218 



Our Surroundings 



This movement of the armature, due to the alternate closing and 

 opening of the circuit, causes the strokes on the bell to be repeated 

 rapidly as long as the button is pressed. 



When a doorbell fails to 

 respond to the push of the 

 button it usually will be 

 found that the battery is ex- 

 hausted and must be replaced. 

 If this is not the case, the 

 cause probably lies in some 

 break in the connections. 



The Telegraph. Another 

 well known application of the 

 electromagnet is the telegraph. 

 The word is derived from the 

 Greek stems graph, meaning 

 write, and tele, meaning afar 

 off. The telegraph, then, means 

 an instrument which writes 

 y far away. It was invented by 

 S. F. B. Morse, an American, 

 in 1832. He demonstrated 

 the fact that messages could 

 be sent over a wire between 

 widely separated places by 

 means of a series of clicks 

 representing different letters 

 of the alphabet. Each click 

 made at one end of the line 

 AN ELECTRIC BELL 



caused a corresponding click at 

 the other end, which was in- 

 dicated on a moving slip of 

 paper by a series of dots and 

 dashes. The dash represented 

 a longer time between the 

 clicks than the dot. By combinations of dots and dashes all the 

 letters of the alphabet were represented. The following table shows 



Pressing the push button, W, completes the 

 circuit and causes a current from the bat- 

 tery to operate the electromagnet, V. The 

 electromagnet attracts the armature, X, 

 moving the hammer against the bell. This 

 breaks the circuit at the screw, Z. Imme- 

 diately the spring Y pulls the armature 

 back, again closing the circuit. This action 

 continues as long as the button is pressed. 



