254 Common Science 



the current flowing in the circuit. And each time the 

 current in the circuit is increased, the disk in the re- 

 ceiver is pulled down, just as the sounder of a tele- 

 graph is pulled down. So every vibration of the disk 

 back of the mouthpiece causes a vibration of the 

 disk in the receiver of the other telephone ; this 

 makes the air over it vibrate just as your voice 

 made the mouthpiece vibrate, and you get the same 

 sound. 



To make a difference between slight vibrations and 

 larger ones in telephones, there are some carbon granules 

 between the mouthpiece disk and a disk behind it ; and 

 there are various other complications, such as the bell- 

 ringing apparatus and the connections in the central 

 office. But the principle of the telephone is almost 

 exactly the same as the principle of the telegraph. Both 

 depend entirely on the fact that an electric current pass- 

 ing around a piece of iron magnetizes the iron. 



Experiment 78. By means of your battery, make an 

 electric bell ring. Examine the bell and trace the current 

 through it. Notice how the current passes around two iron 

 bars and magnetizes them, as it did in the telegraph instru- 

 ment. Notice that the circuit is completed through a little 

 metal attachment on the base of the clapper, and that when 

 the clapper is pulled toward the electromagnet the circuit 

 is broken. The iron bars are then no longer magnetized. 

 Notice that a spring pulls the clapper back into place as 

 soon as the iron stops attracting it. This completes the 

 circuit again and the clapper is pulled down. That breaks 

 the circuit and the clapper springs back. See how this 

 constant making and breaking of the circuit causes the bell 

 clapper to fly back and forth. 





