1894.] 



on the Work of Hertz. 



323 



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 



Fig. 1. 



Fig. 2. 



n\ 



Oscillations of Dumb-bell Hertz 

 Vibrator (after Bjerknes). 



II II I 



Oscillation of Ring-shaped Hertz 

 Resonator excited by the syn- 

 tonic Vibrator which gave the 

 curve Fig. 1 (after Bjerknes). 



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 ethereal 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. 



Fig. 3. 



Oscillation of Ring Resonator similarly excited but not quite syntonic with 

 Radiator. (For method of obtaining these curves see Fig. 14.) 



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. 



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 



z 2 



