Mr. Tesla and Vibratory Currents

The lecture given by Mr. Tesla on Wednesday last week before the Institution of Electrical Engineers, and repeated on the Thursday before the Royal Institution, will live long in the imagination of every person in the brilliant scientific audiences that heard him, opening as it did, to many of them for the first time, apparently limitless possibilities in the applications and control of electricity. Seldom has there been such a gathering of all the foremost electrical authorities of the day, on the tiptoe of expectation to witness the experiments, details of some of which had already been given to us from the other side of the Atlantic, but of which no written account could convey the true significance and beauty. Long before the hour of meeting the hall was crowded, and Mr. Tesla was watched throughout with the keenest interest as he adjusted his apparatus quivering with lightning-like discharges, and now lighted a vacuum tube by grasping it in his hand, now brought to incandescence the filament of an ordinary lamp attached by a single wire, there rendering the air in the interior of a large ring luminous with flame, or sending streams of light from wires stretched over the audience, and, most fascinating of all, after electrifying the whole space of air between his table and an iron plate above him, waving a luminous tube in his hand totally unconnected to any wire whatever. It was, indeed, curious to see the most prominent and noted electricians of the day as interested in Mr. Tesla and his experiments as any child with the first friction machine, asking whether it was safe to do this or that, to touch the wires, and whether they might be allowed to try. For full two hours Mr. Tesla kept his audience spellbound, with easy confidence and the most modest manner possible displaying his experiments, and suggesting, one after another, outlooks for the practical application of his researches; and it was difficult to realise that this memorable lecture was the second only that he had ever delivered. Even at the end Mr. Tesla tantalisingly informed his listeners that he had shown them but one-third of what he was prepared to do, and the whole audience, after Prof. Ayrton had proposed the vote of congratulation and declared the meeting over, yet remained in their seats unwilling to disperse, insisting upon more, and Mr. Tesla had to deliver a supplementary lecture.

It is not for us here to describe the lecture in its details, as a full and illustrated report, revised by Mr. Tesla, is being prepared for the Journal of the Institution of Electrical Engineers, which will appear in due time, and containing much even that could only be glanced at in the lecture itself, and this will be the proper material for electrical engineers to fully discuss. But it may be allowed us to dwell a little upon the scientific modus operandi, and give certain particulars which our readers, no doubt, are looking for with interest.

In the first place, it may be stated, as Mr. Tesla mentioned, but which hardly seems to be realised, that practically the whole of the experiments shown were new, and had never been shown before, and were not merely a repetition of those given in his lecture in America. That of lighting tubes in an electrostatic field and of bringing filaments to incandescence on a single wire were shown before, and being of the most importance, naturally were not left out. But all the other experiments, together with most of the apparatus itself, was novel, and was supplementary to that of his American lecture. In the next place, it is important to understand - as also stated Mr. Tesla at the time, but hardly, perhaps, yet realised - that nearly the whole of his experiments and effects were produced by means of an ordinary alternating current from an ordinary commercial alternating-current dynamo - such, in fact, as can, in many places, be obtained from a central supply company’s mains. These effects can be, and were, brought about also by means of the currents from his special alternator, but the chief benefit of the use of this alternator is in being able to obtain perfectly harmonious currents of a known number of alternations per second, and in research work this knowledge is evidently of immense value.

Without going deeply into the detail or the theory of the working of the apparatus used, it is seen, therefore, that the effects are produced by using an alternating current, either generated direct at high potential and with high frequency of reversal per second, or ordinary currents converted into such currents. Mr. Tesla termed these currents “alternating currents of high potential and high frequency,” but in the same way that the term rotary current (first proposed, we believe, in these columns) has now gained acceptance to indicate alternating currents varying in phase for producing a rotating magnetic field, so we may, perhaps, venture to use the term “vibratory currents” for those of high potential and high frequency. To generate vibratory currents from an ordinary alternating-current circuit, Mr. Tesla uses first an ordinary transformer in oil to transform upwards; the secondary current from this he passes into a second transformer having in its circuit a magnetic spark interrupter, and from the secondary circuit of this second transformer (which it may be noted is of comparatively thick short wire) he obtains discharges in all respects similar to those of the great induction coils, but of very high frequency. Connecting this circuit to a properly-adjusted condenser, in his case a set of Leyden jars, a surging effect is produced on the currents, which raises the frequency of vibrations from 25,000 or so per second to some millions per second, and the potential to some hundreds of thousands, or even, perhaps, millions of volts. The frequency in the case of the use of the Tesla dynamo can of course be accurately determined. The potential, it seems, cannot be accurately arrived at by calculation, or, rather, the calculation does not give an accurate result correspondent with the actualities achieved in these effects - a fact, as we shall see, that may have very important theoretical results upon the wave theory of electricity.

Having at command a vibratory current of this nature, the results shown by Mr. Tesla were the first outcome of continued and careful experiments. Not only does the molecular bombardment of the molecules of highly-exhausted gases in vacuum tubes show phosphorescence, but gases at low states of exhaustion do the same, and even ordinary air at ordinary temperature, as Mr. Tesla showed at his lecture, where the space between two concentric rings glowed with discharge like a vacuum tube itself, while the vacuum tube glowed when at some considerable distance from the plates to which the two poles were connected. In this manner, by vibrating the air molecules at speed correspondent to that of the vibration of light, phosphorescent effects could be shown with ease. Phosphorescence, Mr. Tesla explained, he regarded as incandescence in another form. Ordinary incandescence accrues after the filament has passed interiorly from the state of cold to that of intense heat; while if we regard a bombardment of molecules with sufficient intensity upon the surface of a material, we may conceive an infinitesimal film of that material rendered continuously incandescent without the trouble of heating the whole - in a word, we obtain the light vibrations without passing through the whole gamut of heat vibrations - which has long been the electrical engineer’s most ardent desire. To obtain a concrete idea of the difference between the ordinary alternating current and the vibratory current, we might imagine the first as a large ordinary steam engine reciprocating at 100 revolutions, while the second becomes a smaller and smaller material engine as its reciprocations mount from hundreds to thousands, or hundreds of thousands, in the same unit of time. In the latter we have the greatest efficiency with the smallest of first outlay.

We need not go further into the detail of the experiments shown by Mr. Tesla based upon these considerations, as they will be given, as we have said, fully in his paper. We merely mention here that Mr. Tesla hinted at illumination of houses without wires, transmission of light and power to a distance without wires, the synchronising of various wave-lengths for multiplex telegraphy without wires, the use of motors with but one wire, or even possibly without any, and the recovery of the solar energy radiated around us direct - “gearing,” as he said, “our motors to Nature’s wheels.”

He demonstrated that our ideas upon dielectrics required modification, and that it was a mere question of potential to make every material or gas a conductor. It was shown that the interposition of a plate of ebonite, in fact, facilitated, rather than otherwise, the discharge. He showed that the vanes of the Crookes radiometer would rotate under the influence of his vibratory current, a rotation, curiously enough, the reverse way to that induced by light. We believe a similar result was first pointed out in a paper read before the Institution at Edinburgh by Mr. A. R. Bennett.

Mr. Tesla incidentally showed that the glow discharge in a tube under certain conditions would revolve and then become extremely susceptible to the feeblest magnetism, and he hinted that possibly by this means the rapidity of transmission of telegrams through submarine cables might be greatly increased.

A word remains about one or two personal and scientific problems. In the first place, it will be interesting to know Mr. Tesla’s own ideas as to the future practicability of his researches - ideas which, of course, will have to await their fulfilment for some time for actual application in practice. The question naturally arises, How can the vibratory current be applied to lighting? Should we expect to have incandescent or phosphorescent lamps of a pattern similar to those we know in the Edison-Swan lamps or the Geissler tubes, or should we expect rather to discover a practical means for rendering the whole mass of the air in a room softly and beautifully phosphorescent? Both, Mr. Tesla thinks, if we understand him aright, may occur, but he looks certainly to the possibility of the last and most fascinating project. Further, many of those who witnessed his experiments must have asked themselves a question as to the danger of the vibratory currents, which Mr. Tesla handled so unconcernedly. We took an opportunity of enquiring of Mr. Tesla with reference to this point, how, indeed, he came to dare to take the current through his body? It was the result of a long debate in his mind, it appears, that caused him to attempt the experiment. Reason and calculation showed him that such currents ought not to be dangerous to life any more than the vibrations of light are dangerous. The self-induction and frequency of alternation should be too great for any current to pass, and for a current to be dangerous a certain quantity must pass. Conceive a thin diaphragm in a water-pipe, with to and fro piston-strokes of considerable amplitude the diaphragm will be ruptured at once. With reduced strokes of the same total energy the diaphragm will be less liable to rupture, until with a vibratory impulse of many thousands per second no actual current flows, and the diaphragm is in no danger of rupture. So with the vibratory current - yet in spite of reason and analogy it was with the feelings of a man about to plunge from Brooklyn Bridge (as one might well believe) that Mr. Tesla took his first shock from his apparatus. The result justified his daring, and he suffered no more than a slight shaking in the arms. A spark, of course, passes and this punctures the skin and causes a slight burn, but that is all. This can easily be avoided by holding a conductor of suitable size in the hand and receiving the shock upon that.

There lurks in one sentence of Mr. Tesla’s lecture a statement which will cause much discussion in high scientific circles. We shall not further refer to it here than to say, that if the voltage obtained is not exactly calculable from the data laid down of the condenser and the frequencies - if, in a word, the result is not approximately that calculated, but considerably lower, then this would certainly seem to show that Hertz’s experiments and results are not final, and the way is open to further experiments and research in this direction.

From the way in which Mr. Tesla alluded feelingly to the impulse in research given to himself from perusal of Crookes’s experiments in high vacuum, the effect upon students and scientific men generally of the publication of his own promising researches must be great. We can only hope that others, now that the way has been shown, will take up the work, and before many years have passed produce for the world at large the thoroughly practical outcome hinted at in the “wonder-full” lecture by Mr. Tesla at the Royal Institution, on Wednesday, the 3rd of February, 1892.