IMPRO VEMENTS 



325 



tated by the motion and the resulting clicks of the sounder. 

 By means of these varying clicks of the sounder, the opera- 

 tor interprets the message. 



The Relay. When a telegraph line is very long, the re- 

 sistance of the wire is great, and the current which passes 

 through the electromagnet is correspondingly weak, so feeble 

 indeed that the armature must be made very thin and light 



FIG. 220. Diagram of a modern telegraph system. 



in order to be affected by the makes and breaks in the 

 current. The clicks of an armature light enough to respond 

 to the weak current of a long wire are too faint to be recogniz- 

 able by the ear, and hence in such long circuits some device 

 must be introduced whereby the effect is increased. This is 

 usually done by installing at each station a local battery and 

 a very delicate and sensitive electromagnet called the relay. 

 Under these conditions the current of the main line is not 

 sent through the sounder, but through the relay which opens 

 and closes a local battery in connection with the strong 



