circuits is removed. After a short interval of time the electricity rushes into the condensers and begins to oscillate, the strength of the oscillations rising, after one or two vibrations, to a maximum and then decreasing; the rate of oscillation finally assumes a steady state. The electricity seems to be separated only along the wires at first, and the circuit vibrates more like a closed organ pipe than an open one.

If a unidirectional primary spark excites oscillations in neighbouring circuits which are slightly out of tune, the phenomenon of electrical beats or interferences can be produced in these circuits, and can be shown by photography.

If the primary spark ceases to be unidirectional and is allowed to oscillate, the oscillations of the primary spark tend to compel those of the secondary or neighbouring circuits to follow them; if they are not sufficiently powerful to do this, they beat with the oscillation of the secondary circuit. Moreover, if all capacity is removed from the neighbouring circuits, they oscillate in tune with the primary circuit, following the latter exactly. The secondary circuits without capacity act like sensitive plates and exactly reproduce every disturbance in the primary oscillating circuit.

In Fig. B, frontispiece, S' represents photographs of the unidirectional primary spark. S is the unidirectional spark produced in a neighbouring circuit, B, from which the capacity has been removed. S" is the oscillating spark in the circuit C; the condenser of this circuit was an air condenser. The spark S shows that no oscillation is concealed by the heavy pilot spark of the exciting spark S'. The hotographs S" show that the unidirectional spark S' can set the circuit C into oscillatory movement, and that this oscillatory movement con

tinues long after the exciting blow has ceased. A careful study of many photographs of this nature shows that a circuit containing capacity and self-induction acts at the first instant as if no capacity were in the circuit. It then begins to oscillate with a higher period than it afterward reaches, acting at first like a closed organ pipe and subsequently like a pipe open at both ends..

In Fig. A, frontispiece, S' represents again the oscillating primary circuit, S the oscillating secondary circuit C. The circuits are nearly in geometrical resonance. Slight beats, however, can be observed. The duration of the secondary is nearly the same as that of the primary.

As we advance in our study of the transformations of electricity we perceive that we are driven off, so to

speak, from wires and conductors into the ether. The electrical manifestations refuse to show themselves on the conductors, except on the extreme outer layers of such conductors, while their most vigorous effects are displayed in the ether of space. We have, moreover, directed most of our attention to the effects at the terminals or ends of conductors. Let us now see if we can detect any form of wave motion along the wires or conductors. Returning to the use of a Ruhmkorff coil or step-up transformer, let us arrange two large plate condensers, a and b, parallel to each other (Fig. 45) and

connect them with the terminals of the step-up transformer, providing a spark gap between B and D; then place two other smaller plates, c and d, opposite the plates a and b, and run long wires from these smaller plates fifty or sixty feet away to a spark gap at J. When a spark jumps across the gap between B and D a spark will also jump at J. This latter spark is due to the rate of change of the electrostatic lines between a and b. Now, on walking near the wires E F and applying the ear very close to them, but not touching them, one can find a point where a peculiar crackling sound is loudest; from this point the

sound fades away in both directions. We have evidently detected a wave of electricity on the wires.

in

B

M

Let us now see if we can make it evident to the eyes stead of the ears. Taking a glass tube which has been rarefied to a great degree, let us rest it on the two parallel wires and move it along them. When it rests at the place where our ears detected the greatest sound, the tube lights up; the molecules in it are set into rapid movement. As we move the tube along the wires we find that the brilliancy of its lighting up diminishes as we go in either direction. We have evidently made manifest to the eyes an electrical wave. Let us return to acoustical analogies. If we should connect a silk thread to one prong of a tuning

FIG. 46.

A

fork (Fig. 46), and support the other end on a cylinder at A, and suitably weight the string, it will vibrate with the fork. It is necessary that the time of vibration of the fork should be arranged with reference to the length of the string; in other words, the string must be tuned to the fork. Now, if we should touch the string at a node B we do not disturb the wave form on the string. If, however, we should touch even by a feather the vibrating portions of the string, we will say at M, the wave form can not be re-established; it is broken up. In the same manner, by suitably lengthening or shortening the wires E F (Fig. 45), we can find places where a conducting wire can bridge the two wires and not impair the brilliancy of the light in the exhausted tube. This conducting wire is then placed at the electrical nodes. In examining the conditions of obtaining electrical waves, we find that there are two principal conditions to be observed the number of lines of force or magnetic ripples which emanate in rapidly expanding circles from every unit of length of the wires E F, and which are thus thrust into the space between the parallel wires E and F, and also the number of lines of electrostatic force between a and b and c and d, not to speak of the electrostatic lines which extend from every unit of length of each of the parallel wires. We find by further study that the time of vibration of the spark at J, or, in other words, the time of electrical surging along the wires, is proportioned to the square root of the product of the magnetic lines and the electrostatic lines per unit length. The spark at B D can be said to be the tuning fork which maintains the waves along the wires. The spark can be likened to a tuning fork in resonance with the fork at B D. If we could now photograph the spark at J by means of a rapidly revolving

mirror, and thus spread out its vibrations so that they could be measured, we could obtain the time of oscillation of the electrical waves along the wires EF; if at the same time we measure the distance between the electrical nodes we should get half the wave length. Thus obtaining the wave length, we could obtain the velocity of propagation of the electricity along the wires. For the distance, l, which we call a wave length, is traversed with the velocity, v, of electricity in the time, t, or, to express this by an equation, lv t. Thus measuring and t we can obtain v. In a subsequent chapter I shall show how this has been accomplished, and how a number has been obtained for v which is very close to that of light-about 186,000 miles per second.

=

In the experiment we have described the wave motion of electricity has been apparently confined to wires or conductors. The question naturally arises, Can this wave motion be transmitted through space without wires? Hertz has shown how to detect these electric waves in the air by means of a circle of wire, the ends of which terminate in a micrometer screw (Fig 47), by means of which one can measure a very small spark gap. The dimensions of this circle are so chosen that its time of electrical vibration is the same as that of the circuit containing the exciting spark.

On moving along a straight horizontal line extending from the middle point, M, between the spark gap of the exciter (Fig. 47), a spark will appear at the spark gap, N, of the resonating circle. If now the resonating circle be moved to and fro between the spark gap M and a large metallic plane surface, S, certain nodal points can be found at which the spark in the resonator disappears. We have to do with electrical waves which emanate from the exciter, strike the metallic plane sur