Just as a pebble thrown into a pond excites surface ripples, which can heave up and down floating straws under which they pass, so a struck bell or tuning fork emits energy into the air in the form of what are called sound waves, and this radiant energy is able to set up vibrations in other suitable elastic bodies.
If the body receiving them has its natural or free vibrations violently damped, so that when left to itself it speedily returns to rest ([Fig. 1]), then it can respond fully to notes of almost any pitch. This is the case with your ears and the tones of my voice. Tones must be exceedingly shrill before they cease to excite the ear at all.
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.
Fig. 2.—Oscillation of Ring-shaped Hertz Resonator
excited by Syntonic Vibrator (after Bjerknes).
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.