238 



ELECTRICITY ON A LARGE SCALE 



the first plate by the voice. The vibrations of the far plate 

 thus reproduce the sounds uttered at the opposite end. 



FIG. 109. Thomas Edison, one of the foremost electrical inventors of the present day. 



Cost of electric power. The water power of a stream 



depends upon the quantity of water and the force with which 



it flows. The electric power of a current depends upon the 



L 



>c 



1 



FIG. no. Diagram of a simple telephone circuit. 



quantity of electricity and the force under which it flows. 

 The unit of electric power is called the watt; it is the 



