COST OF ELECTRIC POWER 351 



magnet, current is induced and flows through the wire; if the 

 iron is withdrawn, current is again induced in the wire but 

 flows in the opposite direction. As iron approaches and 

 recedes from the magnet, current is induced in the wire 

 surrounding the magnet. This is in brief the principle of the 

 telephone. When one talks into a receiver, Z, the voice 



FlG. 241. Diagram of a simple telephone circuit. 



throws into vibration a sensitive iron plate standing before an 

 electromagnet. The back and forth motion of the iron plate 

 induces current in the electromagnet c. The current thus 

 induced makes itself evident at the opposite end of the line 

 M, where by its magnetic attraction, it throws a second iron 

 plate into vibrations. The vibrations of the second plate 

 are similar to those produced in the first plate by the voice. 

 The vibrations of the far plate thus reproduce the sounds 

 uttered at the opposite end. 



324. 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 

 quantity of electricity and the force under which it flows. The 

 unit of electric power is called the .watt; it is the power fur- 

 nished by a current of one ampere with a voltage of one volt. 



One watt represents a very small amount of electric power, 

 and for practical purposes a unit 1000 times as large is used, 

 namely, the kilowatt. By experiment it has been found that 

 one kilowatt is equivalent to about i^ horse power. Electric 

 current is charged for by the watt hour. A current of one 



