220 



Our Surroundings 



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B 



H|lH 



Relay 



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A TELEGRAPH CIRCUIT 



I and II represent two telegraph sets connected by a wire. If it is desired to send a 

 message from II to I, the switch E of set I is closed. Then when key A in set II 

 is pressed it completes the main circuit of the two sets and allows an electric current 

 to flow around the coil of the electromagnet in the relay of set I and into the ground 

 at F. This current operates the electromagnet in relay I, causing it to attract the 

 armature located at its left, thus completing the circuit of set I. This allows a power- 

 ful current from battery C to flow through the circuit of set I and operate the electro- 

 magnet in sounder B, attracting the sounder armature at the top. When the switch 

 of set II is closed and the key of set I is pressed, the process is reversed. In this 

 way every movement of the key of one set causes a corresponding movement in the 

 sounder of the other set. 



The Relay. When the current is not strong enough to work 

 a distant sounder, a relay is used. This relay consists of a light 

 armature controlled by a powerful electromagnet, and so deli- 

 cately adjusted that it responds to even a very weak current. 



Key. 



TELEGRAPH INSTRUMENTS 



Sounder. 



Relay. 



When the key in the main circuit is pressed, the weak current 

 causes the electromagnet in the relay to draw its armature into 

 contact with a fixed screw. This closes a local circuit and allows 

 a powerful local battery to operate the sounder. Thus, by the 



