If, on the other hand, the receiving body has a persistent period of vibration, continuing in motion long after it is left to itself (Fig. 2) like another tuning fork or bell, for instance, then far more facility of response exists, but great accuracy of tuning is necessary if it is to be fully called out; for if the receiver is not thus accurately syntonised with the source, it fails more or less completely to resound. [Illustration: FIG. 2.--Oscillation of Ring-shaped Hertz Resonator excited by Syntonic Vibrator (after Bjerknes).] [Illustration: FIG. 3.--Oscillation of Ring Resonator not quite syntonic with Radiator. (For method of obtaining these curves see Fig. 14.)] Conversely, if the _source_ is a persistent vibrator, correct tuning is essential, or it will destroy at one moment (Fig. 3) motion which it originated the previous moment. Whereas, if it is a dead-beat or strongly-damped exciter, almost anything will respond equally well or equally ill to it. What I have said of sounding bodies is true of all vibrators in a medium competent to transmit waves. Now a sending telephone or a microphone, when spoken to, emits waves into the ether, and this radiant energy is likewise able to set up vibration in suitable bodies. But we have no delicate means of directly detecting these electrical or etherial waves; and if they are to produce a perceptible effect at a distance, they must be confined, as by a speaking-tube, prevented from spreading, and concentrated on the distant receiver. This is the function of the telegraph wire; it is to the ether what a speaking-tube is to air. A metal wire in air (_in function_, not in details of analogy) is like a long hollow cavity surrounded by nearly rigid but slightly elastic walls. _Sphere charged from Electrophorus._ Furthermore, any conductor electrically charged or discharged with sufficient suddenness must emit electrical waves into the ether, because the charge given to it will not settle down instantly, but will surge to and fro several times first; and these surgings or electric oscillations must, according to Maxwell, start waves in the ether, because at the end of each half-swing they cause electrostatic, and at the middle of each half-swing they cause electromagnetic effects, and the rapid alternation from one of these modes of energy to the other constitutes etherial waves.[2] If a wire is handy they will run along it, and may be felt a long way off. If no wire exists they will spread out like sound from a bell, or light from a spark, and their intensity will decrease according to the inverse square of the distance.