CHAPTER XVIII.

ELECTRIC WAVES.

WHEN We survey the practical development of electricity, which I have outlined, we are struck with the fact that our minds have been led from a consideration of steady currents of electricity and the phenomena produced by them to what may be termed unsteady or periodic currents. The transformations of energy which are possible with periodic or alternating currents are far more varied than those we can accomplish with steady currents. It would seem that even the development of the applications of the alternating current suggests the electro-magnetic theory of light. The swifter the rate of alternation of our alternating dynamo, the nearer we approach to the manifestations of light and the more varied become the electrical phenomena. This seems to me the most remarkable conclusion to be drawn from Tesla's experiments on high frequency discharges. If we could excite electrical currents which would oscillate some billions of times a second we might produce the sensation of light on the retina of the eye without a spark.

The ordinary Leyden jar is the swiftest alternating machine which we can use at present. Joseph Henry showed conclusively, in 1840, that the discharge of a condenser is, in general, oscillatory. His observations

on this oscillation form an epoch in the study of electricity, and the attention of the scientific world is now closely directed to the manifestations which he discovered, and which Lord Kelvin in 1850 expressed in a mathematical law which forms the basis of Hertz's celebrated work.

The rate of alternation of the Leyden-jar machine depends upon the capacity of the jar and the self-induction of the wire connecting its outer coating with the inner coating. The swiftest rate of alternation we can obtain from an alternating dynamo is barely one hundred thousand alternations per second, and this rate has practically not been reached. With a Leyden-jar discharge we can obtain and make evident by photography the rate of ten millions per second. The lightning discharge is a discharge from the Leyden jar formed by the layers of cloud and the earth, and its destructive effect is in part due to its rapidity of oscillation.

Now two Leyden jars with their circuits of wire can be electrically tuned so as to be in unison with each other, and when one jar is discharged the neighbouring one, which has not been charged, will also give a spark arising from what is termed electrical resonance.

It is important that we should obtain a clear idea of what may be termed electrical tuning or the obtaining of resonance. Let us, in the first place, examine what is termed resonance in the subject of sound. One of the simplest methods of showing acoustical resonance is to sound a tuning fork over the mouth of a long vertical jar and then slowly pour water into the jar. At certain points the column of air in the tube will vibrate in resonance with the tuning fork and a great augmentation of sound results. The particles of air swing

in tune with the prongs of the fork. The best way of showing this phenomenon is to immerse a large glass tube open at both ends in a larger glass jar which is filled with water. By moving the inner glass tube up or down one can lengthen or shorten the column of air at pleasure and thus tune it to the fork. Another simple method showing the effect of resonance consists in mounting two forks which give the same note on hollow boxes closed at one end and open at the other, and placing these boxes with their open ends close together. If the two forks are exactly in tune when one is excited by a violin bow the other will respond, and will continue to vibrate when the exciting fork is brought to rest. If the forks are not in tune they can be brought into resonance by loading the prongs of one of the forks with a little piece of wax. Notes suitably struck on any stringed musical instrument can excite similar notes on another stringed instrument. Electrical tuning or the obtaining of electrical resonance depends upon conditions analogous to those which obtain in the subject of acoustics, and can be illustrated best by considering the photographs of electric sparks (Plate I, frontispiece).

Let us now endeavour to arrange the electrical circuits which shall be in resonance. It will be necessary first to obtain the time of oscillation of one circuit, and then arrange another circuit which shall have the same time of oscillation; the latter circuit will then be in resonance with the first circuit. One obvious way to accomplish this would be to discharge a Leyden jar through the circuit and photograph the spark which is thus produced by means of a revolving concave mirror. Knowing the speed of the revolving mirror and the distance to the sensitive plate, we can obtain the time

of vibration of the circuit, and then we can arrange another electrical circuit which will have the same number of vibrations. If these two circuits are then placed parallel to each other, even twenty feet apart, a spark through one circuit will excite a spark in the other circuit. A simple way of arranging this experiment is as follows: An electrical machine is employed to charge a Leyden jar, A (Fig. 44). The accumulated charge equalizes itself between m and n around the cir

cuit which connects n with the outside of the jar. At the instant a spark passes between m and n a spark is seen to jump between o and p in the circuit connected with the jar B. As I have said, these circuits can be placed from ten to twenty feet apart, and can be made to respond to each other. Two principal factors enter into the phenomenon of electrical resonance: the arrangement of wire in the coils which are opposed to each other, and the number of Leyden jars-or, in other words, the amount of capacity which is connected with these coils. Thus in the above experiment we have the coils and the Leyden jars. The latter serve to accumu

late the charge of electricity and to discharge it through the coils.

When we see an electric spark we must reflect that a magnetic wave reaches our eyes at the same instant as the light. Its velocity in the ether is the same as that of the light rays. When a spark occurs in one circuit a spark will also occur in another circuit; it may be across the room, if the latter circuit is parallel to the first circuit and properly tuned to this circuit. The energy of the first spark is conveyed through the ether in magnetic waves to the second circuit. The second spark appears apparently at the same instant as the exciting spark. The velocity of propagation of the magnetic waves which produce the spark is probably the velocity of light. The velocity of electricity should be measured in free space, and not on conductors, for on metals its propagation is retarded, and it takes time for the current to arrive at its greatest strength. Thus, if we have two parallel circuits with a battery and key in one of the circuits, and if we touch the key so as to send a current along one circuit, we cause lines of force to spread out in circles from the circuit, and these lines cause a current in the neighbouring wire in the opposite direction to the exciting current. The circles of force emanating from this second circuit embrace the first circuit and set up a current in it opposed to the exciting current.

In a paper on the oscillations of lightning discharges,* I expressed the opinion that the method first employed by Spottiswoode, of exciting a Ruhmkorff coil or transformer by means of an alternating-current dynamo, put in the hands of an experimenter a far

* Phil. Mag., October, 1893.