439 



CHAPTER XXXIII. 



SYMPATHETIC VIBRATION AND RESONANCE. 



ympat!ietic vibration. - - If we take two 

 tuning forks (see Fig. 191), tuned to precisely the 

 same pitch, and sound one in the immediate neigh- 

 bourhood of the other, the untouched fork will pick 

 up the sound and vibrate in harmony with it. This 

 is what is called sympathetic vibration. Each fork 

 makes the same number of vibrations per second. 

 When the waves (of condensation and rarefaction) 

 produced by the motion of the first fork batter upon 

 the second, they tend to set it in motion also. Each 

 separate impulse is too feeble to move the fork, but 

 one after another at regular intervals finally sets the 

 second fork in vibration. 



Suppose a child upon a swing and a boy setting 

 the swing in motion. He gives a slight impulse at 

 first, and the swing sways feebly. He waits till it 

 has come back towards him and is just about to sway 

 forwards slightly again. At that instant he gives 

 another push, and waits a similar interval till the 

 swing has again come backwards and is about to 

 move forwards, when he gives another impulse. So 

 in a short time he gets up a good movement of the 

 swing, and can maintain it with slight effort if only he 

 gives the impulse at the proper moment. Suppose, 

 when the swing was half way on its backward course, 

 he gave it a push forwards, he might bring it to a 

 dead stop, and would at least entirely destroy the 

 regular movement. So the tuning-fork, tending to 



o o O 



sway to and fro by condensation and rarefaction 

 reaching it from the sounding fork, is finally set into 



