THE TELEPHONE 



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The telephone. When a magnet is at rest within a closed 

 coil of wire (Fig. 106), current does not flow through the wire. 

 But if a piece of iron is brought near the 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 induced and flows through the wire; if the iron is withdrawn, 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 



FIG. 108. A modern electric machine. 



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, L (Fig. no), the voice throws into vibration a sensi- 

 tive iron plate standing before an electromagnet. The back- 

 and-forth motion of the iron plate induces current in the elec- 

 tromagnet c. The current thus induced makes itself evident 

 at the opposite end of the line Af, where by its magnetic 

 attraction, it throws a second iron plate into vibration. The 

 vibrations of the second plate are similar to those produced in 



