ELECTRICAL APPLIANCES AND MACHINES 269 



called the primary, and the coil on the transformer con- 

 nected with the telephone line is called the secondary. 

 The current from the battery goes to the back of the 

 diaphragm in the mouthpiece, from which the current 

 passes through a little chamber which is filled with 

 fine granular carbon, to the conductor of the trans- 



Linc. or 



Ground 



SIMPLE TELEPHONE SYSTEM 



mitter, and thence through the primary, P, of the induc- 

 tion coil and then back to the battery. 



As the diaphragm vibrates it varies the pressure on 

 the many contact points of the granular carbon through 

 which the current flows; this causes considerable 'varia- 

 tion in the resistance of the local circuit. When the 

 diaphragm moves toward the fine carbon a large current 

 flows through it to the coil, P. When the diaphragm 

 moves back from the carbon a much smaller current 

 flows. These changes of flow in the primary coil produce 

 a change in the magnetism of the soft iron core of the 

 induction coil, and there is a like change in the induced 

 or secondary current coming from the secondary coil and 

 passing over the line to the receiver at the other end. 



