Part 65

This shows that we have only to adjust suitably the tension, length, and weight of a string in order to make it vibrate at any rate we please. Now in the oscillation of currents in the Leyden jar discharge there are conditions which correspond, by analogy at least, with those that determine the vibrations of a stretched string. These conditions are of course electrical, and they are definable in terms of electric units, which need not be discussed here. As we are leading the reader to the modern view of electricity, which sets aside the fluid theories and regards electricity as having no separate existence, but as being merely the manifestation of some condition of a universally pervading medium, the same, in fact, as the luminiferous ether, it is curious to remark that these electrical oscillations would seem to attribute to the incompressible and imponderable ether something very much like the characteristic property of matter we call inertia, by virtue of which the released cord flies past its position of equilibrium to the other side. Or may this quality be dependent on the matter of the dielectric in which the ether is, as it were, entangled?

The oscillatory character of the Leyden jar discharge was elegantly demonstrated before a large audience in a lecture given by Professor O. Lodge at the Royal Institution a few years ago. Clearly it is impossible to render perceptible to the senses the millions of periodic discharges that take place in the marvellously short space of time taken up by a spark, but by doing what is analogous to slackening the tension of the stretched string or increasing its length, that is by increasing the _static capacity_, which means using a large number of jars combined into a battery, and at the same time causing the discharge to pass through coils (the effect of these is to increase the _self-induction_ of the circuit—called also _impedance_), an arrangement corresponding with loading the string, Dr. Lodge was able to bring down the rate of oscillation to 5,000 per second, when, instead of the crack of the ordinary discharge, a very shrill continuous sound was heard. The addition of another coil gave another load, and when the rate was thus reduced to about 500, the note emitted was that of the C above the middle A of the piano. With the rate of oscillation thus reduced, it became easy to render the discontinuity of the discharge visible by means of revolving mirrors, as in the well-known acoustical demonstrations.

Professor Lodge has devised an experiment which again shows the analogy of electrical oscillations with those by which sound is produced. It is well known that a vibrating tuning-fork will set another fork of the same pitch to vibrate also by mere approximation. A and B (Fig. 280_k_) are two exactly similar Leyden jars, the inner and outer coatings of each being connected by a wire enclosing a considerable area in its circuit, which in the case of A contains an air gap across which sparks pass when the coatings are connected with the poles of an electrical machine. The circuit of B is provided with an adjustable sliding piece C, and the coatings are almost connected with each other by a strip of tinfoil hanging over the rim but not quite reaching to the outer coating. When the jars are placed so that their wire circuits are parallel, and sparks are passing across the air interval of A’s circuit, a position of the slider on the other can be found when sparks also pass between the tin-foil and the outer coating. But if the slider be moved from this position, the two circuits will no longer be in unison, and the sparks in B will cease. This response of the oscillations in one jar to those set up in another of the same vibratory period is called _electrical resonance_.

Dr. Hertz, a professor in the University of Bonn, has opened out new paths to investigators by a brilliant series of researches which have shown that in the dielectric surrounding an electrical system executing very rapid oscillations there are waves of electro-motive and magnetic force. These researches are not capable of any condensed description here, and the reasoning is of a kind that appears mainly to the expert physicist. One of his modes of investigation required oscillations of extreme rapidity, and he obtained them by attaching to each pole of an induction coil a metal plate, and between these plates, which were in the same vertical plane, passed a stout wire interrupted by an air gap in its centre provided with small brass balls. The rate of oscillation of this arrangement was calculated as the hundred-millionth part of 1·4 second. In conjunction with this system Hertz made use of a very simple apparatus he called a resonator, which consisted merely of a piece of copper wire bent into a circle of about 28 inches diameter. The ends of the wire did not, however, meet, but were fitted with two balls, or with a ball and a point, and an arrangement by which the air gap between them could be very finely adjusted and measured. This resonator was, of course, prepared as to be in electrical tune with the original vibrator, and with it Hertz was able to examine the condition of the surrounding space. When held in the hand near the vibrator he found that sparks crossed the air space in the resonator, and that the length of the air space across which the sparks would pass varied with the position of the resonator. When the plane of the resonator was parallel with the metal planes of the vibrator and its axis in the horizontal line drawn perpendicularly through the vibrator’s air space, the sparks passed readily when the air space of the resonator was at the same time vertically above or below its centre, but they ceased entirely when it was level with the centre. He obtained these sparks when the resonator was held—in free space, be it understood—in the above-mentioned position even at a distance from the vibrator of 13 yds., the length of the apartment. By examining the results with other positions of his resonator and by other and varied experiments, Hertz was able to prove the existence of definite waves of electro-magnetic and electro-motive forces, to measure their lengths, and to show that they are capable of reflection, refraction, and even polarization by the same laws that hold with the extremely short but enormously rapid vibrations constituting light. It may here be mentioned that the existence of currents in the resonator can be shown by a Geissler tube being made to take the place of the air space, which tube is thus lighted up without any metallic or visible connection with any electrical apparatus whatever, the only requisite conditions being that its circuit be tuned to the vibrator, and in a certain position in relation to the axis of the spark space of the latter. Hertz has also shown that electro-magnetic disturbances (transversal waves) are propagated in space with a determinate velocity akin to that of light, and in short the outcome of his investigations, as well as of those undertaken by others, has been a vindication of Clerk Maxwell’s splendid theory by which light is regarded as an electro-magnetic action. Professor Righi of Bologna, having succeeded in obtaining shorter electrical waves than anyone before—namely, 4/10ths of an inch instead of about 20 inches—was able with them to repeat all the phenomena of optics such as reflection, refraction, circular polarization, interference, &c. It appears then almost certain that light and electro-magnetic waves or radiations are but one and the same affection of a pervading medium we call the ether.

By following up in certain directions lines of research suggested by the investigations of Maxwell, Lodge, Hertz and others, and by an unreserved acceptance of the ether theory of light, electricity and magnetism, some wonderful practical results have recently been obtained by M. Nikola Tesla, an electrical engineer now resident in New York. The experiments shown by Tesla in his public lectures have excited great interest in scientific circles, and have by many persons been witnessed with something like astonishment.

One of the first objects of M. Tesla was to obtain alternating currents of high tension and great frequency. It may be seen from Fig. 272 that the movement of coils of wire in a magnetic field generates currents, and it has been stated that these currents are in alternately opposite directions as the coils approach or recede from the magnetic poles. In the machine represented in Fig. 280_a_, each revolution would produce 16 reversals of current. Tesla constructed a rotatory machine which gave 20,000 alternations of current in one second, because it had 400 poles and could be rotated at a very high speed. But of course the number of poles and the speed of the machine could not be increased beyond certain practical limits. By a happy application of the known principle of harmonic oscillations, in which all the rotatory movements of fly-wheels, coils and poles could be dispensed with, Tesla simplified the alternate current generator, reducing the moving parts to the minimum at the same time that he obtained a greater number of alternations and almost perfect regularity in their periodicity. The way in which this has been accomplished may be gathered from a careful inspection of Fig. 280_l_ compared with the following explanation. This illustration, it should be understood, is merely a diagram in which details of mechanism are altogether omitted, and only so much shown as will serve to explain the principle. We shall take the mechanical part first, and direct the reader’s attention to the means by which an iron rod is made to perform very rapid to-and-fro movements in the direction of its length, and to do that with perfectly isochronous periods, which may be made longer or shorter at will, and which are quite independent of very considerable variations in the motive power. The diagram represents the apparatus in section, and the central part of it marked by letters P and P´ is a piston through which passes what may be called a piston-rod A, which projects some distance out of the cylinder at both ends. The piston is shown in the diagram in its central position, where the impelling power has no action to move it as will presently be seen. This moving power we may assume to be the compressed air applied through the ports I I´. Just to the right of the upper one of these on the diagram will be observed in the piston a slot S opening into a hollow T, which communicates directly with the space on the left of the piston. The same arrangement, with directions reversed, is seen on the other side of the piston. If now the piston were pushed a little to the left of the position shown in the diagram, the compressed air rushing from I through the slot into the opening S T would impel the piston towards the right, and it would be carried onward by its inertia beyond the position shown in the figure towards the right, but in doing this the access of the compressed air on the left would be cut off, and the slot communicating with the space on the right hand would allow the compressed air to act in the space P, checking the further advance of the piston to the right, acting like a spring or elastic cushion, and again driving the piston to the left, during which movement the air that has done its work is allowed to escape at the outlet O O. The same cycle of operations will be rapidly repeated, but the rate of oscillation admits of control, for the larger the air chamber in which the air is compressed by the momentum of the piston and rod, the less will it be compressed and the less powerfully it will resist, while with a smaller capacity of air-chamber the more powerful will be the back spring of the imprisoned air. On the other hand, the mass that is moved may be increased; that is the weight of the rod, &c., may be increased. In any case the oscillations will be perfectly regular, because the force which tends to bring the piston to its position of equilibrium will be always proportionate to its distance from that point. So that we have here a rod shooting in and out shuttle-wise with the utmost regularity and with almost any desired rapidity, controllable under precisely the like conditions as the stretched string already mentioned, for as the tension of the string is the measure of the force with which it strives to regain its position of equilibrium, so the compression of air in the chamber behind the piston; and as the loaded string vibrates slower, so will the loaded piston. So much for the mechanical part of this machine, for we may omit all details of valves, &c. The electrical arrangement is very simple and of the greatest efficiency. On each projecting end of the piston are wound coils of insulated copper wire, which being shot in and out across a powerful magnetic field between the jaws of very large electro-magnets M M´ cut the “lines of force” to the best advantage, and from these coils alternating currents of high tension and frequency are gathered up. The vibrating rod is steadied by working in bearings (not shown). The electro-magnets are actuated as usual by coils of insulated wire surrounding their iron cores. In the motion of the moving coils there are electrical forces called into play which in mechanical effect control the movement in the same way as the air-springs, and as these electrical forces admit of certain adjustments and have calculable effects, the _mechanical period_ of the machine and the _electrical_ one can be made to accord, and thus to, as it were, sustain each other, and assure a perfectly isochronous periodicity, even with considerable variations of the impelling force. Though we have supposed compressed air as the actuating agent, steam has been applied in some slightly modified forms of the machine, and sometimes at the high pressure of 350 lbs. per square inch. Such is Tesla’s alternating current producer, or the _Tesla Oscillator_, as it has been called. This, of course, is a very different thing from the vibrator of disruptive discharge already mentioned in connection with the experiments of Professor Hertz. Tesla also uses the disruptive discharge, and what with the high frequency and the great tension of his currents, he obtains electric oscillations of hitherto unequalled rapidity, calculable at thousands of millions per second. He claims, indeed, to be able to agitate the ether at rates of undulation comparable with those of light itself (500 billions per second). Some of the experiments he has shown certainly lend support to such an explanation. The lighting of electric lamps with but one metallic connection, and that held in a person’s hand, and causing Geissler tubes to light up without any metallic connections whatever, and making gas at ordinary pressures luminous, a lump of charcoal contained in a closed glass vessel to become red-hot while the vessel is merely held in the hand, are certainly phenomena that cannot be explained on the old lines. The space between two large surfaces of metal 15 feet apart, and forming the poles of an oscillatory system, is shown to be full of light-forming influences, as when phosphorescent substances contained in closed glass vessels glow intensely, the glass being apparently no obstacle. According to Tesla, you make space and matter equally permeable to ethereal undulations when these are tuned, so to speak, to the proper frequency.

Many of the strange effects Tesla has shown are referable to the principle of electric resonance; such are the powers of a coil with no metallic connections with any other apparatus and removed, by a distance of many feet, from any current-conveying wires. Tesla’s workshop was an apartment 40 feet long and 20 wide, and the wires connecting the poles of his oscillator were carried round the walls, while in the centre of the workshop stood a very large but entirely insulated coil, between the terminals of which an ordinary incandescent lamp was placed. This lamp was brilliantly illuminated when the oscillator was in action. The electric qualities of this coil were so adjusted that its currents came into tune with the ethereal vibrations propagated from the conductor round the room. But further, a single hoop of copper wire of the proper diameter and thickness could be brought into unison with the coil, and when held in the hand over the latter, even at a considerable distance, incandescent lamps attached to it were lighted up by the induced currents. Many other novel experiments have been shown by M. Tesla, but they need not here be described, as they have yet to be connected with the logical study of the entire class of phenomena. M. Tesla speaks somewhat sanguinely of being ultimately able to convey signals, and even power, to a distance, not merely with one wire but with no wires at all! Another thing he looks forward to is to set the electricity, or rather the ether that interpenetrates the matter of the whole earth, into a state of agitation. This seems what is commercially termed “a large order;” but we have seen that every Leyden jar, every coil, and in fact every electrical system, has its own period, and if by any possibility we could discover, or by chance hit upon the earth’s electric vibration period, it is not antecedently impossible that even the comparatively small efforts of such oscillatory vibrations as we could produce, would by their accumulation agitate the earth’s ether. It is well known that very small impulses, so tuned as to correspond with the natural period of a considerable mass, will produce striking mechanical effects. Thus, a troop of soldiers passing over a bridge have often been known to break down a structure that would have supported their mere weight many times over, because they were all marching together and with a step corresponding in time with the oscillatory period of the bridge. It is now always enjoined in the military orders that troops in crossing a bridge must “break step.” Another familiar illustration of the accumulation of small synchronous impulses is the experiment of singing into a glass goblet the note corresponding with its vibration period. The singer merely by sustaining this note for a short time often succeeds in shivering the glass into fragments. M. Tesla believes that he has already succeeded in agitating the earth’s ether to some extent; he does at least obtain flaming purple streamers passing into the air from one end of a coil, while the other is connected with the earth.

These discoveries and theories appear likely to lead to many unforeseen results, valuable for both science and its applications, and such as may far surpass the expectations of those who take less enthusiastic views of the matter than M. Tesla and his friends do. The theoretical properties of the ether and the conditions of it, which are held capable of making it the scene and the medium of all the hitherto so-called ponderable and imponderable forces, have not been completely worked out. The experiments that have been already made show that disturbances of very different kinds may be propagated in the ether by undulations of any length from less than 1/60000th part of an inch, as in the case of violet light already spoken of, to the 1,200 miles attributed to certain electrical conditions.

The foregoing sentences, describing the discoveries of Hertz and others, had not long been penned before it had become possible to announce that they had borne fruit in as extraordinary an invention as could have distinguished the close of an extraordinary century. It is the realization of what the most accomplished electrician would not long before have pronounced a dream—namely, _wireless telegraphy_. The general principle of it should not be obscure after the account of the “Hertzian waves”; but our space does not permit a description of details of its working out in a practical form by a young Italian electrician, Signor Marconi. We have already seen that a Geissler tube, when its circuit is properly attuned, can be lighted up by the magneto-electric disturbance propagated without material contacts, and this itself would constitute a method of signalling to a distance. On the same principle, a discharge may be determined by the “wave” between conductors in certain adjustable conditions of electric tension, and in this way local circuits may be brought into play, and ordinary telegraphic effects produced, as described in the following article. The actual apparatus to receive the ethereal impulses is extremely simple—merely a little fine metallic dust (nickel and silver) in a glass tube included in the resonator circuit by a wire at each end, touching the dust. This gathers together, or coheres (hence the apparatus is called the _coherer_), under the magneto-electric influence, a local battery discharge then passes, completing a circuit, and the dust has to be shaken loose again by a mechanical agitation. Marconi has been able to signal over a distance of forty-three miles.

THE ELECTRIC TELEGRAPH.

More than two centuries ago a learned Italian Jesuit, named Strada, gave a fanciful account of a method by which he supposed two persons might communicate with each other, however far they might be separated. He conceived two needles magnetized by a loadstone of such virtue, that the needles balanced on separate pivots ever afterwards pointed in parallel directions; and if one were turned to any point, the other also sympathetically moved in complete accordance with it. The happy possessors of these sympathetic needles, each having his needle mounted on a dial marked with the same letters and words similarly inscribed, would be able to communicate their thoughts to each other at preconcerted hours, by movements and pauses of the wonderful needles. The poet Akenside, when describing, in his “Pleasures of the Imagination,” the effect of association in bringing ideas before our minds, illustrates his point by a happy allusion to Strada’s conceit. Here is the passage:

“For when the different images of things, By chance combined, have struck the attentive soul With deeper impulse, or, connected long, Have drawn her frequent eye; howe’er distinct The external scenes, yet oft the ideas gain From that conjunction an eternal tie And sympathy unbroken. Let the mind Recall one partner of the various league— Immediate, lo! the firm confederates rise. ‘Twas thus, if ancient fame the truth unfold, Two faithful needles, from the informing touch Of the same parent stone, together drew Its mystic virtue, and at first conspired With fatal impulse quivering to the pole. Then—though disjoined by kingdoms, though the main Rolled its broad surge betwixt, and different stars Beheld their wakeful motions—yet preserved The former friendship, and remembered still The alliance of their birth. Whate’er the line Which one possessed, nor pause nor quiet knew The sure associate, ere, with trembling speed, He found its path, and fixed unerring there.”