Now suppose the circuit is closed by some one press- 

 ing the push button. A current flows through the entire 

 circuit. The electromagnet attracts the soft iron arma- 

 ture and pulls it away from the contact point at P, mak- 

 ing a gap or break 

 in the circuit. The 

 current instantly 

 stops, the magnet- 

 ism disappears, the 

 spring (S) brings the 

 armature back and 

 closes the circuit, a 

 current flows again, 

 and everything is re- 

 peated. This back- 

 ward and forward 

 movement of the 

 armature causes a 

 similar vibration of the hammer which strikes the gong 

 on every pull of the magnet. This vibration and result- 

 ing ringing of the bell continues as long as the button 

 is pressed. 



Sources of current. The dry cell has about the same 

 strength as the wet cell, which is still used to some ex- 

 tent. For the usual house circuit two cells will be ample 

 and they should be joined, carbon (inside pole) of one to 

 zinc (outside pole) of the other. Either one of these types 

 of cells needs renewal or replacement from time to time. 

 Where electricity is used for lighting it is possible by in- 

 stalling a special device for reducing its pressure, to use 

 it for the doorbell system. The device used is called a 

 transformer. A transformer made for bell ringing circuits 



The electric bell system. 



