Discoveries and Inventions of the Nineteenth Century

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, 542having succeeded in obtaining shorter electrical waves than anyone before—namely, 410ths 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. Fig. 280l.—The Tesla Oscillator. Fig. 280m.—M. Nikola Tesla. 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. 280a, 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. 280l 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 544with 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, 545sustain 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 546small 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