,​ at very low degrees of exhaustion, when the gas is well
 +conducting, the metal tube not only does not act as an electrostatic
 +screen, but even is a drawback, aiding to a considerable extent the
 +dissipation of the energy laterally from the leading-in wire. This, of
 +course, is to be expected. In this case, namely, the metal tube is in
 +good electrical connection with the leading-in wire, and most of the
 +bombardment is directed upon the tube. As long as the electrical
 +connection is not good, the conducting tube is always of some
 +advantage, for although it may not greatly economize energy, still it
 +protects the support of the refractory button, and is a means for
 +concentrating more energy upon the same.</​p>​
 +<p>To whatever extent the aluminium tube performs the function of a
 +screen, its usefulness is therefore limited to very high degrees of
 +exhaustion when it is insulated from the electrode&​mdash;​that is, when the
 +gas as a whole is non-conducting,​ and the molecules, or atoms, act as
 +independent carriers of electric charges.</​p>​
 +<p>In addition to acting as a more or less effective screen, in the true
 +meaning of the word, the conducting tube or coating may also act, by
 +reason of its conductivity,​ as a sort of equalizer or dampener of the
 +bombardment against the stem. To be explicit, I assume the action as
 +follows: Suppose a rhythmical bombardment to occur against the
 +conducting tube by reason of its imperfect action as a screen,
 +<!-- Page 78 -->
 +it certainly must happen that some molecules, or atoms, strike the tube
 +sooner than others. Those which come first in contact with it give up
 +their superfluous charge, and the tube is electrified,​ the
 +electrification instantly spreading over its surface. But this must
 +diminish the energy lost in the bombardment for two reasons: first,
 +the charge given up by the atoms spreads over a great area, and hence
 +the electric density at any point is small, and the atoms are repelled
 +with less energy than they would be if they would strike against a
 +good insulator: secondly, as the tube is electrified by the atoms
 +which first come in contact with it, the progress of the following
 +atoms against the tube is more or less checked by the repulsion which
 +the electrified tube must exert upon the similarly electrified atoms.
 +This repulsion may perhaps be sufficient to prevent a large portion of
 +the atoms from striking the tube, but at any rate it must diminish the
 +energy of their impact. It is clear that when the exhaustion is very
 +low, and the rarefied gas well conducting, neither of the above
 +effects can occur, and, on the other hand, the fewer the atoms, with
 +the greater freedom they move; in other words, the higher the degree
 +of exhaustion, up to a limit, the more telling will be both the
 +<​p>​What I have just said may afford an explanation of the phenomenon
 +observed by Prof. Crookes, namely, that a discharge through a bulb is
 +established with much greater facility when an insulator than when a
 +conductor is present in the same. In my opinion, the conductor acts as
 +a dampener of the motion of the atoms in the two ways pointed out;
 +hence, to cause a visible discharge to pass
 +<!-- Page 79 -->
 +through the bulb, a much higher potential is needed if a conductor,
 +especially of much surface, be present.</​p>​
 +<​p>​For the sake of clearness of some of the remarks before made, I must
 +now refer to Figs. 18, 19 and 20, which illustrate various
 +arrangements with a type of bulb most generally used.</​p>​
 +<img src="​images/​acfig18.gif"​ width="​315"​ height="​560"​ border="​0"​ hspace="​10"​ align="​left"​
 +Fig. 18 is a section through a spherical bulb <​i>​L</​i>,​ with the glass stem
 +<​i>​s</​i>,​ containing the leading-in wire <​i>​w</​i>;​ which has a lamp filament <​i>​l</​i>​
 +fastened to it, serving to support the refractory button <​i>​m</​i>​ in the centre. <​i>​M</​i>​ is a sheet of thin
 +<!-- Page 80 -->
 +mica wound in several layers around the stem <​i>​s</​i>,​ and <​i>​a</​i>​ is the aluminium tube.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig19.gif"​ width="​263"​ height="​563"​ border="​0"​ align="​left"​ hspace="​10"​
 +Fig. 19 illustrates such a bulb in a somewhat more advanced stage of
 +perfection. A metallic tube <​i>​S</​i>​ is fastened by means of some cement to
 +the neck of the tube. In the tube is screwed a plug <​i>​P</​i>,​ of insulating
 +material, in the centre of which is fastened a metallic terminal <​i>​t</​i>,​
 +for the connection to the leading-in wire <​i>​w</​i>​. This terminal must be
 +well insulated from the metal tube <​i>​S</​i>,​ therefore, if the cement used
 +is conducting&​mdash;​and most generally it is sufficiently so&​mdash;​the space
 +between the plug <​i>​P</​i>​ and the neck of the bulb should be filled with
 +some good insulating material, as mica powder.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig20.gif"​ width="​274"​ height="​564"​ border="​0"​ align="​left"​ hspace="​10"​
 +Fig. 20 shows a bulb made for experimental purposes. In this bulb the
 +aluminium tube is provided with an external connection, which serves
 +to investigate the effect of the tube under various conditions. It is
 +referred to chiefly to suggest a line of experiment followed.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig21.gif"​ width="​278"​ height="​562"​ border="​0"​ align="​left"​ hspace="​10"​
 +Since the bombardment against the stem containing the leading-in wire is due to
 +the inductive action of the latter upon the rarefied gas, it is of advantage to reduce this action
 +<!-- Page 81 -->
 +as far as practicable by employing a very thin wire, surrounded by a very thick insulation
 +of glass or other material, and by making the wire passing through the
 +rarefied gas as short as practicable. To combine these features I
 +employ a large tube <​i>​T</​i>​ (Fig. 21), which protrudes into the bulb to
 +some distance, and carries on the top a very short glass stem <​i>​s</​i>,​
 +into which is sealed the leading-in wire <​i>​w</​i>,​ and I protect the top of
 +the glass stem against the heat by a small, aluminium tube <​i>​a</​i>​ and a
 +layer of mica underneath the same, as usual. The wire <​i>​w</​i>,​ passing
 +through the large tube to the outside of the bulb, should be well
 +insulated&​mdash;​with a glass tube, for instance&​mdash;​and the space between
 +ought to be filled out with some excellent insulator. Among many
 +insulating powders I have tried, I have found that mica powder is the
 +best to employ. If this precaution is not taken, the tube <​i>​T</​i>,​
 +protruding into the bulb, will surely be cracked in consequence of the
 +heating by the brushes which are apt to form in the upper part of the
 +tube, near the exhausted globe, especially if the vacuum be excellent,
 +and therefore the potential necessary to operate the lamp very high.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig22.gif"​ width="​252"​ height="​570"​ border="​0"​ align="​left"​ hspace="​10"​
 +Fig. 22 illustrates a similar arrangement,​ with a large tube <​i>​T</​i>​
 +protruding in to the part of the bulb containing the refractors button
 +<​i>​m</​i>​. In this case the wire leading from the outside into the bulb is
 +omitted, the energy required being supplied through condenser coatings
 +<​i>​C&​nbsp;​C</​i>​. The insulating packing <​i>​P</​i>​ should in this construction be
 +tightly fitting to the glass, and rather wide, or otherwise the
 +discharge might avoid passing through the wire <​i>​w</​i>,​ which connects the
 +inside condenser coating to the incandescent button <​i>​m</​i>​.
 +<!-- Page 82 -->
 +The molecular bombardment against the glass stem in the bulb is a source
 +of great trouble. As illustration I will cite a phenomenon only too
 +frequently and unwillingly observed. A bulb, preferably a large one,
 +may be taken, and a good conducting body, such as a piece of carbon,
 +may be mounted in it upon a platinum wire sealed in the glass stem.
 +The bulb may be exhausted to a fairly high degree, nearly to the point
 +when phosphorescence begins to appear.</​p>​
 +<!-- Page 83 -->
 +<br clear="​all">&​nbsp;<​br>​
 +<​p>​When the bulb is connected with the coil, the piece of carbon, if
 +small, may become highly incandescent at first, but its brightness
 +immediately diminishes, and then the discharge may break through the
 +glass somewhere in the middle of the stem, in the form of bright
 +sparks, in spite of the fact that the platinum wire is in good
 +electrical connection with the rarefied gas through the piece of
 +carbon or metal at the top. The first sparks are singularly bright,
 +recalling those drawn from a clear surface of mercury. But, as they
 +heat the glass rapidly, they, of course, lose their brightness, and
 +cease when the glass at the ruptured place becomes incandescent,​ or
 +generally sufficiently hot to conduct. When observed for the first
 +time the phenomenon must appear very curious, and shows in a striking
 +manner how radically different alternate currents, or impulses, of
 +high frequency behave, as compared with steady currents, or currents
 +of low frequency. With such currents&​mdash;​namely,​ the latter&​mdash;​the
 +phenomenon would of course not occur. When frequencies such as are
 +obtained by mechanical means are used, I think that the rupture of the
 +glass is more or less the consequence of the bombardment,​ which warms
 +it up and impairs its insulating power; but with frequencies
 +obtainable with condensers I have no doubt that the glass may give way
 +without previous heating. Although this appears most singular at
 +first, it is in reality what we might expect to occur. The energy
 +supplied to the wire leading into the bulb is given off partly by
 +direct action through the carbon button, and partly by inductive
 +action through the glass surrounding the wire. The case is thus
 +analogous to that in which a condenser shunted by a
 +<!-- Page 84 -->
 +conductor of low resistance is connected to a source of alternating currents.
 +As long as the frequencies are low, the conductor gets the most, and the
 +condenser is perfectly safe: but when the frequency becomes excessive,
 +the <​i>​r&​ocirc;​le</​i>​ of the conductor may become quite insignificant. In the
 +latter case the difference of potential at the terminals of the
 +condenser may become so great as to rupture the dielectric,
 +notwithstanding the fact that the terminals are joined by a conductor
 +of low resistance.</​p>​
 +<!-- Page 85 -->
 +<p>It is, of course, not necessary, when it is desired to produce the
 +incandescence of a body inclosed in a bulb by means of these currents,
 +that the body should be a conductor, for even a perfect non-conductor
 +may be quite as readily heated. For this purpose it is sufficient to
 +surround a conducting electrode with a non-conducting material, as,
 +for instance, in the bulb described before in Fig. 21, in which a thin
 +incandescent lamp filament is coated with a non-conductor,​ and
 +supports a button of the same material on the top. At the start the
 +bombardment goes on by inductive action through the non-conductor,​
 +until the same is sufficiently heated to become conducting, when the
 +bombardment continues in the ordinary way.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig23.gif"​ width="​452"​ height="​566"​ border="​0"​
 +alt="​FIG. 23.&​mdash;​EFFECT PRODUCED BY A RUBY DROP.">​
 +<p>A different arrangement used in some of the bulbs constructed is
 +illustrated in Fig. 23. In this instance a non-conductor <​i>​m</​i>​ is
 +mounted in a piece of common arc light carbon so as to project some
 +small distance above the latter. The carbon piece is connected to the
 +leading-in wire passing through a glass stem, which is wrapped with
 +several layers of mica. An aluminium tube <​i>​a</​i>​ is employed as usual for
 +screening. It is so arranged that it reaches very nearly as high as
 +the carbon and only the non-conductor <​i>​m</​i>​ projects a little above it.
 +The bombardment goes at first against the upper surface of carbon, the
 +lower parts being protected by the aluminium tube. As soon, however,
 +as the non-conductor <​i>​m</​i>​ is heated it is rendered good conducting, and
 +then it becomes the centre of the bombardment,​ being most exposed to
 +the same.</​p>​
 +<p>I have also constructed during these experiments many such single-wire
 +bulbs with or without internal electrode,
 +<!-- Page 86 -->
 +in which the radiant matter was projected against, or focused upon, the body
 +to be rendered incandescent. Fig. 24 illustrates one of the bulbs used. It consists
 +of a spherical globe <​i>​L</​i>,​ provided with a long neck <​i>​n</​i>,​ on the top,
 +for increasing the action in some cases by the application of an
 +external conducting coating. The globe <​i>​L</​i>​ is blown out on the bottom
 +into a very small bulb <​i>​b</​i>,​ which serves to hold it firmly in a socket
 +<​i>​S</​i>​ of insulating material into which it is cemented. A fine lamp
 +filament <​i>​f</​i>,​ supported on a wire <​i>​w</​i>,​ passes through the centre of
 +the globe <​i>​L</​i>​. The filament is rendered incandescent in the middle
 +portion, where the bombardment proceeding from the lower inside
 +surface of the globe is most intense. The lower portion of the globe,
 +as far as the socket <​i>​S</​i>​ reaches, is rendered conducting, either by a
 +tinfoil coating or otherwise, and the external electrode is connected
 +to a terminal of the coil.</​p>​
 +<​p>​The arrangement diagrammatically indicated in Fig. 24 was found to be
 +an inferior one when it was desired to render incandescent a filament
 +or button supported in the centre of the globe, but it was convenient
 +when the object was to excite phosphorescence.</​p>​
 +<p>In many experiments in which bodies of a different kind were
 +mounted in the bulb as, for instance, indicated in Fig. 23, some observations
 +of interest were made.</​p>​
 +<p>It was found, among other things, that in such cases, no matter where
 +the bombardment began, just as soon as a high temperature was reached
 +there was generally one of the bodies which seemed to take most of the
 +bombardment upon itself, the other, or others, being thereby relieved.
 +This quality appeared to depend principally on the point of
 +<!-- Page 87 -->
 +fusion, and on the facility with which the body was &​quot;​evaporated,&​quot;​ or,
 +generally speaking, disintegrated&​mdash;​meaning by the latter term not only
 +the throwing off of atoms, but likewise of larger lumps. The
 +observation made was in accordance with generally accepted notions. In
 +a highly exhausted bulb electricity is carried off from the electrode
 +by independent carriers, which are partly the atoms, or molecules, of
 +the residual atmosphere, and partly the atoms, molecules, or lumps
 +thrown off from the electrode. If the electrode is composed of bodies
 +of different character, and if one of these is more easily
 +disintegrated than the others, most of the electricity supplied is
 +carried off from that body, which is then brought to a higher
 +temperature than the others, and this the more, as upon an increase of
 +the temperature the body is still more easily disintegrated.</​p>​
 +<p>It seems to me quite probable that a similar process takes place in
 +the bulb even with a homogeneous electrode, and I think it to be the
 +principal cause of the disintegration. There is bound to be some
 +irregularity,​ even if the surface is highly polished, which, of
 +course, is impossible with most of the refractory bodies employed as
 +electrodes. Assume that a point of the electrode gets hotter,
 +instantly most of the discharge passes through that point, and a
 +minute patch is probably fused and evaporated. It is now possible that
 +in consequence of the violent disintegration the spot attacked sinks
 +in temperature,​ or that a counter force is created, as in an arc; at
 +any rate, the local tearing off meets with the limitations incident to
 +the experiment, whereupon the same process occurs on another place. To
 +the eye the electrode appears uniformly brilliant,
 +<!-- Page 88 -->
 +but there are upon it points constantly shifting and wandering around,
 +of a temperature far above the mean, and this materially hastens the process
 +of deterioration. That some such thing occurs, at least when the
 +electrode is at a lower temperature,​ sufficient experimental evidence
 +can be obtained in the following manner: Exhaust a bulb to a very high
 +degree, so that with a fairly high potential the discharge cannot
 +pass&​mdash;​that is, not a <​i>​luminous</​i>​ one, for a weak invisible discharge
 +occurs always, in all probability. Now raise slowly and carefully the
 +potential, leaving the primary current on no more than for an instant.
 +At a certain point, two, three, or half a dozen phosphorescent spots
 +will appear on the globe. These places of the glass are evidently more
 +violently bombarded than others, this being due to the unevenly
 +distributed electric density, necessitated,​ of course, by sharp
 +projections,​ or, generally speaking, irregularities of the electrode.
 +But the luminous patches are constantly changing in position, which is
 +especially well observable if one manages to produce very few, and
 +this indicates that the configuration of the electrode is rapidly
 +<​p>​From experiences of this kind I am led to infer that, in order to be
 +most durable, the refractory button in the bulb should be in the form
 +of a sphere with a highly polished surface. Such a small sphere could
 +be manufactured from a diamond or some other crystal, but a better way
 +would be to fuse, by the employment of extreme degrees of temperature,​
 +some oxide&​mdash;​as,​ for instance, zirconia&​mdash;​into a small drop, and then
 +keep it in the bulb at a temperature somewhat below its point of
 +<!-- Page 89 -->
 +<​p>​Interesting and useful results can no doubt be reached in the
 +direction of extreme degrees of heat. How can such high temperatures
 +be arrived at? How are the highest degrees of heat reached in nature?
 +By the impact of stars, by high speeds and collisions. In a collision
 +any rate of heat generation may be attained. In a chemical process we
 +are limited. When oxygen and hydrogen combine, they fall,
 +metaphorically speaking, from a definite height. We cannot go very far
 +with a blast, nor by confining heat in a furnace, but in an exhausted
 +bulb we can concentrate any amount of energy upon a minute button.
 +Leaving practicability out of consideration,​ this, then, would be the
 +means which, in my opinion, would enable us to reach the highest
 +temperature. But a great difficulty when proceeding in this way is
 +encountered,​ namely, in most cases the body is carried off before it
 +can fuse and form a drop. This difficulty exists principally with an
 +oxide such as zirconia, because it cannot be compressed in so hard a
 +cake that it would not be carried off quickly. I endeavored repeatedly
 +to fuse zirconia, placing it in a cup or arc light carbon as indicated
 +in Fig. 23. It glowed with a most intense light, and the stream of the
 +particles projected out of the carbon cup was of a vivid white: but
 +whether it was compressed in a cake or made into a paste with carbon,
 +it was carried off before it could be fused. The carbon cup containing
 +the zirconia had to be mounted very low in the neck of a large bulb,
 +as the heating of the glass by the projected particles of the oxide
 +was so rapid that in the first trial the bulb was cracked almost in an
 +instant when the current was turned on. The heating of the glass
 +<!-- Page 90 -->
 +by the projected particles was found to be always greater when the carbon
 +cup contained a body which was rapidly carried off&​mdash;​I presume because
 +in such cases, with the same potential, higher speeds were reached,
 +and also because, per unit of time, more matter was projected&​mdash;​that
 +is, more particles would strike the glass.</​p>​
 +<​p>​The before mentioned difficulty did not exist, however, when the body
 +mounted in the carbon cup offered great resistance to deterioration.
 +For instance, when an oxide was first fused in an oxygen blast and
 +then mounted in the bulb, it melted very readily into a drop.</​p>​
 +<​p>​Generally during the process of fusion magnificent light effects were
 +noted, of which it would be difficult to give an adequate idea. Fig.
 +23 is intended to illustrate the effect observed with a ruby drop. At
 +first one may see a narrow funnel of white light projected against the
 +top of the globe, where it produces an irregularly outlined
 +phosphorescent patch. When the point of the ruby fuses the
 +phosphorescence becomes very powerful; but as the atoms are projected
 +with much greater speed from the surface of the drop, soon the glass
 +gets hot and &​quot;​tired,&​quot;​ and now only the outer edge of the patch glows.
 +In this manner an intensely phosphorescent,​ sharply defined line, <​i>​l</​i>,​
 +corresponding to the outline of the drop, is produced, which spreads
 +slowly over the globe as the drop gets larger. When the mass begins to
 +boil, small bubbles and cavities are formed, which cause dark colored
 +spots to sweep across the globe. The bulb may be turned downward
 +without fear of the drop falling off, as the mass possesses
 +considerable viscosity.</​p>​
 +<p>I may mention here another feature of some interest,
 +<!-- Page 91 -->
 +which I believe to have noted in the course of these experiments,​ though
 +the observations do not amount to a certitude. It <​i>​appeared</​i>​ that under
 +the molecular impact caused by the rapidly alternating potential the
 +body was fused and maintained in that state at a lower temperature in
 +a highly exhausted bulb than was the case at normal pressure and
 +application of heat in the ordinary way&​mdash;​that is, at least, judging
 +from the quantity of the light emitted. One of the experiments
 +performed may be mentioned here by way of illustration. A small piece
 +of pumice stone was stuck on a platinum wire, and first melted to it
 +in a gas burner. The wire was next placed between two pieces of
 +charcoal and a burner applied so as to produce an intense heat,
 +sufficient to melt down the pumice stone into a small glass-like
 +button. The platinum wire had to be taken of sufficient thickness to
 +prevent its melting in the fire. While in the charcoal fire, or when
 +held in a burner to get a better idea of the degree of heat, the
 +button glowed with great brilliancy. The wire with the button was then
 +mounted in a bulb, and upon exhausting the same to a high degree, the
 +current was turned on slowly so as to prevent the cracking of the
 +button. The button was heated to the point of fusion, and when it
 +melted it did not, apparently, glow with the same brilliancy as
 +before, and this would indicate a lower temperature. Leaving out of
 +consideration the observer'​s possible, and even probable, error, the
 +question is, can a body under these conditions be brought from a solid
 +to a liquid state with evolution of <​i>​less</​i>​ light?</​p>​
 +<​p>​When the potential of a body is rapidly alternated it is
 +<!-- Page 92 -->
 +certain that the structure is jarred. When the potential is very high, although the
 +vibrations may be few&​mdash;​say 20,000 per second&​mdash;​the effect upon
 +the structure may be considerable. Suppose, for example, that a ruby is
 +melted into a drop by a steady application of energy. When it forms a
 +drop it will emit visible and invisible waves, which will be in a
 +definite ratio, and to the eye the drop will appear to be of a certain
 +brilliancy. Next, suppose we diminish to any degree we choose the
 +energy steadily supplied, and, instead, supply energy which rises and
 +falls according to a certain law. Now, when the drop is formed, there
 +will be emitted from it three different kinds of vibrations&​mdash;​the
 +ordinary visible, and two kinds of invisible waves: that is, the
 +ordinary dark waves of all lengths, and, in addition, waves of a well
 +defined character. The latter would not exist by a steady supply of
 +the energy; still they help to jar and loosen the structure. If this
 +really be the case, then the ruby drop will emit relatively less
 +visible and more invisible waves than before. Thus it would seem that
 +when a platinum wire, for instance, is fused by currents alternating
 +with extreme rapidity, it emits at the point of fusion less light and
 +more invisible radiation than it does when melted by a steady current,
 +though the total energy used up in the process of fusion is the same
 +in both cases. Or, to cite another example, a lamp filament is not
 +capable of withstanding as long with currents of extreme frequency as
 +it does with steady currents, assuming that it be worked at the same
 +luminous intensity. This means that for rapidly alternating currents
 +the filament should be shorter and thicker. The higher the
 +<!-- Page 93 -->
 +frequency&​mdash;​that is, the greater the departure from the steady
 +flow&​mdash;​the worse it would be for the filament. But if the truth of this
 +remark were demonstrated,​ it would be erroneous to conclude that such
 +a refractory button as used in these bulbs would be deteriorated
 +quicker by currents of extremely high frequency than by steady or low
 +frequency currents. From experience I may say that just the opposite
 +holds good: the button withstands the bombardment better with currents
 +of very high frequency. But this is due to the fact that a high
 +frequency discharge passes through a rarefied gas with much greater
 +freedom than a steady or low frequency discharge, and this will say
 +that with the former we can work with a lower potential or with a less
 +violent impact. As long, then, as the gas is of no consequence,​ a
 +steady or low frequency current is better; but as soon as the action
 +of the gas is desired and important, high frequencies are preferable.</​p>​
 +<p>In the course of these experiments a great many trials were made with
 +all kinds of carbon buttons. Electrodes made of ordinary carbon
 +buttons were decidedly more durable when the buttons were obtained by
 +the application of enormous pressure. Electrodes prepared by
 +depositing carbon in well known ways did not show up well; they
 +blackened the globe very quickly. From many experiences I conclude
 +that lamp filaments obtained in this manner can be advantageously used
 +only with low potentials and low frequency currents. Some kinds of
 +carbon withstand so well that, in order to bring them to the point of
 +fusion, it is necessary to employ very small buttons. In this case the
 +observation is rendered very
 +<!-- Page 94 -->
 +difficult on account of the intense heat produced. Nevertheless there can be
 +no doubt that all kinds of carbon are fused under the molecular bombardment,​
 +but the liquid state must be one of great instability. Of all the bodies tried there were
 +two which withstood best&​mdash;​diamond and carborundum. These two showed up
 +about equally, but the latter was preferable, for many reasons. As it
 +is more than likely that this body is not yet generally known, I will
 +venture to call your attention to it.</​p>​
 +<p>It has been recently produced by Mr. E.G. Acheson, of Monongahela
 +City, Pa., U.S.A. It is intended to replace ordinary diamond powder
 +for polishing precious stones, etc., and I have been informed that it
 +accomplishes this object quite successfully. I do not know why the
 +name &​quot;​carborundum&​quot;​ has been given to it, unless there is something in
 +the process of its manufacture which justifies this selection. Through
 +the kindness of the inventor, I obtained a short while ago some
 +samples which I desired to test in regard to their qualities of
 +phosphorescence and capability of withstanding high degrees of heat.</​p>​
 +<​p>​Carborundum can be obtained in two forms&​mdash;​in the form of &​quot;​crystals&​quot;​
 +and of powder. The former appear to the naked eye dark colored, but
 +are very brilliant; the latter is of nearly the same color as ordinary
 +diamond powder, but very much finer. When viewed under a microscope
 +the samples of crystals given to me did not appear to have any definite form,
 +but rather resembled pieces of broken up egg coal of fine quality. The majority
 +were opaque, but there were some which were transparent and colored.
 +The crystals are a kind of carbon containing some impurities; they are
 +<!-- Page 95 -->
 +extremely hard, and withstand for a long time even an oxygen blast. When
 +the blast is directed against them they at first form a cake of some compactness,​
 +probably in consequence of the fusion of impurities they contain. The mass
 +withstands for a very long time the blast without further fusion; but a slow
 +carrying off, or burning, occurs, and, finally, a small quantity of a
 +glass-like residue is left, which, I suppose, is melted alumina. When
 +compressed strongly they conduct very well, but not as well as
 +ordinary carbon. The powder, which is obtained from the crystals in
 +some way, is practically non-conducting. It affords a magnificent
 +polishing material for stones.</​p>​
 +<​p>​The time has been too short to make a satisfactory study of the
 +properties of this product, but enough experience has been gained in a
 +few weeks I have experimented upon it to say that it does possess some
 +remarkable properties in many respects. It withstands excessively high
 +degrees of heat, it is little deteriorated by molecular bombardment,​
 +and it does not blacken the globe as ordinary carbon does. The only
 +difficulty which I have found in its use in connection with these
 +experiments was to find some binding material which would resist the
 +heat and the effect of the bombardment as successfully as carborundum
 +itself does.</​p>​
 +<p>I have here a number of bulbs which I have provided with buttons of
 +carborundum. To make such a button of carborundum crystals I proceed
 +in the following manner: I take an ordinary lamp filament and dip its
 +point in tar, or some other thick substance or paint which may be
 +readily carbonized. I next pass the point of the filament through the
 +crystals, and then hold it vertically over a hot
 +<!-- Page 96 -->
 +plate. The tar softens and forms a drop on the point of the filament, the
 +crystals adhering to the surface of the drop. By regulating the distance
 +from the plate the tar is slowly dried out and the button becomes solid.
 +I then once more dip the button in tar and hold it again over a plate
 +until the tar is evaporated, leaving only a hard mass which firmly
 +binds the crystals. When a larger button is required I repeat the
 +process several times, and I generally also cover the filament a
 +certain distance below the button with crystals. The button being
 +mounted in a bulb, when a good vacuum has been reached, first a weak
 +and then a strong discharge is passed through the bulb to carbonize
 +the tar and expel all gases, and later it is brought to a very intense
 +<​p>​When the powder is used I have found it best to proceed as follows: I
 +make a thick paint of carborundum and tar, and pass a lamp filament
 +through the paint. Taking then most of the paint off by rubbing the
 +filament against a piece of chamois leather, I hold it over a hot
 +plate until the tar evaporates and the coating becomes firm. I repeat
 +this process as many times as it is necessary to obtain a certain
 +thickness of coating. On the point of the coated filament I form a
 +button in the same manner.</​p>​
 +<​p>​There is no doubt that such a button&​mdash;​properly prepared under great
 +pressure&​mdash;​of carborundum,​ especially of powder of the best quality,
 +will withstand the effect of the bombardment fully as well as anything
 +we know. The difficulty is that the binding material gives way, and
 +the carborundum is slowly thrown off after some time. As it does not
 +seem to blacken the globe in the least, it might be
 +<!-- Page 97 -->
 +found useful for coating the filaments of ordinary incandescent lamps, and I think
 +that it is even possible to produce thin threads or sticks of carborundum
 +which will replace the ordinary filaments in an incandescent lamp. A
 +carborundum coating seems to be more durable than other coatings, not
 +only because the carborundum can withstand high degrees of heat, but
 +also because it seems to unite with the carbon better than any other
 +material I have tried. A coating of zirconia or any other oxide, for
 +instance, is far more quickly destroyed. I prepared buttons of diamond
 +dust in the same manner as of carborundum,​ and these came in
 +durability nearest to those prepared of carborundum,​ but the binding
 +paste gave way much more quickly in the diamond buttons: this,
 +however, I attributed to the size and irregularity of the grains of
 +the diamond.</​p>​
 +<p>It was of interest to find whether carborundum possesses the quality
 +of phosphorescence. One is, of course, prepared to encounter two
 +difficulties:​ first, as regards the rough product, the &​quot;​crystals,&​quot;​
 +they are good conducting, and it is a fact that conductors do not
 +phosphoresce;​ second, the powder, being exceedingly fine, would not be
 +apt to exhibit very prominently this quality, since we know that when
 +crystals, even such as diamond or ruby, are finely powdered, they lose
 +the property of phosphorescence to a considerable degree.</​p>​
 +<​p>​The question presents itself here, can a conductor phosphoresce?​
 +What is there in such a body as a metal, for instance, that would deprive
 +it of the quality of phosphorescence,​ unless it is that property which
 +characterizes it as a
 +<!-- Page 98 -->
 +conductor? for it is a fact that most of the phosphorescent bodies lose that
 +quality when they are sufficiently heated to become more or less conducting.
 +Then, if a metal be in a large measure, or perhaps entirely, deprived of that property,
 +it should be capable of phosphorescence. Therefore it is quite possible
 +that at some extremely high frequency, when behaving practically as a
 +non-conductor,​ a metal or any other conductor might exhibit the
 +quality of phosphorescence,​ even though it be entirely incapable of
 +phosphorescing under the impact of a low-frequency discharge. There
 +is, however, another possible way how a conductor might at least
 +<​i>​appear</​i>​ to phosphoresce.</​p>​
 +<​p>​Considerable doubt still exists as to what really is phosphorescence,​
 +and as to whether the various phenomena comprised under this head are
 +due to the same causes. Suppose that in an exhausted bulb, under the
 +molecular impact, the surface of a piece of metal or other conductor
 +is rendered strongly luminous, but at the same time it is found that
 +it remains comparatively cool, would not this luminosity be called
 +phosphorescence?​ Now such a result, theoretically at least, is
 +possible, for it is a mere question of potential or speed. Assume the
 +potential of the electrode, and consequently the speed of the
 +projected atoms, to be sufficiently high, the surface of the metal
 +piece against which the atoms are projected would be rendered highly
 +incandescent,​ since the process of heat generation would be
 +incomparably faster than that of radiating or conducting away from the
 +surface of the collision. In the eye of the observer a single impact of the
 +atoms would cause an instantaneous flash, but if the impacts were repeated
 +<!-- Page 99 -->
 +with sufficient rapidity they would produce a continuous impression upon his retina.
 +To him then the surface of the metal would appear continuously incandescent and of
 +constant luminous intensity, while in reality the light would be either intermittent or
 +at least changing periodically in intensity. The metal piece would
 +rise in temperature until equilibrium was attained&​mdash;​that is until the
 +energy continuously radiated would equal that intermittently supplied.
 +But the supplied energy might under such conditions not be sufficient
 +to bring the body to any more than a very moderate mean temperature,​
 +especially if the frequency of the atomic impacts be very low&​mdash;​just
 +enough that the fluctuation of the intensity of the light emitted
 +could not be detected by the eye. The body would now, owing to the
 +manner in which the energy is supplied, emit a strong light, and yet
 +be at a comparatively very low mean temperature. How could the
 +observer call the luminosity thus produced? Even if the analysis of
 +the light would teach him something definite, still he would probably
 +rank it under the phenomena of phosphorescence. It is conceivable that
 +in such a way both conducting and non-conducting bodies may be
 +maintained at a certain luminous intensity, but the energy required
 +would very greatly vary with the nature and properties of the bodies.</​p>​
 +<​p>​These and some foregoing remarks of a speculative nature were made
 +merely to bring out curious features of alternate currents or electric
 +impulses. By their help we may cause a body to emit <​i>​more</​i>​ light,
 +while at a certain mean temperature,​ than it would emit if brought to
 +that temperature by a steady supply; and, again, we may bring
 +<!-- Page 100 -->
 +a body to the point of fusion, and cause it to emit <​i>​less</​i>​ light than when
 +fused by the application of energy in ordinary ways. It all depends on
 +how we supply the energy, and what kind of vibrations we set up: in
 +one case the vibrations are more, in the other less, adapted to affect
 +our sense of vision.</​p>​
 +<​p>​Some effects, which I had not observed before, obtained with
 +carborundum in the first trials, I attributed to phosphorescence,​ but
 +in subsequent experiments it appeared that it was devoid of that
 +quality. The crystals possess a noteworthy feature. In a bulb provided
 +with a single electrode in the shape of a small circular metal disc,
 +for instance, at a certain degree of exhaustion the electrode is
 +covered with a milky film, which is separated by a dark space from the
 +glow filling the bulb. When the metal disc is covered with carborundum
 +crystals, the film is far more intense, and snow-white. This I found
 +later to be merely an effect of the bright surface of the crystals,
 +for when an aluminium electrode was highly polished it exhibited more
 +or less the same phenomenon. I made a number of experiments with the
 +samples of crystals obtained, principally because it would have been
 +of special interest to find that they are capable of phosphorescence,​
 +on account of their being conducting. I could not produce
 +phosphorescence distinctly, but I must remark that a decisive opinion
 +cannot be formed until other experimenters have gone over the same
 +<​p>​The powder behaved in some experiments as though it contained alumina,
 +but it did not exhibit with sufficient distinctness the red of the latter. Its dead color brightens
 +<!-- Page 101 -->
 +considerably under the molecular impact, but I am now convinced it does not phosphoresce.
 +Still, the tests with the powder are not conclusive, because powdered carborundum
 +probably does not behave like a phosphorescent sulphide, for example,
 +which could be finely powdered without impairing the phosphorescence,​
 +but rather like powdered ruby or diamond, and therefore it would be
 +necessary, in order to make a decisive test, to obtain it in a large
 +lump and polish up the surface.</​p>​
 +<p>If the carborundum proves useful in connection with these and similar
 +experiments,​ its chief value will be found in the production of
 +coatings, thin conductors, buttons, or other electrodes capable of
 +withstanding extremely high degrees of heat.</​p>​
 +<​p>​The production of a small electrode capable of withstanding enormous
 +temperatures I regard as of the greatest importance in the manufacture
 +of light. It would enable us to obtain, by means of currents of very
 +high frequencies,​ certainly 20 times, if not more, the quantity of
 +light which is obtained in the present incandescent lamp by the same
 +expenditure of energy. This estimate may appear to many exaggerated,​
 +but in reality I think it is far from being so. As this statement
 +might be misunderstood I think it necessary to expose clearly the
 +problem with which in this line of work we are confronted, and the
 +manner in which, in my opinion, a solution will be arrived at.</​p>​
 +<​p>​Any one who begins a study of the problem will be apt to think that
 +what is wanted in a lamp with an electrode is a very high degree of incandescence of
 +<!-- Page 102 -->
 +the electrode. There he will be mistaken. The high incandescence
 +of the button is a necessary evil, but what is really wanted is the high
 +incandescence of the gas surrounding the button. In other words,
 +the problem in such a lamp is to bring a mass of gas to the highest
 +possible incandescence. The higher the incandescence,​ the
 +quicker the mean vibration, the greater is the economy of the light
 +production. But to maintain a mass of gas at a high degree of
 +incandescence in a glass vessel, it will always be necessary to keep
 +the incandescent mass away from the glass; that is, to confine it as
 +much as possible to the central portion of the globe.</​p>​
 +<p>In one of the experiments this evening a brush was produced at the end
 +of a wire. This brush was a flame, a source of heat and light. It did
 +not emit much perceptible heat, nor did it glow with an intense light;
 +but is it the less a flame because it does not scorch my hand? Is it
 +the less a flame because it does not hurt my eye by its brilliancy?
 +The problem is precisely to produce in the bulb such a flame, much
 +smaller in size, but incomparably more powerful. Were there means at
 +hand for producing electric impulses of a sufficiently high frequency,
 +and for transmitting them, the bulb could be done away with, unless it
 +were used to protect the electrode, or to economize the energy by
 +confining the heat. But as such means are not at disposal, it becomes
 +necessary to place the terminal in a bulb and rarefy the air in the
 +same. This is done merely to enable the apparatus to perform the work
 +which it is not capable of performing at ordinary air pressure. In the
 +bulb we are able to intensify the action to any degree&​mdash;​so far that
 +the brush emits a powerful light.
 +<!-- Page 103 -->
 +The intensity of the light emitted depends principally on the frequency and
 +potential of the impulses, and on the electric density of the surface of the electrode.
 +It is of the greatest importance to employ the smallest possible button, in
 +order to push the density very far. Under the violent impact of the
 +molecules of the gas surrounding it, the small electrode is of course
 +brought to an extremely high temperature,​ but around it is a mass of
 +highly incandescent gas, a flame photosphere,​ many hundred times the
 +volume of the electrode. With a diamond, carborundum or zirconia
 +button the photosphere can be as much as one thousand times the volume
 +of the button. Without much reflecting one would think that in pushing
 +so far the incandescence of the electrode it would be instantly volatilized.
 +But after a careful consideration he would find that, theoretically,​ it should
 +not occur, and in this fact&​mdash;​which,​ however, is experimentally
 +demonstrated&​mdash;​lies principally the future value of such a lamp.</​p>​
 +<p>At first, when the bombardment begins, most of the work is performed
 +on the surface of the button, but when a highly conducting photosphere
 +is formed the button is comparatively relieved. The higher the
 +incandescence of the photosphere the more it approaches in
 +conductivity to that of the electrode, and the more, therefore, the
 +solid and the gas form one conducting body. The consequence is that
 +the further is forced the incandescence the more work, comparatively,​
 +is performed on the gas, and the less on the electrode. The formation
 +of a powerful photosphere is consequently the very means for
 +protecting the electrode. This protection, of course, is a relative one,
 +<!-- Page 104 -->
 +and it should not be thought that by pushing the incandescence
 +higher the electrode is actually less deteriorated. Still,
 +theoretically,​ with extreme frequencies,​ this result must be reached,
 +but probably at a temperature too high for most of the refractory
 +bodies known. Given, then, an electrode which can withstand to a very
 +high limit the effect of the bombardment and outward strain, it would
 +be safe no matter how much it is forced beyond that limit. In an
 +incandescent lamp quite different considerations apply. There the gas
 +is not at all concerned: the whole of the work is performed on the
 +filament; and the life of the lamp diminishes so rapidly with the
 +increase of the degree of incandescence that economical reasons compel
 +us to work it at a low incandescence. But if an incandescent lamp is
 +operated with currents of very high frequency, the action of the gas
 +cannot be neglected, and the rules for the most economical working
 +must be considerably modified.</​p>​
 +<p>In order to bring such a lamp with one or two electrodes to a great
 +perfection, it is necessary to employ impulses of very high frequency.
 +The high frequency secures, among others, two chief advantages, which
 +have a most important bearing upon the economy of the light
 +production. First, the deterioration of the electrode is reduced by
 +reason of the fact that we employ a great many small impacts, instead
 +of a few violent ones, which shatter quickly the structure; secondly,
 +the formation of a large photosphere is facilitated.</​p>​
 +<p>In order to reduce the deterioration of the electrode to the minimum,
 +it is desirable that the vibration be harmonic,
 +<!-- Page 105 -->
 +for any suddenness hastens the process of destruction. An electrode lasts
 +much longer when kept at incandescence by currents, or impulses, obtained
 +from a high-frequency alternator, which rise and fall more or less
 +harmonically,​ than by impulses obtained from a disruptive discharge
 +coil. In the latter case there is no doubt that most of the damage is
 +done by the fundamental sudden discharges.</​p>​
 +<​p>​One of the elements of loss in such a lamp is the bombardment of the
 +globe. As the potential is very high, the molecules are projected with
 +great speed; they strike the glass, and usually excite a strong
 +phosphorescence. The effect produced is very pretty, but for
 +economical reasons it would be perhaps preferable to prevent, or at
 +least reduce to the minimum, the bombardment against the globe, as in
 +such case it is, as a rule, not the object to excite phosphorescence,​
 +and as some loss of energy results from the bombardment. This loss in
 +the bulb is principally dependent on the potential of the impulses and
 +on the electric density on the surface of the electrode. In employing
 +very high frequencies the loss of energy by the bombardment is greatly
 +reduced, for, first, the potential needed to perform a given amount of
 +work is much smaller; and, secondly, by producing a highly conducting
 +photosphere around the electrode, the same result is obtained as
 +though the electrode were much larger, which is equivalent to a smaller
 +electric density. But be it by the diminution of the maximum potential or
 +of the density, the gain is effected in the same manner, namely, by
 +avoiding violent shocks, which strain the glass much beyond its limit of
 +<!-- Page 106 -->
 +elasticity. If the frequency could be brought high enough,
 +the loss due to the imperfect elasticity of the glass would be
 +entirely negligible. The loss due to bombardment of the globe may,
 +however, be reduced by using two electrodes instead of one. In such
 +case each of the electrodes may be connected to one of the terminals;
 +or else, if it is preferable to use only one wire, one electrode may
 +be connected to one terminal and the other to the ground or to an
 +insulated body of some surface, as, for instance, a shade on the lamp.
 +In the latter case, unless some judgment is used, one of the
 +electrodes might glow more intensely than the other.</​p>​
 +<​p>​But on the whole I find it preferable when using such high frequencies
 +to employ only one electrode and one connecting wire. I am convinced
 +that the illuminating device of the near future will not require for
 +its operation more than one lead, and, at any rate, it will have no
 +leading-in wire, since the energy required can be as well transmitted
 +through the glass. In experimental bulbs the leading-in wire is most
 +generally used on account of convenience,​ as in employing condenser
 +coatings in the manner indicated in Fig. 22, for example, there is
 +some difficulty in fitting the parts, but these difficulties would not
 +exist if a great many bulbs were manufactured;​ otherwise the energy
 +can be conveyed through the glass as well as through a wire, and with
 +these high frequencies the losses are very small. Such illuminating
 +devices will necessarily involve the use of very high potentials, and
 +this, in the eyes of practical men, might be an objectionable feature.
 +Yet, in reality, high potentials are not objectionable&​mdash;​certainly not
 +<!-- Page 107 -->
 +in the least as far as the safety of the devices is concerned.</​p>​
 +<​p>​There are two ways of rendering an electric appliance safe. One is to
 +use low potentials, the other is to determine the dimensions of the
 +apparatus so that it is safe no matter how high a potential is used.
 +Of the two the latter seems to me the better way, for then the safety
 +is absolute, unaffected by any possible combination of circumstances
 +which might render even a low-potential appliance dangerous to life
 +and property. But the practical conditions require not only the
 +judicious determination of the dimensions of the apparatus; they
 +likewise necessitate the employment of energy of the proper kind. It
 +is easy, for instance, to construct a transformer capable of giving,
 +when operated from an ordinary alternate current machine of low
 +tension, say 50,000 volts, which might be required to light a highly
 +exhausted phosphorescent tube, so that, in spite of the high
 +potential, it is perfectly safe, the shock from it producing no
 +inconvenience. Still, such a transformer would be expensive, and in
 +itself inefficient;​ and, besides, what energy was obtained from it
 +would not be economically used for the production of light. The
 +economy demands the employment of energy in the form of extremely
 +rapid vibrations. The problem of producing light has been likened to
 +that of maintaining a certain high-pitch note by means of a bell. It
 +should be said a <​i>​barely audible</​i>​ note; and even these words would not
 +express it, so wonderful is the sensitiveness of the eye. We may
 +deliver powerful blows at long intervals, waste a good deal of energy,
 +and still not get what we want; or we may keep up the note
 +<!-- Page 108 -->
 +by delivering frequent gentle taps, and get nearer to the object sought
 +by the expenditure of much less energy. In the production of light, as
 +far as the illuminating device is concerned, there can be only one
 +rule&​mdash;​that is, to use as high frequencies as can be obtained; but the
 +means for the production and conveyance of impulses of such character
 +impose, at present at least, great limitations. Once it is decided to
 +use very high frequencies,​ the return wire becomes unnecessary,​ and
 +all the appliances are simplified. By the use of obvious means the
 +same result is obtained as though the return wire were used. It is
 +sufficient for this purpose to bring in contact with the bulb, or
 +merely in the vicinity of the same, an insulated body of some surface.
 +The surface need, of course, be the smaller, the higher the frequency
 +and potential used, and necessarily,​ also, the higher the economy of
 +the lamp or other device.</​p>​
 +<​p>​This plan of working has been resorted to on several occasions this
 +evening. So, for instance, when the incandescence of a button was
 +produced by grasping the bulb with the hand, the body of the
 +experimenter merely served to intensify the action. The bulb used was
 +similar to that illustrated in Fig. 19, and the coil was excited to a
 +small potential, not sufficient to bring the button to incandescence
 +when the bulb was hanging from the wire; and incidentally,​ in order to
 +perform the experiment in a more suitable manner, the button was taken
 +so large that a perceptible time had to elapse before, upon grasping
 +the bulb, it could be rendered incandescent. The contact with the bulb
 +was, of course, quite unnecessary. It is easy, by using a rather large
 +bulb with an exceedingly small electrode, to adjust
 +<!-- Page 109 -->
 +the conditions so that the latter is brought to bright incandescence by the
 +mere approach of the experimenter within a few feet of the bulb, and that
 +the incandescence subsides upon his receding.</​p>​
 +<!-- Page 110 -->
 +<p>In another experiment, when phosphorescence was excited, a similar
 +bulb was used. Here again, originally, the potential was not
 +sufficient to excite phosphorescence until the action was
 +intensified&​mdash;​in this case, however, to present a different feature, by
 +touching the socket with a metallic object held in the hand. The
 +electrode in the bulb was a carbon button so large that it could not
 +be brought to incandescence,​ and thereby spoil the effect produced by
 +<​p>​Again,​ in another of the early experiments,​ a bulb was used as
 +illustrated in Fig. 12. In this instance, by touching the bulb with
 +one or two fingers, one or two shadows of the stem inside were
 +projected against the glass, the touch of the finger producing the
 +same result as the application of an external negative electrode under
 +ordinary circumstances.</​p>​
 +<p>In all these experiments the action was intensified by
 +<!-- Page 111 -->
 +augmenting the capacity at the end of the lead connected to the terminal.
 +As a rule, it is not necessary to resort to such means, and would be quite
 +unnecessary with still higher frequencies;​ but when it <​i>​is</​i>​ desired,
 +the bulb, or tube, can be easily adapted to the purpose.</​p>​
 +<img src="​images/​acfig24.gif"​ width="​489"​ height="​648"​ border="​0"​ align="​left"​ hspace="​10"​
 +In Fig. 24, for example, an experimental bulb <​i>​L</​i>​ is shown, which is
 +provided with a neck <​i>​n</​i>​ on the top for the application of an external
 +tinfoil coating, which may be connected to a body of larger surface.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig25.gif"​ width="​269"​ height="​662"​ border="​0"​ align="​left"​ hspace="​10"​
 +alt="​FIG. 25.&​mdash;​IMPROVED EXPERIMENTAL BULB.">​
 +Such a lamp as illustrated in Fig. 25 may also be lighted by
 +connecting the tinfoil coating on the neck <​i>​n</​i>​ to the terminal, and
 +the leading-in wire <​i>​w</​i>​ to an insulated plate. If the bulb stands in a
 +socket upright, as shown in the cut, a shade of conducting material
 +may be slipped in the neck <​i>​n</​i>,​ and the action thus magnified.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<div align="​center">​
 +<img src="​images/​acfig26.gif"​ width="​586"​ height="​389"​ border="​0"​
 +<p>A more perfected arrangement used in some of these bulbs is
 +illustrated in Fig. 26. In this case the construction
 +<!-- Page 112 -->
 +of the bulb is as shown and described before, when reference was made to Fig. 19.
 +A zinc sheet <​i>​Z</​i>,​ with a tubular extension <​i>​T</​i>,​ is slipped over the
 +metallic socket <​i>​S</​i>​. The bulb hangs downward from the terminal <​i>​t</​i>,​
 +the zinc sheet <​i>​Z</​i>,​ performing the double office of intensifier and
 +reflector. The reflector is separated from the terminal <​i>​t</​i>​ by an
 +extension of the insulating plug <​i>​P</​i>​.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig27.gif"​ width="​500"​ height="​557"​ border="​0"​
 +<p>A similar disposition with a phosphorescent tube is illustrated
 +<!-- Page 113 -->
 +in Fig. 27. The tube <​i>​T</​i>​ is prepared from two short tubes of a different
 +diameter, which are sealed on the ends. On the lower end is placed an
 +outside conducting coating <​i>​C</​i>,​ which connects to the wire <​i>​w</​i>​. The
 +wire has a hook on the upper end for suspension, and passes through
 +the centre of the inside tube, which is filled with some good and
 +tightly packed insulator. On the outside of the upper end of the tube
 +<​i>​T</​i>​ is another conducting coating <​i>​C</​i><​sub>​1</​sub>​ upon which is slipped a
 +metallic reflector <​i>​Z</​i>,​ which should be separated by a thick
 +insulation from the end of wire <​i>​w</​i>​.</​p>​
 +<​p>​The economical use of such a reflector or intensifier would require
 +that all energy supplied to an air condenser should be recoverable,​
 +or, in other words, that there should not be any losses, neither in
 +the gaseous medium nor through its action elsewhere. This is far from
 +being so, but, fortunately,​ the losses may be reduced to anything
 +desired. A few remarks are necessary on this subject, in order to make
 +the experiences gathered in the course of these investigations
 +perfectly clear.</​p>​
 +<​p>​Suppose a small helix with many well insulated turns, as in experiment
 +Fig. 17, has one of its ends connected to one of the terminals of the
 +induction coil, and the other to a metal plate, or, for the sake of
 +simplicity, a sphere, insulated in space. When the coil is set to
 +work, the potential of the sphere is alternated, and the small helix
 +now behaves as though its free end were connected to the other
 +terminal of the induction coil. If an iron rod be held within the
 +small helix it is quickly brought to a high temperature,​ indicating
 +the passage of a strong current through the helix. How does the
 +insulated sphere act in this case?
 +<!-- Page 114 -->
 +It can be a condenser, storing and returning the energy supplied to it,
 +or it can be a mere sink of energy, and the conditions of the experiment
 +determine whether it is more one or the other. The sphere being charged to
 +a high potential, it acts inductively upon the surrounding air, or whatever gaseous
 +medium there might be. The molecules, or atoms, which are near the
 +sphere are of course more attracted, and move through a greater
 +distance than the farther ones. When the nearest molecules strike the
 +sphere they are repelled, and collisions occur at all distances within
 +the inductive action of the sphere. It is now clear that, if the
 +potential be steady, but little loss of energy can be caused in this
 +way, for the molecules which are nearest to the sphere, having had an
 +additional charge imparted to them by contact, are not attracted until
 +they have parted, if not with all, at least with most of the
 +additional charge, which can be accomplished only after a great many
 +collisions. From the fact that with a steady potential there is but
 +little loss in dry air, one must come to such a conclusion. When the
 +potential of the sphere, instead of being steady, is alternating,​ the
 +conditions are entirely different. In this case a rhythmical
 +bombardment occurs, no matter whether the molecules after coming in
 +contact with the sphere lose the imparted charge or not; what is more,
 +if the charge is not lost, the impacts are only the more violent.
 +Still if the frequency of the impulses be very small, the loss caused
 +by the impacts and collisions would not be serious unless the
 +potential were excessive. But when extremely high frequencies and more
 +or less high potentials are used, the loss may be very great. The
 +total energy lost per unit of time is proportionate
 +<!-- Page 115 -->
 +to the product of the number of impacts per second, or the frequency and the
 +energy lost in each impact. But the energy of an impact must be proportionate
 +to the square of the electric density of the sphere, since the charge
 +imparted to the molecule is proportionate to that density. I conclude
 +from this that the total energy lost must be proportionate to the
 +product of the frequency and the square of the electric density; but
 +this law needs experimental confirmation. Assuming the preceding
 +considerations to be true, then, by rapidly alternating the potential
 +of a body immersed in an insulating gaseous medium, any amount of
 +energy may be dissipated into space. Most of that energy then, I
 +believe, is not dissipated in the form of long ether waves, propagated
 +to considerable distance, as is thought most generally, but is
 +consumed&​mdash;​in the case of an insulated sphere, for example&​mdash;​in impact
 +and collisional losses&​mdash;​that is, heat vibrations&​mdash;​on the surface and
 +in the vicinity of the sphere. To reduce the dissipation it is
 +necessary to work with a small electric density&​mdash;​the smaller the
 +higher the frequency.</​p>​
 +<​p>​But since, on the assumption before made, the loss is diminished with
 +the square of the density, and since currents of very high frequencies
 +involve considerable waste when transmitted through conductors, it
 +follows that, on the whole, it is better to employ one wire than two.
 +Therefore, if motors, lamps, or devices of any kind are perfected,
 +capable of being advantageously operated by currents of extremely high
 +frequency, economical reasons will make it advisable to use only one
 +wire, especially if the distances are great. </p>
 +<!-- Page 116 -->
 +<​p>​When energy is absorbed in a condenser the same behaves as though its
 +capacity were increased. Absorption always exists more or less, but
 +generally it is small and of no consequence as long as the frequencies
 +are not very great. In using extremely high frequencies,​ and,
 +necessarily in such case, also high potentials, the absorption&​mdash;​or,​
 +what is here meant more particularly by this term, the loss of energy
 +due to the presence of a gaseous medium&​mdash;​is an important factor to be
 +considered, as the energy absorbed in the air condenser may be any
 +fraction of the supplied energy. This would seem to make it very
 +difficult to tell from the measured or computed capacity of an air
 +condenser its actual capacity or vibration period, especially if the
 +condenser is of very small surface and is charged to a very high
 +potential. As many important results are dependent upon the
 +correctness of the estimation of the vibration period, this subject
 +demands the most careful scrutiny of other investigators. To reduce
 +the probable error as much as possible in experiments of the kind
 +alluded to, it is advisable to use spheres or plates of large surface,
 +so as to make the density exceedingly small. Otherwise, when it is
 +practicable,​ an oil condenser should be used in preference. In oil or
 +other liquid dielectrics there are seemingly no such losses as in
 +gaseous media. It being impossible to exclude entirely the gas in
 +condensers with solid dielectrics,​ such condensers should be immersed
 +in oil, for economical reasons if nothing else; they can then be
 +strained to the utmost and will remain cool. In Leyden jars the loss
 +due to air is comparatively small, as the tinfoil coatings are large,
 +close together, and the charged
 +<!-- Page 117 -->
 +surfaces not directly exposed; but when the potentials are very high,
 +the loss may be more or less considerable at, or near, the upper edge
 +of the foil, where the air is principally acted upon. If the jar be immersed
 +in boiled-out oil, it will be capable of performing four times the amount
 +of work which it can for any length of time when used in the ordinary way,
 +and the loss will be inappreciable.</​p>​
 +<p>It should not be thought that the loss in heat in an air condenser is
 +necessarily associated with the formation of <​i>​visible</​i>​ streams or
 +brushes. If a small electrode, inclosed in an unexhausted bulb, is
 +connected to one of the terminals of the coil, streams can be seen to
 +issue from the electrode and the air in the bulb is heated; if,
 +instead of a small electrode, a large sphere is inclosed in the bulb,
 +no streams are observed, still the air is heated.</​p>​
 +<​p>​Nor should it be thought that the temperature of an air condenser
 +would give even an approximate idea of the loss in heat incurred, as
 +in such case heat must be given off much more quickly, since there is,
 +in addition to the ordinary radiation, a very active carrying away of
 +heat by independent carriers going on, and since not only the
 +apparatus, but the air at some distance from it is heated in
 +consequence of the collisions which must occur.</​p>​
 +<​p>​Owing to this, in experiments with such a coil, a rise of temperature
 +can be distinctly observed only when the body connected to the coil is
 +very small. But with apparatus on a larger scale, even a body of
 +considerable bulk would be heated, as, for instance, the body of a
 +person; and I think that skilled physicians might make observations of
 +utility in such experiments,​ which, if the apparatus were
 +<!-- Page 118 -->
 +judiciously designed, would not present the slightest danger.</​p>​
 +<p>A question of some interest, principally to meteorologists,​ presents
 +itself here. How does the earth behave? The earth is an air condenser,
 +but is it a perfect or a very imperfect one&​mdash;​a mere sink of energy?
 +There can be little doubt that to such small disturbance as might be
 +caused in an experiment the earth behaves as an almost perfect
 +condenser. But it might be different when its charge is set in
 +vibration by some sudden disturbance occurring in the heavens. In such
 +case, as before stated, probably only little of the energy of the
 +vibrations set up would be lost into space in the form of long ether
 +radiations, but most of the energy, I think, would spend itself in
 +molecular impacts and collisions, and pass off into space in the form
 +of short heat, and possibly light, waves. As both the frequency of the
 +vibrations of the charge and the potential are in all probability
 +excessive, the energy converted into heat may be considerable. Since
 +the density must be unevenly distributed,​ either in consequence of the
 +irregularity of the earth'​s surface, or on account of the condition of
 +the atmosphere in various places, the effect produced would
 +accordingly vary from place to place. Considerable variations in the
 +temperature and pressure of the atmosphere may in this manner be
 +caused at any point of the surface of the earth. The variations may be
 +gradual or very sudden, according to the nature of the general
 +disturbance,​ and may produce rain and storms, or locally modify the
 +weather in any way.</​p>​
 +<​p>​From the remarks before made one may see what an important
 +<!-- Page 119 -->
 +factor of loss the air in the neighborhood of a charged surface becomes when
 +the electric density is great and the frequency of the impulses excessive.
 +But the action as explained implies that the air is insulating&​mdash;​that
 +is, that it is composed of independent carriers immersed in an
 +insulating medium. This is the case only when the air is at something
 +like ordinary or greater, or at extremely small, pressure. When the
 +air is slightly rarefied and conducting, then true conduction losses
 +occur also. In such case, of course, considerable energy may be
 +dissipated into space even with a steady potential, or with impulses
 +of low frequency, if the density is very great.</​p>​
 +<​p>​When the gas is at very low pressure, an electrode is heated more
 +because higher speeds can be reached. If the gas around the electrode
 +is strongly compressed, the displacements,​ and consequently the
 +speeds, are very small, and the heating is insignificant. But if in
 +such case the frequency could be sufficiently increased, the electrode
 +would be brought to a high temperature as well as if the gas were at
 +very low pressure; in fact, exhausting the bulb is only necessary
 +because we cannot produce (and possibly not convey) currents of the
 +required frequency.</​p>​
 +<​p>​Returning to the subject of electrode lamps, it is obviously of
 +advantage in such a lamp to confine as much as possible the heat to
 +the electrode by preventing the circulation of the gas in the bulb. If
 +a very small bulb be taken, it would confine the heat better than a
 +large one, but it might not be of sufficient capacity to be operated
 +from the coil, or, if so, the glass might get too hot. A simple way to
 +improve in this direction is to employ a globe of the required
 +<!-- Page 120 -->
 +size, but to place a small bulb, the diameter of which is properly
 +estimated, over the refractory button contained in the globe. This
 +arrangement is illustrated in Fig. 28.</​p>​
 +<img src="​images/​acfig28.gif"​ width="​490"​ height="​565"​ border="​0"​ align="​left"​ hspace="​10"​
 +<​p>​The globe <​i>​L</​i>​ has in this case a large neck <​i>​n</​i>,​ allowing the small
 +bulb <​i>​b</​i>​ to slip through. Otherwise the construction is the same as
 +shown in Fig. 18, for example. The small bulb is conveniently
 +supported upon the stem <​i>​s</​i>,​ carrying
 +<!-- Page 121 -->
 +the refractory button <​i>​m</​i>​. It is separated from the aluminium tube <​i>​a</​i>​
 +by several layers of mica <​i>​M</​i>,​ in order to prevent the cracking of the neck by the
 +rapid heating of the aluminium tube upon a sudden turning on of the current. The
 +inside bulb should be as small as possible when it is desired to
 +obtain light only by incandescence of the electrode. If it is desired
 +to produce phosphorescence,​ the bulb should be larger, else it would
 +be apt to get too hot, and the phosphorescence would cease. In this
 +arrangement usually only the small bulb shows phosphorescence,​ as
 +there is practically no bombardment against the outer globe. In some
 +of these bulbs constructed as illustrated in Fig. 28 the small tube
 +was coated with phosphorescent paint, and beautiful effects were
 +obtained. Instead of making the inside bulb large, in order to avoid
 +undue heating, it answers the purpose to make the electrode <​i>​m</​i>​
 +larger. In this case the bombardment is weakened by reason of the
 +smaller electric density.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig29.gif"​ width="​503"​ height="​563"​ border="​0"​ align="​left"​ hspace="​10"​
 +<​p>​Many bulbs were constructed on the plan illustrated in Fig. 29. Here a
 +small bulb <​i>​b</​i>,​ containing the refractory button <​i>​m</​i>,​ upon being
 +exhausted to a very high degree was sealed in a large globe <​i>​L</​i>,​ which
 +was then moderately exhausted and sealed off. The principal advantage
 +of this construction was that it allowed of reaching extremely high vacua, and,
 +at the same time use a large bulb. It was found, in the course of experiences
 +with bulbs such as illustrated in Fig. 29, that it was well to make the stem
 +<​i>​s</​i>​ near the seal at <​i>​e</​i>​ very thick, and the leading-in wire <​i>​w</​i>​ thin,
 +as it occurred sometimes that the stem at <​i>​e</​i>​ was heated and the bulb
 +was cracked. Often the outer globe <​i>​L</​i>​ was exhausted
 +<!-- Page 122 -->
 +only just enough to allow the discharge to pass through, and the space
 +between the bulbs appeared crimson, producing a curious effect.
 +In some cases, when the exhaustion in globe <​i>​L</​i>​ was
 +very low, and the air good conducting, it was found necessary, in
 +order to bring the button <​i>​m</​i>​ to high incandescence,​ to place,
 +preferably on the upper part of the neck of the globe, a tinfoil
 +coating which was connected to an insulated body, to the ground, or to
 +the other terminal of the coil, as the highly conducting air weakened
 +<!-- Page 123 -->
 +the effect somewhat, probably by being acted upon inductively from
 +the wire <​i>​w</​i>,​ where it entered the bulb at <​i>​e</​i>​. Another
 +difficulty&​mdash;​which,​ however, is always present when the refractory
 +button is mounted in a very small bulb&​mdash;​existed in the construction
 +illustrated in Fig. 29, namely, the vacuum in the bulb <​i>​b</​i>​ would be
 +impaired in a comparatively short time.</​p>​
 +<br clear="​all">​
 +<​p>​The chief idea in the two last described constructions was to confine
 +the heat to the central portion of the globe by preventing the
 +exchange of air. An advantage is secured, but owing to the heating of
 +the inside bulb and slow evaporation of the glass the vacuum is hard
 +to maintain, even if the construction illustrated in Fig. 28 be
 +chosen, in which both bulbs communicate.</​p>​
 +<​p>​But by far the better way&​mdash;​the ideal way&​mdash;​would be to reach
 +sufficiently high frequencies. The higher the frequency the slower
 +would be the exchange of the air, and I think that a frequency may be
 +reached at which there would be no exchange whatever of the air
 +molecules around the terminal. We would then produce a flame in which
 +there would be no carrying away of material, and a queer flame it
 +would be, for it would be rigid! With such high frequencies the
 +inertia of the particles would come into play. As the brush, or flame,
 +would gain rigidity in virtue of the inertia of the particles, the
 +exchange of the latter would be prevented. This would necessarily
 +occur, for, the number of the impulses being augmented, the potential
 +energy of each would diminish, so that finally only atomic vibrations
 +could be set up, and the motion of translation through measurable
 +space would cease. Thus an ordinary gas burner
 +<!-- Page 124 -->
 +connected to a source of rapidly alternating potential might have its efficiency
 +augmented to a certain limit, and this for two reasons&​mdash;​because of the
 +additional vibration imparted, and because of a slowing down of the
 +process of carrying off. But the renewal being rendered difficult, and
 +renewal being necessary to maintain the <​i>​burner</​i>,​ a continued increase
 +of the frequency of the impulses, assuming they could be transmitted
 +to and impressed upon the flame, would result in the &​quot;​extinction&​quot;​ of
 +the latter, meaning by this term only the cessation of the chemical
 +<p>I think, however, that in the case of an electrode immersed in a fluid
 +insulating medium, and surrounded by independent carriers of electric
 +charges, which can be acted upon inductively,​ a sufficiently high
 +frequency of the impulses would probably result in a gravitation of
 +the gas all around toward the electrode. For this it would be only
 +necessary to assume that the independent bodies are irregularly
 +shaped; they would then turn toward the electrode their side of the
 +greatest electric density, and this would be a position in which the
 +fluid resistance to approach would be smaller than that offered to the
 +<​p>​The general opinion, I do not doubt, is that it is out of the question
 +to reach any such frequencies as might&​mdash;​assuming some of the views
 +before expressed to be true&​mdash;​produce any of the results which I have
 +pointed out as mere possibilities. This may be so, but in the course
 +of these investigations,​ from the observation of many phenomena I have
 +gained the conviction that these frequencies would be much lower than
 +one is apt to estimate at first. In a flame we set up light vibrations
 +by causing molecules, or atoms, to collide.
 +<!-- Page 125 -->
 +But what is the ratio of the frequency of the collisions and that of the vibrations set up?
 +Certainly it must be incomparably smaller than that of the knocks of
 +the bell and the sound vibrations, or that of the discharges and the
 +oscillations of the condenser. We may cause the molecules of the gas
 +to collide by the use of alternate electric impulses of high
 +frequency, and so we may imitate the process in a flame; and from
 +experiments with frequencies which we are now able to obtain, I think
 +that the result is producible with impulses which are transmissible
 +through a conductor.</​p>​
 +<p>In connection with thoughts of a similar nature, it appeared to me of
 +great interest to demonstrate the rigidity of a vibrating gaseous
 +column. Although with such low frequencies as, say 10,000 per second,
 +which I was able to obtain without difficulty from a specially
 +constructed alternator, the task looked discouraging at first, I made
 +a series of experiments. The trials with air at ordinary pressure led
 +to no result, but with air moderately rarefied I obtain what I think
 +to be an unmistakable experimental evidence of the property sought
 +for. As a result of this kind might lead able investigators to
 +conclusions of importance I will describe one of the experiments
 +<p>It is well known that when a tube is slightly exhausted the discharge
 +may be passed through it in the form of a thin luminous thread. When
 +produced with currents of low frequency, obtained from a coil operated
 +as usual, this thread is inert. If a magnet be approached to it, the
 +part near the same is attracted or repelled, according to the
 +direction of the lines of force of the magnet. It occurred to
 +<!-- Page 126 -->
 +me that if such a thread would be produced with currents of very high
 +frequency, it should be more or less rigid, and as it was visible it
 +could be easily studied. Accordingly I prepared a tube about 1 inch in
 +diameter and 1 metre long, with outside coating at each end. The tube
 +was exhausted to a point at which by a little working the thread
 +discharge could be obtained. It must be remarked here that the general
 +aspect of the tube, and the degree of exhaustion, are quite different
 +than when ordinary low frequency currents are used. As it was found
 +preferable to work with one terminal, the tube prepared was suspended
 +from the end of a wire connected to the terminal, the tinfoil coating
 +being connected to the wire, and to the lower coating sometimes a
 +small insulated plate was attached. When the thread was formed it
 +extended through the upper part of the tube and lost itself in the
 +lower end. If it possessed rigidity it resembled, not exactly an
 +elastic cord stretched tight between two supports, but a cord
 +suspended from a height with a small weight attached at the end. When
 +the finger or a magnet was approached to the upper end of the luminous
 +thread, it could be brought locally out of position by electrostatic
 +or magnetic action; and when the disturbing object was very quickly
 +removed, an analogous result was produced, as though a suspended cord
 +would be displaced and quickly released near the point of suspension.
 +In doing this the luminous thread was set in vibration, and two very
 +sharply marked nodes, and a third indistinct one, were formed. The
 +vibration, once set up, continued for fully eight minutes, dying
 +gradually out. The speed of the vibration
 +<!-- Page 127 -->
 +often varied perceptibly,​ and it could be observed that the electrostatic attraction
 +of the glass affected the vibrating thread; but it was clear that the
 +electrostatic action was not the cause of the vibration, for the
 +thread was most generally stationary, and could always be set in
 +vibration by passing the finger quickly near the upper part of the
 +tube. With a magnet the thread could be split in two and both parts
 +vibrated. By approaching the hand to the lower coating of the tube, or
 +insulated plate if attached, the vibration was quickened; also, as far
 +as I could see, by raising the potential or frequency. Thus, either
 +increasing the frequency or passing a stronger discharge of the same
 +frequency corresponded to a tightening of the cord. I did not obtain
 +any experimental evidence with condenser discharges. A luminous band
 +excited in a bulb by repeated discharges of a Leyden jar must possess
 +rigidity, and if deformed and suddenly released should vibrate. But
 +probably the amount of vibrating matter is so small that in spite of
 +the extreme speed the inertia cannot prominently assert itself.
 +Besides, the observation in such a case is rendered extremely
 +difficult on account of the fundamental vibration.</​p>​
 +<​p>​The demonstration of the fact&​mdash;​which still needs better experimental
 +confirmation&​mdash;​that a vibrating gaseous column possesses rigidity,
 +might greatly modify the views of thinkers. When with low frequencies
 +and insignificant potentials indications of that property may be
 +noted, how must a gaseous medium behave under the influence of
 +enormous electrostatic stresses which may be active in the
 +interstellar space, and which may alternate with inconceivable
 +<!-- Page 128 -->
 +rapidity? The existence of such an electrostatic,​ rhythmically
 +throbbing force&​mdash;​of a vibrating electrostatic field&​mdash;​would show a
 +possible way how solids might have formed from the ultra-gaseous
 +uterus, and how transverse and all kinds of vibrations may be
 +transmitted through a gaseous medium filling all space. Then, ether
 +might be a true fluid, devoid of rigidity, and at rest, it being
 +merely necessary as a connecting link to enable interaction. What
 +determines the rigidity of a body? It must be the speed and the amount
 +of moving matter. In a gas the speed may be considerable,​ but the
 +density is exceedingly small; in a liquid the speed would be likely to
 +be small, though the density may be considerable;​ and in both cases
 +the inertia resistance offered to displacement is practically <​i>​nil</​i>​.
 +But place a gaseous (or liquid) column in an intense, rapidly
 +alternating electrostatic field, set the particles vibrating with
 +enormous speeds, then the inertia resistance asserts itself. A body
 +might move with more or less freedom through the vibrating mass, but
 +as a whole it would be rigid.</​p>​
 +<​p>​There is a subject which I must mention in connection with these
 +experiments:​ it is that of high vacua. This is a subject the study of
 +which is not only interesting,​ but useful, for it may lead to results
 +of great practical importance. In commercial apparatus, such as
 +incandescent lamps, operated from ordinary systems of distribution,​ a
 +much higher vacuum than obtained at present would not secure a very
 +great advantage. In such a case the work is performed on the filament
 +and the gas is little concerned; the improvement,​ therefore, would be but trifling.
 +But when we begin to use very high frequencies and potentials, the action
 +<!-- Page 129 -->
 +of the gas becomes all important, and the degree of exhaustion materially modifies
 +the results. As long as ordinary coils, even very large ones, were used, the study of
 +the subject was limited, because just at a point when it became most
 +interesting it had to be interrupted on account of the &​quot;​non-striking&​quot;​
 +vacuum being reached. But presently we are able to obtain from a small
 +disruptive discharge coil potentials much higher than even the largest
 +coil was capable of giving, and, what is more, we can make the
 +potential alternate with great rapidity. Both of these results enable
 +us now to pass a luminous discharge through almost any vacua
 +obtainable, and the field of our investigations is greatly extended.
 +Think we as we may, of all the possible directions to develop a
 +practical illuminant, the line of high vacua seems to be the most
 +promising at present. But to reach extreme vacua the appliances must
 +be much more improved, and ultimate perfection will not be attained
 +until we shall have discarded the mechanical and perfected an
 +<​i>​electrical</​i>​ vacuum pump. Molecules and atoms can be thrown out of a
 +bulb under the action of an enormous potential: <​i>​this</​i>​ will be the
 +principle of the vacuum pump of the future. For the present, we must
 +secure the best results we can with mechanical appliances. In this
 +respect, it might not be out of the way to say a few words about the
 +method of, and apparatus for, producing excessively high degrees of
 +exhaustion of which I have availed myself in the course of these
 +investigations. It is very probable that other experimenters have used
 +similar arrangements;​ but as it is possible that there may be an item
 +of interest in their description,​ a few remarks, which
 +<!-- Page 130 -->
 +will render this investigation more complete, might be permitted.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig30.gif" ​ width="​495"​ height="​566"​ border="​0"​
 +<​p>​The apparatus is illustrated in a drawing shown in Fig. 30. <​i>​S</​i>​
 +represents a Sprengel pump, which has been specially constructed to
 +better suit the work required. The stop-cock which is usually employed has been
 +omitted, and instead of it a hollow stopper <​i>​s</​i>​ has been fitted in the neck
 +<!-- Page 131 -->
 +of the reservoir <​i>​R</​i>​. This stopper has a small hole <​i>​h</​i>,​ through which
 +the mercury descends; the size of the outlet <​i>​o</​i>​ being properly determined
 +with respect to the section of the fall tube <​i>​t</​i>,​ which is sealed to the reservoir
 +instead of being connected to it in the usual manner. This arrangement overcomes
 +the imperfections and troubles which often arise from the use of the
 +stopcock on the reservoir and the connection of the latter with the fall tube.</​p>​
 +<​p>​The pump is connected through a U-shaped tube <​i>​t</​i>​ to a very large
 +reservoir <​i>​R</​i><​sub>​1</​sub>​. Especial care was taken in fitting the grinding
 +surfaces of the stoppers <​i>​p</​i>​ and <​i>​p</​i><​sub>​1</​sub>,​ and both of these and the
 +mercury caps above them were made exceptionally long. After the
 +U-shaped tube was fitted and put in place, it was heated, so as to
 +soften and take off the strain resulting from imperfect fitting. The
 +U-shaped tube was provided with a stopcock <​i>​C</​i>,​ and two ground
 +connections <​i>​g</​i>​ and <​i>​g</​i><​sub>​1</​sub>&​mdash;​one for a small bulb <​i>​b</​i>,​
 +usually containing caustic potash, and the other for the receiver <​i>​r</​i>,​ to be
 +<​p>​The reservoir <​i>​R</​i><​sub>​1</​sub>​ was connected by means of a rubber tube to a
 +slightly larger reservoir <​i>​R</​i><​sub>​2</​sub>,​ each of the two reservoirs being
 +provided with a stopcock <​i>​C</​i><​sub>​1</​sub>​ and <​i>​C</​i><​sub>​2</​sub>,​ respectively.
 +The reservoir <​i>​R</​i><​sub>​1</​sub>​ could be raised and lowered by a wheel and rack,
 +and the range of its motion was so determined that when it was filled with mercury
 +and the stopcock <​i>​C</​i><​sub>​2</​sub>​ closed, so as to form a Torricellian vacuum in
 +it when raised, it could be lifted so high that the mercury in reservoir <​i>​R</​i><​sub>​1</​sub>​
 +would stand a little above stopcock <​i>​C</​i><​sub>​1</​sub>;​ and when this stopcock was
 +closed and the reservoir <​i>​R</​i><​sub>​2</​sub>​ descended, so as to form a Torricellian vacuum in
 +<!-- Page 132 -->
 +reservoir <​i>​R</​i><​sub>​1</​sub>,​ it could be lowered so far as to
 +completely empty the latter, the mercury filling the reservoir <​i>​R</​i><​sub>​2</​sub>​
 +up to a little above stopcock <​i>​C</​i><​sub>​2</​sub>​.</​p>​
 +<​p>​The capacity of the pump and of the connections was taken as small as
 +possible relatively to the volume of reservoir <​i>​R</​i><​sub>​1</​sub>,​ since, of course,
 +the degree of exhaustion depended upon the ratio of these quantities.</​p>​
 +<​p>​With this apparatus I combined the usual means indicated by former
 +experiments for the production of very high vacua. In most of the
 +experiments it was convenient to use caustic potash. I may venture to
 +say, in regard to its use, that much time is saved and a more perfect
 +action of the pump insured by fusing and boiling the potash as soon
 +as, or even before, the pump settles down. If this course is not
 +followed the sticks, as ordinarily employed, may give moisture off at
 +a certain very slow rate, and the pump may work for many hours without
 +reaching a very high vacuum. The potash was heated either by a spirit
 +lamp or by passing a discharge through it, or by passing a current
 +through a wire contained in it. The advantage in the latter case was
 +that the heating could be more rapidly repeated.</​p>​
 +<​p>​Generally the process of exhaustion was the following:&​mdash;​At the start,
 +the stop-cocks <​i>​C</​i>​ and <​i>​C</​i><​sub>​1</​sub>​ being open, and all other connections
 +closed, the reservoir <​i>​R</​i><​sub>​2</​sub>​ was raised so far that the mercury filled the
 +reservoir <​i>​R</​i><​sub>​1</​sub>​ and a part of the narrow connecting U-shaped tube. When
 +the pump was set to work, the mercury would, of course, quickly rise in the tube, and
 +reservoir <​i>​R</​i><​sub>​2</​sub>​ was lowered, the experimenter keeping the mercury
 +at about the same level.
 +<!-- Page 133 -->
 +The reservoir <​i>​R</​i><​sub>​2</​sub>​ was balanced by a long spring which facilitated
 +the operation, and the friction of the parts was generally sufficient to keep it almost in any position.
 +When the Sprengel pump had done its work, the reservoir <​i>​R</​i><​sub>​2</​sub>​ was
 +further lowered and the mercury descended in <​i>​R</​i><​sub>​1</​sub>​ and filled <​i>​R</​i><​sub>​2</​sub>,​
 +whereupon stopcock <​i>​C</​i><​sub>​2</​sub>​ was closed. The air adhering to the walls of
 +<​i>​R</​i><​sub>​1</​sub>​ and that absorbed by the mercury was carried off, and to free the
 +mercury of all air the reservoir <​i>​R</​i><​sub>​2</​sub> ​ was for a long time worked up and
 +down. During this process some air, which would gather below stopcock
 +<​i>​C</​i><​sub>​2</​sub>,​ was expelled from <​i>​R</​i><​sub>​2</​sub>​ by lowering it far enough and
 +opening the stopcock, closing the latter again before raising the reservoir. When
 +all the air had been expelled from the mercury, and no air would
 +gather in <​i>​R</​i><​sub>​2</​sub>​ when it was lowered, the caustic potash was resorted to.
 +The reservoir <​i>​R</​i><​sub>​2</​sub>​ was now again raised until the mercury in
 +<​i>​R</​i><​sub>​1</​sub>​ stood above stopcock <​i>​C</​i><​sub>​1</​sub>​. The caustic potash
 +was fused and boiled, and the moisture partly carried off by the pump and partly re-absorbed;​
 +and this process of heating and cooling was repeated many times, and each
 +time, upon the moisture being absorbed or carried off, the reservoir
 +<​i>​R</​i><​sub>​2</​sub>​ was for a long time raised and lowered. In this manner all the
 +moisture was carried off from the mercury, and both the reservoirs
 +were in proper condition to be used. The reservoir <​i>​R</​i><​sub>​2</​sub>​ was then again
 +raised to the top, and the pump was kept working for a long time. When
 +the highest vacuum obtainable with the pump had been reached the
 +potash bulb was usually wrapped with cotton which was sprinkled with
 +ether so as to keep the potash at a very low temperature,​ then the
 +reservoir <​i>​R</​i><​sub>​2</​sub>​ was lowered, and
 +<!-- Page 134 -->
 +upon reservoir <​i>​R</​i><​sub>​1</​sub>​ being emptied the receiver <​i>​r</​i>​ was
 +quickly sealed up.</​p>​
 +<​p>​When a new bulb was put on, the mercury was always raised above
 +stopcock <​i>​C</​i><​sub>​1</​sub>​ which was closed, so as to always keep the mercury and
 +both the reservoirs in fine condition, and the mercury was never
 +withdrawn from <​i>​R</​i><​sub>​1</​sub>​ except when the pump had reached the highest
 +degree of exhaustion. It is necessary to observe this rule if it is
 +desired to use the apparatus to advantage.</​p>​
 +<p>By means of this arrangement I was able to proceed very quickly, and
 +when the apparatus was in perfect order it was possible to reach the
 +phosphorescent stage in a small bulb in less than 15 minutes, which is
 +certainly very quick work for a small laboratory arrangement requiring
 +all in all about 100 pounds of mercury. With ordinary small bulbs the
 +ratio of the capacity of the pump, receiver, and connections,​ and that
 +of reservoir <​i>​R</​i>​ was about 1-20, and the degrees of exhaustion reached
 +were necessarily very high, though I am unable to make a precise and
 +reliable statement how far the exhaustion was carried.</​p>​
 +<​p>​What impresses the investigator most in the course of these
 +experiences is the behavior of gases when subjected to great rapidly
 +alternating electrostatic stresses. But he must remain in doubt as to
 +whether the effects observed are due wholly to the molecules, or atoms,
 +of the gas which chemical analysis discloses to us, or whether there enters
 +into play another medium of a gaseous nature, comprising atoms, or molecules,
 +immersed in a fluid pervading the space. Such a medium surely must exist,
 +and I am convinced that, for instance, even if air were absent, the surface
 +<!-- Page 135 -->
 +and neighborhood of a body in space would be heated by rapidly alternating the
 +potential of the body; but no such heating of the surface or neighborhood could occur
 +if all free atoms were removed and only a homogeneous,​ incompressible,​ and elastic
 +fluid&​mdash;​such as ether is supposed to be&​mdash;​would remain, for then there
 +would be no impacts, no collisions. In such a case, as far as the body
 +itself is concerned, only frictional losses in the inside could occur.</​p>​
 +<p>It is a striking fact that the discharge through a gas is established
 +with ever increasing freedom as the frequency of the impulses is
 +augmented. It behaves in this respect quite contrarily to a metallic
 +conductor. In the latter the impedance enters prominently into play as
 +the frequency is increased, but the gas acts much as a series of
 +condensers would: the facility with which the discharge passes through
 +seems to depend on the rate of change of potential. If it act so, then
 +in a vacuum tube even of great length, and no matter how strong the
 +current, self-induction could not assert itself to any appreciable
 +degree. We have, then, as far as we can now see, in the gas a
 +conductor which is capable of transmitting electric impulses of any
 +frequency which we may be able to produce. Could the frequency be
 +brought high enough, then a queer system of electric distribution,​
 +which would be likely to interest gas companies, might be realized:
 +metal pipes filled with gas&​mdash;​the metal being the insulator, the gas
 +the conductor&​mdash;​supplying phosphorescent bulbs, or perhaps devices as
 +yet uninvented. It is certainly possible to take a hollow core of
 +copper, rarefy the gas in the same, and by passing impulses of
 +sufficiently high frequency through a circuit around it, bring the gas inside to
 +<!-- Page 136 -->
 +a high degree of incandescence;​ but as to the nature of the
 +forces there would be considerable uncertainty,​ for it would be
 +doubtful whether with such impulses the copper core would act as a
 +static screen. Such paradoxes and apparent impossibilities we
 +encounter at every step in this line of work, and therein lies, to a
 +great extent, the claim of the study.</​p>​
 +<p>I have here a short and wide tube which is exhausted to a high degree
 +and covered with a substantial coating of bronze, the coating allowing
 +barely the light to shine through. A metallic clasp, with a hook for
 +suspending the tube, is fastened around the middle portion of the
 +latter, the clasp being in contact with the bronze coating. I now want
 +to light the gas inside by suspending the tube on a wire connected to
 +the coil. Any one who would try the experiment for the first time, not
 +having any previous experience, would probably take care to be quite
 +alone when making the trial, for fear that he might become the joke of
 +his assistants. Still, the bulb lights in spite of the metal coating,
 +and the light can be distinctly perceived through the latter. A long
 +tube covered with aluminium bronze lights when held in one hand&​mdash;​the
 +other touching the terminal of the coil&​mdash;​quite powerfully. It might be
 +objected that the coatings are not sufficiently conducting; still,
 +even if they were highly resistant, they ought to screen the gas. They
 +certainly screen it perfectly in a condition of rest, but not by far
 +perfectly when the charge is surging in the coating. But the loss of
 +energy which occurs within the tube, notwithstanding the screen,
 +is occasioned principally by the presence of the gas. Were
 +<!-- Page 137 -->
 +we to take a large hollow metallic sphere and fill it with a perfect incompressible
 +fluid dielectric, there would be no loss inside of the sphere, and
 +consequently the inside might be considered as perfectly screened,
 +though the potential be very rapidly alternating. Even were the sphere
 +filled with oil, the loss would be incomparably smaller than when the
 +fluid is replaced by a gas, for in the latter case the force produces
 +displacements;​ that means impact and collisions in the inside.</​p>​
 +<p>No matter what the pressure of the gas may be, it becomes an important
 +factor in the heating of a conductor when the electric density is
 +great and the frequency very high. That in the heating of conductors
 +by lightning discharges air is an element of great importance, is
 +almost as certain as an experimental fact. I may illustrate the action
 +of the air by the following experiment: I take a short tube which is
 +exhausted to a moderate degree and has a platinum wire running through
 +the middle from one end to the other. I pass a steady or low frequency
 +current through the wire, and it is heated uniformly in all parts. The
 +heating here is due to conduction, or frictional losses, and the gas
 +around the wire has&​mdash;​as far as we can see&​mdash;​no function to perform.
 +But now let me pass sudden discharges, or a high frequency current,
 +through the wire. Again the wire is heated, this time principally on
 +the ends and least in the middle portion; and if the frequency of the
 +impulses, or the rate of change, is high enough, the wire might as
 +well be cut in the middle as not, for practically all the heating is due to the
 +rarefied gas. Here the gas might only act as a conductor of no impedance
 +<!-- Page 138 -->
 +diverting the current from the wire as the impedance of the latter is
 +enormously increased, and merely heating the ends of the
 +wire by reason of their resistance to the passage of the discharge.
 +But it is not at all necessary that the gas in the tube should he
 +conducting; it might be at an extremely low pressure, still the ends
 +of the wire would be heated&​mdash;​as,​ however, is ascertained by
 +experience&​mdash;​only the two ends would in such, case not be electrically
 +connected through the gaseous medium. Now what with these frequencies
 +and potentials occurs in an exhausted tube occurs in the lightning
 +discharges at ordinary pressure. We only need remember one of the
 +facts arrived at in the course of these investigations,​ namely, that
 +to impulses of very high frequency the gas at ordinary pressure
 +behaves much in the same manner as though it were at moderately low
 +pressure. I think that in lightning discharges frequently wires or
 +conducting objects are volatilized merely because air is present and
 +that, were the conductor immersed in an insulating liquid, it would be
 +safe, for then the energy would have to spend itself somewhere else.
 +From the behavior of gases to sudden impulses of high potential I am
 +led to conclude that there can be no surer way of diverting a
 +lightning discharge than by affording it a passage through a volume of
 +gas, if such a thing can be done in a practical manner.</​p>​
 +<​p>​There are two more features upon which I think it necessary to dwell
 +in connection with these experiments&​mdash;​the &​quot;​radiant state&​quot;​ and the
 +&​quot;​non-striking vacuum.&​quot;</​p>​
 +<​p>​Any one who has studied Crookes'​ work must have received the
 +impression that the &​quot;​radiant state&​quot;​ is a property
 +<!-- Page 139 -->
 +of the gas inseparably connected with an extremely high degree of exhaustion.
 +But it should be remembered that the phenomena observed in an exhausted
 +vessel are limited to the character and capacity of the apparatus
 +which is made use of. I think that in a bulb a molecule, or atom, does
 +not precisely move in a straight line because it meets no obstacle,
 +but because the velocity imparted to it is sufficient to propel it in
 +a sensibly straight line. The mean free path is one thing, but the
 +velocity&​mdash;​the energy associated
 +<!-- Page 140 -->
 +with the moving body&​mdash;​is another, and under ordinary circumstances I believe
 +that it is a mere question of potential or speed. A disruptive discharge coil, when the
 +potential is pushed very far, excites phosphorescence and projects shadows, at
 +comparatively low degrees of exhaustion. In a lightning discharge,
 +matter moves in straight lines as ordinary pressure when the mean free
 +path is exceedingly small, and frequently images of wires or other
 +metallic objects have been produced by the particles thrown off in straight lines.</​p>​
 +<div align="​center">​
 +<img src="​images/​acfig31.gif"​ width="​492"​ height="​526"​ border="​0"​
 +<p>I have prepared a bulb to illustrate by an experiment the correctness
 +of these assertions. In a globe <​i>​L</​i>​ (Fig. 31) I have mounted upon a
 +lamp filament <​i>​f</​i>​ a piece of lime <​i>​l</​i>​. The lamp filament is connected
 +with a wire which leads into the bulb, and the general construction of
 +the latter is as indicated in Fig. 19, before described. The bulb
 +being suspended from a wire connected to the terminal of the coil, and
 +the latter being set to work, the lime piece <​i>​l</​i>​ and the projecting
 +parts of the filament <​i>​f</​i>​ are bombarded. The degree of exhaustion is
 +just such that with the potential the coil is capable of giving phosphorescence
 +of the glass is produced, but disappears as soon as the vacuum is impaired.
 +The lime containing moisture, and moisture being given off as soon as heating
 +occurs, the phosphorescence lasts only for a few moments. When the lime
 +has been sufficiently heated, enough moisture has been given off to impair
 +materially the vacuum of the bulb. As the bombardment goes on, one point
 +of the lime piece is more heated than other points, and the result is that finally
 +practically all the discharge passes through
 +<!-- Page 141 -->
 +that point which is intensely heated, and a white stream of lime particles (Fig. 31)
 +then breaks forth from that point. This stream is composed of &​quot;​radiant&​quot;​
 +matter, yet the degree of exhaustion is low. But the particles move in
 +straight lines because the velocity imparted to them is great, and
 +this is due to three causes&​mdash;​to the great electric density, the high
 +temperature of the small point, and the fact that the particles of the
 +lime are easily torn and thrown off&​mdash;​far more easily than those of
 +carbon. With frequencies such as we are able to obtain, the particles
 +are bodily thrown off and projected to a considerable distance; but
 +with sufficiently high frequencies no such thing would occur: in such
 +case only a stress would spread or a vibration would be propagated
 +through the bulb. It would be out of the question to reach any such
 +frequency on the assumption that the atoms move with the speed of
 +light; but I believe that such a thing is impossible; for this an
 +enormous potential would be required. With potentials which we are
 +able to obtain, even with a disruptive discharge coil, the speed must
 +be quite insignificant.</​p>​
 +<p>As to the &​quot;​non-striking vacuum,&​quot;​ the point to be noted is
 +that it can occur only with low frequency impulses, and it is necessitated by the
 +impossibility of carrying off enough energy with such impulses in high
 +vacuum since the few atoms which are around the terminal upon coming
 +in contact with the same are repelled and kept at a distance for a
 +comparatively long period of time, and not enough work can be performed
 +to render the effect perceptible to the eye. If the difference of potential between
 +the terminals is raised, the dielectric breaks down. But with very high
 +<!-- Page 142 -->
 +frequency impulses there is no necessity for such breaking down, since
 +any amount of work can be performed by continually agitating the atoms in the
 +exhausted vessel, provided the frequency is high enough. It is easy to
 +reach&​mdash;​even with frequencies obtained from an alternator as here
 +used&​mdash;​a stage at which the discharge does not pass between two
 +electrodes in a narrow tube, each of these being connected to one of
 +the terminals of the coil, but it is difficult to reach a point at
 +which a luminous discharge would not occur around each electrode.</​p>​
 +<p>A thought which naturally presents itself in connection with high
 +frequency currents, is to make use of their powerful electro-dynamic
 +inductive action to produce light effects in a sealed glass globe. The
 +leading-in wire is one of the defects of the present incandescent
 +lamp, and if no other improvement were made, that imperfection at
 +least should be done away with. Following this thought, I have carried
 +on experiments in various directions, of which some were indicated in
 +my former paper. I may here mention one or two more lines of
 +experiment which have been followed up.</​p>​
 +<​p>​Many bulbs were constructed as shown in Fig. 32 and Fig. 33.</​p>​
 +<img src="​images/​acfig32.gif"​ width="​236"​ height="​594"​ border="​0"​ align="​left"​ hspace="​10"​
 +In Fig. 32 a wide tube <​i>​T</​i>​ was sealed to a smaller W-shaped tube <​i>​U</​i>,​
 +of phosphorescent glass. In the tube <​i>​T</​i>​ was placed a coil <​i>​C</​i>​ of
 +aluminium wire, the ends of which were provided with small spheres <​i>​t</​i>​
 +and <​i>​t</​i><​sub>​1</​sub>​ of aluminium, and reached into the <​i>​U</​i>​ tube.
 +The tube <​i>​T</​i>​ was slipped into a socket containing a primary coil
 +through which usually the discharges of Leyden jars were directed, and
 +<!-- Page 143 -->
 +the rarefied gas in the small <​i>​U</​i>​ tube was excited to strong luminosity
 +by the high-tension currents induced in the coil <​i>​C</​i>​. When Leyden jar
 +discharges were used to induce currents in the coil <​i>​C</​i>,​ it was found
 +necessary to pack the tube <​i>​T</​i>​ tightly with insulating powder, as a
 +discharge would occur frequently between the turns of the coil, especially
 +<!-- Page 144 -->
 +when the primary was thick and the air gap, through which the jars discharged,
 +large, and no little trouble was experienced in this way.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig33.gif"​ width="​260"​ height="​543"​ border="​0"​ align="​left"​ hspace="​10"​
 +In Fig. 33 is illustrated another form of the bulb constructed. In
 +this case a tube <​i>​T</​i>​ is sealed to a globe <​i>​L</​i>​. The tube contains a
 +coil <​i>​C</​i>,​ the ends of which pass through two small glass tubes <​i>​t</​i>​ and
 +<​i>​t</​i><​sub>​1</​sub>,​ which are sealed to the tube <​i>​T</​i>​. Two refractory buttons <​i>​m</​i>​
 +and <​i>​m</​i><​sub>​1</​sub>​ are mounted on lamp filaments which are fastened to the ends
 +of the wires passing through the glass tubes <​i>​t</​i>​ and <​i>​t</​i><​sub>​1</​sub>​. Generally
 +in bulbs made on this plan the globe <​i>​L</​i>​ communicated with the tube
 +<​i>​T</​i>​. For this purpose the ends of the small tubes <​i>​t</​i>​ and <​i>​t</​i><​sub>​1</​sub>​ were
 +just a trifle heated in the burner, merely to hold the wires, but not
 +to interfere with the communication. The tube <​i>​T</​i>,​ with the small
 +tubes, wires through the same, and the refractory buttons <​i>​m</​i>​ and
 +<​i>​m</​i><​sub>​1</​sub>,​ was first prepared, and then sealed to globe <​i>​L</​i>,​ whereupon
 +the coil <​i>​C</​i>​ was slipped in and the connections made to its ends. The tube
 +was then packed with insulating powder, jamming the latter as tight as
 +possible up to very nearly the end, then it was closed and only a
 +small hole left through which the remainder of the powder was
 +introduced, and finally the end of the tube was closed. Usually in
 +bulbs constructed as shown in Fig. 33 an aluminium tube <​i>​a</​i>​ was
 +fastened to the upper end <​i>​s</​i>​ of each of the tubes <​i>​t</​i>​ and <​i>​t</​i><​sub>​1</​sub>,​ in
 +order to protect that end against the heat. The buttons <​i>​m</​i>​ and <​i>​m</​i><​sub>​1</​sub>​
 +could be brought to any degree of incandescence by passing the
 +discharges of Leyden jars around the coil <​i>​C</​i>​. In such bulbs with two
 +buttons a very curious effect is produced by the formation of the
 +shadows of each of the two buttons. </p>
 +<!-- Page 145 -->
 +<br clear="​all">​
 +<​p>​Another line of experiment, which has been assiduously followed, was
 +to induce by electro-dynamic induction a current or luminous discharge
 +in an exhausted tube or bulb. This matter has received such able
 +treatment at the hands of Prof. J.J. Thomson that I could add but
 +little to what he has made known, even had I made it the special
 +subject of this lecture. Still, since experiences in this line have
 +gradually led me to the present views and results, a few words must be
 +devoted here to this subject.</​p>​
 +<p>It has occurred, no doubt, to many that as a vacuum tube is made
 +longer the electromotive force per unit length of the tube, necessary
 +to pass a luminous discharge through the latter, gets continually
 +smaller; therefore, if the exhausted tube be made long enough, even
 +with low frequencies a luminous discharge could be induced in such a
 +tube closed upon itself. Such a tube might be placed around a ball or
 +on a ceiling, and at once a simple appliance capable of giving
 +considerable light would be obtained. But this would be an appliance
 +hard to manufacture and extremely unmanageable. It would not do to
 +make the tube up of small lengths, because there would be with
 +ordinary frequencies considerable loss in the coatings, and besides,
 +if coatings were used, it would be better to supply the current
 +directly to the tube by connecting the coatings to a transformer. But
 +even if all objections of such nature were removed, still, with low
 +frequencies the light conversion itself would be inefficient,​ as I
 +have before stated. In using extremely high frequencies the length of
 +the secondary&​mdash;​in other words, the size of the vessel&​mdash;​can
 +be reduced as far as desired, and the efficiency
 +<!-- Page 146 -->
 +of the light conversion is increased, provided that means are invented for efficiently
 +obtaining such high frequencies. Thus one is led, from theoretical and practical
 +considerations,​ to the use of high frequencies,​ and this means high
 +electromotive forces and small currents in the primary. When he works
 +with condenser charges&​mdash;​and they are the only means up to the present
 +known for reaching these extreme frequencies&​mdash;​he gets to electromotive
 +forces of several thousands of volts per turn of the primary. He
 +cannot multiply the electro-dynamic inductive effect by taking more
 +turns in the primary, for he arrives at the conclusion that the best
 +way is to work with one single turn&​mdash;​though he must sometimes depart
 +from this rule&​mdash;​and he must get along with whatever inductive effect
 +he can obtain with one turn. But before he has long experimented with
 +the extreme frequencies required to set up in a small bulb an
 +electromotive force of several thousands of volts he realizes the
 +great importance of electrostatic effects, and these effects grow
 +relatively to the electro-dynamic in significance as the frequency is
 +<​p>​Now,​ if anything is desirable in this case, it is to increase the
 +frequency, and this would make it still worse for the electro-dynamic
 +effects. On the other hand, it is easy to exalt the electrostatic
 +action as far as one likes by taking more turns on the secondary, or
 +combining self-induction and capacity to raise the potential. It
 +should also be remembered that, in reducing the current to the
 +smallest value and increasing the potential, the electric impulses of
 +high frequency can be more easily transmitted through a conductor. </p>
 +<!-- Page 147 -->
 +<​p>​These and similar thoughts determined me to devote more attention to
 +the electrostatic phenomena, and to endeavor to produce potentials as
 +high as possible, and alternating as fast as they could be made to
 +alternate. I then found that I could excite vacuum tubes at
 +considerable distance from a conductor connected to a properly
 +constructed coil, and that I could, by converting the oscillatory
 +current of a condenser to a higher potential, establish electrostatic
 +alternating fields which acted through the whole extent of a room,
 +lighting up a tube no matter where it was held in space. I thought I
 +recognized that I had made a step in advance, and I have persevered in
 +this line; but I wish to say that I share with all lovers of science
 +and progress the one and only desire&​mdash;​to reach a result of utility to
 +men in any direction to which thought or experiment may lead me. I
 +think that this departure is the right one, for I cannot see, from the
 +observation of the phenomena which manifest themselves as the
 +frequency is increased, what there would remain to act between two
 +circuits conveying, for instance, impulses of several hundred millions
 +per second, except electrostatic forces. Even with such trifling
 +frequencies the energy would be practically all potential, and my
 +conviction has grown strong that, to whatever kind of motion light may
 +be due, it is produced by tremendous electrostatic stresses vibrating
 +with extreme rapidity.</​p>​
 +<p>Of all these phenomena observed with currents, or electric impulses,
 +of high frequency, the most fascinating for an audience are certainly those
 +which are noted in an electrostatic field acting through considerable distance, and the
 +<!-- Page 148 -->
 +best an unskilled lecturer can do is to begin and finish with the exhibition of these
 +singular effects. I take a tube in the hand and move it about, and it is lighted
 +wherever I may hold it; throughout space the invisible forces act. But I may
 +take another tube and it might not light, the vacuum being very high.
 +I excite it by means of a disruptive discharge coil, and now it will
 +light in the electrostatic field. I may put it away for a few weeks or
 +months, still it retains the faculty of being excited. What change
 +have I produced in the tube in the act of exciting it? If a motion
 +imparted to the atoms, it is difficult to perceive how it can persist
 +so long without being arrested by frictional losses; and if a strain
 +exerted in the dielectric, such as a simple electrification would
 +produce, it is easy to see how it may persist indefinitely,​ but very
 +difficult to understand why such a condition should aid the excitation
 +when we have to deal with potentials which are rapidly alternating.</​p>​
 +<​p>​Since I have exhibited these phenomena for the first time, I have
 +obtained some other interesting effects. For instance, I have produced
 +the incandescence of a button, filament, or wire enclosed in a tube.
 +To get to this result it was necessary to economize the energy which
 +is obtained from the field and direct most of it on the small body to
 +be rendered incandescent. At the beginning the task appeared
 +difficult, but the experiences gathered permitted me to reach the
 +result easily. In Fig. 34 and Fig. 35 two such tubes are illustrated
 +which are prepared for the occasion.</​p>​
 +<img src="​images/​acfig34.gif"​ width="​232"​ height="​591"​ border="​0"​ align="​left"​ hspace="​10"​
 +In Fig. 34 a short tube <​i>​T</​i><​sub>​1</​sub>,​ sealed to another long tube <​i>​T</​i>,​
 +is provided with a stem <​i>​s</​i>,​ with a platinum wire sealed in the latter.
 +A very thin lamp filament <​i>​l</​i>​ is fastened to this
 +<!-- Page 149 -->
 +wire, and connection to the outside is made through a thin copper wire <​i>​w</​i>​.
 +The tube is provided with outside and inside coatings, <​i>​C</​i>​ and
 +<​i>​C</​i><​sub>​1</​sub>​ respectively,​ and is filled as far as the coatings reach
 +with conducting, and the space above with insulating powder. These coatings are
 +merely used to enable me to perform two experiments with the
 +<!-- Page 150 -->
 +tube&​mdash;​namely,​ to produce the effect desired
 +either by direct connection of the body of the experimenter or of
 +another body to the wire <​i>​w</​i>,​ or by acting inductively through the
 +glass. The stem <​i>​s</​i>​ is provided with an aluminium tube <​i>​a</​i>,​ for
 +purposes before explained, and only a small part of the filament
 +reaches out of this tube. By holding the tube <​i>​T</​i><​sub>​1</​sub>​ anywhere in the
 +electrostatic field the filament is rendered incandescent.</​p>​
 +<br clear="​all">&​nbsp;<​br>​
 +<img src="​images/​acfig35.gif"​ width="​259"​ height="​592"​ border="​0"​ align="​left"​ hspace="​10"​
 +A more interesting piece of apparatus is illustrated in Fig. 35. The
 +construction is the same as before, only instead of the lamp filament
 +a small platinum wire <​i>​p</​i>,​ sealed in a stem <​i>​s</​i>,​ and bent above it in
 +a circle, is connected to the copper wire <​i>​w</​i>,​ which is joined to an
 +inside coating <​i>​C</​i>​. A small stem <​i>​s</​i><​sub>​1</​sub>​ is provided with a needle, on
 +the point of which is arranged to rotate very freely a very light fan
 +of mica <​i>​v</​i>​. To prevent the fan from falling out, a thin stem of glass
 +<​i>​g</​i>​ is bent properly and fastened to the aluminium tube. When the
 +glass tube is held anywhere in the electrostatic field the platinum
 +wire becomes incandescent,​ and the mica vanes are rotated very fast.</​p>​
 +<br clear="​all">​
 +<​p>​Intense phosphorescence may be excited in a bulb by merely connecting
 +it to a plate within the field, and the plate need not be any larger
 +than an ordinary lamp shade. The phosphorescence excited with these
 +currents is incomparably more powerful than with ordinary apparatus. A
 +small phosphorescent bulb, when attached to a wire connected to a
 +coil, emits sufficient light to allow reading ordinary print at a distance of
 +five to six paces. It was of interest to see how some of the phosphorescent
 +bulbs of Professor Crookes would behave with these currents, and
 +<!-- Page 151 -->
 +he has had the kindness to lend me a few for the occasion.
 +The effects produced are magnificent,​ especially by the
 +sulphide of calcium and sulphide of zinc. From the disruptive
 +discharge coil they glow intensely merely by holding them in the hand
 +and connecting the body to the terminal of the coil.</​p>​
 +<p>To whatever results investigations of this kind may lead, their chief
 +interest lies for the present in the possibilities they offer for the
 +production of an efficient illuminating device. In no branch of
 +electric industry is an advance more desired than in the manufacture
 +of light. Every thinker, when considering the barbarous methods
 +employed, the deplorable losses incurred in our best systems of light
 +production, must have asked himself, What is likely to be the light of
 +the future? Is it to be an incandescent solid, as in the present lamp,
 +or an incandescent gas, or a phosphorescent body, or something like a
 +burner, but incomparably more efficient?</​p>​
 +<​p>​There is little chance to perfect a gas burner; not, perhaps, because
 +human ingenuity has been bent upon that problem for centuries without
 +a radical departure having been made&​mdash;​though this argument is not
 +devoid of force-but because in a burner the higher vibrations can
 +never be reached except by passing through all the low ones. For how
 +is a flame produced unless by a fall of lifted weights? Such process
 +cannot be maintained without renewal, and renewal is repeated passing
 +from low to high vibrations. One way only seems to be open to improve
 +a burner, and that is by trying to reach higher degrees of incandescence.
 +Higher incandescence is equivalent to a quicker vibration;
 +<!-- Page 152 -->
 +that means more light from the same material, and that,
 +again, means more economy. In this direction some improvements have
 +been made, but the progress is hampered by many limitations.
 +Discarding, then, the burner, there remain the three ways first
 +mentioned, which are essentially electrical.</​p>​
 +<​p>​Suppose the light of the immediate future to be a solid rendered
 +incandescent by electricity. Would it not seem that it is better to
 +employ a small button than a frail filament? From many considerations
 +it certainly must be concluded that a button is capable of a higher
 +economy, assuming, of course, the difficulties connected with the
 +operation of such a lamp to be effectively overcome. But to light such
 +a lamp we require a high potential; and to get this economically we
 +must use high frequencies.</​p>​
 +<​p>​Such considerations apply even more to the production of light by the
 +incandescence of a gas, or by phosphorescence. In all cases we require
 +high frequencies and high potentials. These thoughts occurred to me a
 +long time ago.</​p>​
 +<​p>​Incidentally we gain, by the use of very high frequencies,​ many
 +advantages, such as a higher economy in the light production, the
 +possibility of working with one lead, the possibility of
 +doing away with the leading-in wire, etc.</​p>​
 +<​p>​The question is, how far can we go with frequencies?​ Ordinary
 +conductors rapidly lose the facility of transmitting electric impulses
 +when the frequency is greatly increased. Assume the means for the
 +production of impulses of very great frequency brought to the utmost
 +perfection, every one will naturally ask how to transmit them when the
 +necessity arises. In transmitting such impulses through
 +<!-- Page 153 -->
 +conductors we must remember that we have to deal with <​i>​pressure</​i>​
 +and <​i>​flow</​i>,​ in the ordinary interpretation of these terms. Let the pressure
 +increase to an enormous value, and let the flow correspondingly diminish, then
 +such impulses&​mdash;​variations merely of pressure, as it were&​mdash;​can no doubt
 +be transmitted through a wire even if their frequency be many hundreds
 +of millions per second. It would, of course, be out of question to
 +transmit such impulses through a wire immersed in a gaseous medium,
 +even if the wire were provided with a thick and excellent insulation
 +for most of the energy would be lost in molecular bombardment and
 +consequent heating. The end of the wire connected to the source would
 +be heated, and the remote end would receive but a trifling part of the
 +energy supplied. The prime necessity, then, if such electric impulses
 +are to be used, is to find means to reduce as much as possible the
 +<​p>​The first thought is, employ the thinnest possible wire surrounded by
 +the thickest practicable insulation. The next thought is to employ
 +electrostatic screens. The insulation of the wire may be covered with
 +a thin conducting coating and the latter connected to the ground.
 +But this would not do, as then all the energy would pass through the
 +conducting coating to the ground and nothing would get to the end of
 +the wire. If a ground connection is made it can only be made through a
 +conductor offering an enormous impedance, or though a condenser of
 +extremely small capacity. This, however, does not do away with other
 +<p>If the wave length of the impulses is much smaller than
 +<!-- Page 154 -->
 +the length of the wire, then corresponding short waves will be sent up in
 +the conducting coating, and it will be more or less the same as though
 +the coating were directly connected to earth. It is therefore necessary to
 +cut up the coating in sections much shorter than the wave length. Such
 +an arrangement does not still afford a perfect screen, but it is ten
 +thousand times better than none. I think it preferable to cut up the
 +conducting coating in small sections, even if the current waves be
 +much longer than the coating.</​p>​
 +<p>If a wire were provided with a perfect electrostatic screen, it would
 +be the same as though all objects were removed from it at infinite
 +distance. The capacity would then be reduced to the capacity of the
 +wire itself, which would be very small. It would then be possible to
 +send over the wire current vibrations of very high frequencies at
 +enormous distance without affecting greatly the character of the
 +vibrations. A perfect screen is of course out of the question, but I
 +believe that with a screen such as I have just described telephony
 +could be rendered practicable across the Atlantic. According
 +to my ideas, the gutta-percha covered wire should be provided with a third
 +conducting coating subdivided in sections. On the top of this should
 +be again placed a layer of gutta-percha and other insulation, and on
 +the top of the whole the armor. But such cables will not be
 +constructed,​ for ere long intelligence&​mdash;​transmitted without
 +wires&​mdash;​will throb through the earth like a pulse through a living
 +organism. The wonder is that, with the present state of knowledge and
 +the experiences gained, no attempt is being made to disturb
 +<!-- Page 155 -->
 +the electrostatic or magnetic condition of the earth, and transmit, if
 +nothing else, intelligence.</​p>​
 +<p>It has been my chief aim in presenting these results to point out
 +phenomena or features of novelty, and to advance ideas which I am
 +hopeful will serve as starting points of new departures. It has been
 +my chief desire this evening to entertain you with some novel
 +experiments. Your applause, so frequently and generously accorded, has
 +told me that I have succeeded.</​p>​
 +<p>In conclusion, let me thank you most heartily for your kindness and
 +attention, and assure you that the honor I have had in addressing such
 +a distinguished audience, the pleasure I have had in presenting these
 +results to a gathering of so many able men&​mdash;​and among them also some
 +of those in whose work for many years past I have found enlightenment
 +and constant pleasure&​mdash;​I shall never forget.</​p>​
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