CHAP, xii.] ELECTRICITY AND MAGNETISM. 407 



of lines-of-force through the coil C. Many of these pass 

 into the iron disc. When the iron disc in vibrating 

 moves towards the magnet-pole, more lines-of-force meet 

 it ; when it recedes, fewer lines-of-force meet it. Its 

 motion to or fro will therefore alter the number of lines- 

 of-force which pass through the hollow of the coil C, and 

 will therefore (Art. 394) generate in the wire of the coils 

 currents whose strength is proportional to the rate of 

 change in the number of the lines-of-force which- pass 

 through the coil. Bell's telephone, when used as a 

 transmitter, may there- 

 fore be regarded as a 

 sort of magneto -electric 

 generator, which, by vi- 

 brating to and fro, pumps 

 currents in alternate direc- 

 tions into the wire. At 

 the distant end the cur- 

 rents as they arrive flow 

 round the coils either in , 

 one direction or the other, j 

 and therefore either add f 

 momentarily to or take Fig> l66 



from the strength of the 



magnet. When the current in the coils is in such a 

 direction as to reinforce the magnet, the magnet attracts 

 the iron disc in front of it more strongly than before. If 

 the current is in the opposite direction the disc is less 

 attracted and flies back. Hence, whatever movement is 

 imparted to the disc of the transmitting telephone, the 

 disc of the distant receiving telephone is forced to repeat, 

 and it therefore throws the air into similar vibrations, 

 and so reproduces the sound. 



436. Edison's Telephone. Edison constructed a . 

 telephone for transmitting speech, in which the vibrations 

 of the voice, actuating a thin disc, made it press with 

 more or less force against a button of prepared carbon 



