Experiments with Alternate Currents of High Potential and High Frequency (1892) — part 1 of 8

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EXPERIMENTS

WITH

ALTERNATE CUBRENTS

OF

HIGH POTENTIAL AND HIGH FREQUENCY.

BY

NIKOLA TESLA.

A LECTURE}

DELIVERED BEFORE THE

INSTITUTION OF ELECTRICAL ENGINEERS, LONDON.

With a Portrait and Biographical Sketch of the Author.

NEW YORK :

The W. J. Johnston Company, Ltd., 167-176 Times Building.

• l

■

.

531 5

Biographical Sketch of Nikola Tesla.

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 “unspeak¬ able Turk” and reclaiming gradually the fair lands that endure the curse of Mohammedan rule. For a long time the Slav people — who, after the battle of Kosovopjolje, in which the Turks defeated the Servians, retired to the con¬ fines of the present Montenegro, Dalmatia, Herzegovina and Bosnia, and “Borderland” of Austria — knew what it was to deal, as our Western pioneers did, with foes cease¬ lessly 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.

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 elec¬ trical pursuits and of the country where electricity enjoys ds widest application.

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IV

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 repre¬ sented. His mother enjoyed great fame throughout the country side for her skill and originality in needlework, and doubtless transmitted her ingenuity to Nikola; though it naturally took another and more masculine direction.

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 quickwitted boys go through, although he varied the programme on one oc¬ casion by getting imprisoned in a remote mountain chapel rarely visited for service; and on another occasion by fall¬ ing 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.

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 en¬ gine 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

V

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 be¬ come passionately fond of expeiimenting, and as much as his means and leisure permitted devoted his ener¬ gies 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 recog¬ nizing the value of linguistic acquirements, he went to Prague and then to Buda-Pesth with the view of master¬ ing 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

• • J*

between his dispatches and the corresponding remittance

from home; and when the mathematical expression for

VI

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 engineer¬ ing 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 in¬ vention 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 inven¬ tions, 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. Long¬ ing 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

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 em¬ ployment as an electrical engineer with one of the largest electric lighting companies. The next year lie 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 at¬ tention, and once again staking everything on a single throw, he crossed the Atlantic.

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 prop¬ osition was made to him to start his own company, and, accepting the terms, he at once worked up a practical sys¬ tem of arc lighting, as well as a potential method of dy¬ namo regulation, which in one form is now known as the “ third brush regulation.” He also devised a thermo-mag¬ netic 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 cur¬ rents, 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 be¬ fore the American Institute of Electrical Engineers, Pro¬ fessor Ferraris, in Europe, published his discovery of prin-

VIII

ciples analogous to those ennunciated 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 Fer- raris’ work as it had not been published. Professor Fer¬ raris stated himself, with becoming modesty, that he did not think Tesla could have known of his (Ferraris’) experi¬ ments 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.

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 Westingliouse Elec¬ tric 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 The Electrical World 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.

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 phenom¬ enal 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 “pours out heart affluence in dis-

IX

cursive talk,” 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.

In the year 1890 he severed his connection with the West- inghouse Company, since which time he has devoted him¬ self 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 electric¬ ity, which he may be said to have created, was delivered be¬ fore 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 The Electrical World, July 11, 1891. Its publication excited such interest abroad that he received numerous re¬ quests 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.

The present lecture presupposes a knowledge of the for¬ mer, but it may be read and understood by any one even though he has not read the earlier one. It forms a sort cf continuation of the latter, and includes chiefly the results of his researches since that time.

EXPERIMENTS

WITH

Alternate Currents of High Potential and High Frequency-

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.

v

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 — names of those among you who are recognized as the leaders in this enchanting science ; but one, at least, I must mention — a name which could not be omitted in a demonstration of this kind. It is a name associated with the most beautiful in¬ vention ever made : it is Crookes !

When I was at college, a good time ago, I read, in a translation (for then I was not familiar with your magnifi¬ cent language), the description of his experiments on radiant matter. I read it only once in my life — that time — yet every

2

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.

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 — which even by myself cannot be considered trifling, since they are so appreciated by you — 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.

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.

Give me leave to introduce the subject in a few words.

A short time ago I had the honor to bring before our American Institute of Electrical Engineers* 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

*For Mr. Tesla s American lecture on this subject see The Elec¬ trical World of July 11, 1891, and for a report of his French lee ture see The Electrical World of March 26, 1892.

8

in this work have "been for me a great reward and encour¬ agement. 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 even¬ ing's lecture consistent.

This investigation, then, it goes without saying, deals with alternating currents, and, to be more precise, with alter¬ nating currents of high potential and high frequency. Just in how much a very high frequency is essential for the pro¬ duction of the results presented is a question which, even with my present experience, would embarrass me to an¬ swer. 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 in¬ duction apparatus employed, the high potentials, which in their turn are necessary to the demonstration of most of the experiments here contemplated.

Of the various branches of electrical investigation, per¬ haps the most interesting and immediately the most promis¬ ing 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 ex¬ periences are met with and new avenues of research are opened. Even at this hour possibilities not dreamed of be¬ fore are, by the use of these currents, partly realized. As in nature all is ebb and tide, all is wave motion, so it seems

4

that in all branches of industry alternating currents — elec¬ tric wave motion — will have the sway.

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, w'th great speed and power— with the energy possibly con¬ veyed from a great distance. We observe how the energy of an alternating cuirent traversing the wire manifests itself — not so much in the wire as in the surrounding space — 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, — in the hope that some one, no matter who, may find a solution of one of the pending great problems, — and each succeeding day we return to our task with renewed ardor ; and even if we are 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.

We may take — at random, if you choose — any of the many experiments which may be performed with alternat-

mg currents; a few of which only, and by no means the most striking, form the subject of this evening’s demon¬ stration ; they are all equally interesting, equally inciting to thought.

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 per¬ sists with undiminished brightness.

Here is an exhausted bulb suspended from a single wire. Standing on an insulated support, I grasp it, and a plati¬ num button mounted in it is brought to vivid incandescence.

Here, attached to a leading wire, is another bulb, which, as I touch its metallic socket, is filled with magnificent colors of phosphorescent light.

Here still another, which by my fingers’ touch casts a shadow — the Crookes shadow, of the stem inside of it.

Here, again, insulated as I stand on this platform, I bring my body in contact with one of the terminals of the sec¬ ondary of this induction coil — with the end of a wire many miles long — and you see streams of light break forth from its distant end, which is set in violent vibration.

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 be¬ tween the plates. I insert a thick plate of one of the best dielectrics between them, and instead of rendering alto¬ gether impossible, as we are used to expect, I aid the pas-

e

sage of the discharge, which, as I insert the plate, merely changes in appearance and assumes the form of luminous streams.

Is there, I ask, can there be, a more interesting study than that of alternating currents?

In all these investigations, in all these experiments, which are so very, very interesting, for many years past — ever since the greatest experimenter who lec¬ tured in this hall discovered its principle — we have had a steady companion, an appliance familiar to every one, a plaything once, a thing of momentous importance now — 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 dis¬ closed to our ravished eyes. So well known is this appa¬ ratus, so familiar are these phenomena to every one, that my courage nearly fails me when I think' that I have ven¬ tured to address so able an audience, that I have ventured to entertain you with that same old subject. Here in real¬ ity is the same apparatus, and here are the same phenom ena, only the apparatus is operated somewhat differently, the phenomena are presented in a different aspect. Som( of the results we find as expected, others surprise us, bul all captivate our attention, for in scientific investigatior each novel result achieved may be the centre of a new de parture, each novel fact learned may lead to important de velopments.

r/

Usually in operating an induction coil we have set up a vibration cf moderate frequency in the primary, either by means of an interrupter or break, or by the use of an alter¬ nator. 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 preceive many of the curious effects which have since been observed.

In the experiments such as performed this evening, we operate the coil either from a specially constructed alter¬ nator capable of giving many thousands of reversals of current per second, or, by disruptively discharging a con¬ denser through the primary, we set up a vibration in the secondary circuit of a frequency of many hundred thou¬ sand or millions per second, if we so desire; and in using either of these means we enter a field as yet unexplored.

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 appli¬ cable to the subject of this lecture the many curious and unexpected phenomena which we observe afford a con¬ vincing proof. By way of illustration, take for instance the most obvious phenomena, those of the discharge of the induction coil.

Here is a coil which is operated by currents vibrating with extreme rapidity, obtained by disruptively discharg¬ ing a Leyden jar. It would not surprise a student were

I

8

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 antici¬ pated 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.)

Now compare this phenomenon which you have just witnessed with the discharge of a Holtz or Wimsliurst machine — that other interesting appliance so dear to the experimenter. What a difference there is between these phenomena ! And yet, had I made the necessary arrange¬ ments — which could have been made easily, were it not that they would interfere with other experiments — 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 fric¬ tion machine. This may be done in many ways — for instance, by operating the induction coil which charges the con-

9

\N '“V •->

denser 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 ob

Fig. 1.— Discharge Between Two Wires with Frequen¬ cies of a Few Hundred Thousand per Second.

tain in the secondary circuit, if the knobs are of the required size and properly set, a more or less rapid succession of sparks of great intensity and small quantity, which possess

10

the same brilliancy, and are accompanied by the same sharp crackling sound, as those obtained from a friction or

influence machine.

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 dis¬ charges into the primary circuit of a second coil through a small air gap, which is necessary to produce a sud¬ den rush of current through the pri¬ mary. The scheme of connections in the present experiment is indicated in Fig. 2.

G is an ordinarily constructed alternator, supplying the primary Pof an induction coil, the secondary S of which

Fig. 2.— Imitating the Spark of a Holtz Machine.

K K

11

charges the condensers or jars C C. The terminals of the secondary are connected to the inside coatings of the jars, the outer coatings being connected to the ends of the pri¬ mary pp of a second induction coil. This primary p p has a small air gap a b.

The secondary s of this coil is provided with kpobs or spheres K K of the proper size and set at a distance suit¬ able for the experiment.

A long arc is established between the terminals A B of the first induction coil. M M are the mica plates.

Each time the arc is broken between A and B the jars are quickly charged and discharged through the primary p p, producing a snapping spark between the knobs K K. Upon the arc forming between A and B the potential falls, and the jars cannot be charged to such high potential as to break through the air gap a b until the arc is again broken by the draught.

In this manner sudden impulses, at long intervals, are produced in the primary p p , which in the secondary s give a corresponding number of impulses of great intensity. If the secondary knobs or spheres, K K, are of the proper size, the sparks show much resemblance to those of a Holtz machine.

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 interest.

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

12

of the phenomena is rendered much more easy. In such case, in varying the strength and frequency of the cur¬ rents through the primary, we may observe five distinct forms of discharge, which I have described in my former paper on the subject* before the American Institute of Elec¬ trical Engineers, May 20, 1891.

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 press¬ ure. We know that the phenomenon is due to the agita¬ tion 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. In¬ deed, we hnd 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 — save, perhaps, that chemical affinity might not be electrical in its nature.

As the production of heat and light is here due to the im¬ pact 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 ob-

*See The Electrical World, July 11, 1891,

13

tained 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 al¬ ways 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 fre¬ quencies, which we may yet find means to produce effi¬ ciently, and provided that electric impulses of such high frequencies could be transmitted through a conductor, the electrical characteristics of the brush discharge would com¬ pletely vanish— no spark would pass, no shock would be felt — yet we would still have to deal with an electric phe¬ nomenon, 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 interest.

When a coil is operated with currents of very high fre¬ quency, beautiful brush effects may be produced, even if the coil be of comparatively small dimensions. The ex-

14

perimenter 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 — in fact, often better with one than with two.

But of all the discharge phenomena observed, the most pleasing to the eye, and the most instructive, are those ob¬ served with a coil which is operated by means of the dis¬ ruptive 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 com¬ plete mass of fire.

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 de¬ stroy the insulation it is necessary to prevent them. This is best done by immersing the coil in a good liquid insula¬ tor, such as boiled-out oil. Immersion in a liquid maybe considered almost an absolute necessity for the continued and successful working of such a coil.

It is of course out of the question, in an experimental lec¬ ture, with only a few minutes at disposal for the perfor¬ mance of each experiment, to show these discharge phe¬ nomena 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 even-

15

ing, they are sufficiently striking to interest an intelligent audience.

Before showing some of these curious effects I must, for the sake of completeness, give a short description of the

Fig. 3.— Disruptive Discharge Coil.

coil and other apparatus used in the experiments with the disruptive discharge this evening.

It is contained in a box B (Fig. 3) of thick boards of hard wood, covered on the outside with zinc sheet Z, which is

16