| √t | |
| n = | |
| 2l√w |
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.
Fig. 280k.
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. [280k]) 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.
Fig. 280l.—The Tesla Oscillator.