264 



GENERAL SCIENCE 



RELAY 



184. Relay and Sounder. On account of the great 

 resistance in the external circuit of long telegraph lines, 



it is not possible to 

 drive a current of suffi- 

 cient strength through 

 them to operate the 

 electromagnet of the 

 sounder so that the ar- 

 mature is drawn against 

 the post hard enough to make a sound that can be 

 heard distinctly. Therefore an instrument is used which 

 is very similar to the sounder. It is called the relay. 

 The coil of the electro- 

 magnet of the relay is 

 made of many thousand 

 turns of very fine wire, 

 and the movable arma- 

 ture is very light in SENDING KEY 

 order that the feeble 



current going through the coil of the magnet can move it. 



Since the clicks of this light armature on the relay 



cannot be heard, there is at each station a local circuit 



with a local battery, 

 and another instrument 

 called the sounder, which 

 has a heavy armature 

 that can be distinctly 

 heard when moved by 

 a strong electromagnet 

 connected with the local 

 battery. The armature 



on the relay makes and breaks the local circuit so that 

 the sounder repeats the movements of the relay, but with 



