THE TELEPHONE 



339 



receiver at the other end and produce corresponding varia- 

 tions in the vibrations of the diaphragm. 



The induction coil just referred to is another addition to 

 the original telephone. This is necessary in order to pro- 

 duce a current of sufficient force to operate over long dis- 

 tances. The current is usually furnished by a battery at 

 the central stations, instead of by a local battery. 



The telephone is a very sensitive instrument. In some 

 cases the distance through which the diaphragm moves is 



Tine 



Transmitter 



H-nii Receiver 



FIG. 140. Local battery telephone system. 



only one ten-millionth of an inch. To send a telephone 

 message requires less than a hundred-millionth part of the 

 current required for a telegraphic message. So small is the 

 amount of current necessary to operate the receiver, that if 

 all the work done in lifting the receiver off the hook one foot 

 were changed into electric energy, it would furnish enough 

 electricity to operate the receiver continually for 100,000 

 years. 



DEMONSTRATION 26 



Purpose. To study the working of the telephone. 



Apparatus. Galvanometer, bar magnet, coil of wire wound 

 in form of a spool. Demonstration form of dissectible tele- 

 phone. 



Directions, i. Secure a coil of wire wound on a spool and 

 provided with binding posts. Connect these with a galvanom- 

 eter. Place a bar magnet inside the coil. Bring a piece of 

 soft iron near the magnet. Does the needle move? Take 

 the iron away. Is any current generated ? 



