324 



SCIENCE OF HOME AND COMMUNITY 



Earth 



FIG. 132. Diagram of relay 

 telegraph circuit. 



line are attached to pieces of metal sunk in the ground so 

 as to make good connections. 



The relay (figures 132 and 133) is somewhat like the 

 sounder. It consists of an electromagnet and an armature. 

 When a current passes through the 

 magnet, the armature is drawn down 

 and contact between two points is 

 made. This closes the local circuit, 

 and the local battery is brought into 

 play, which operates the sounder. By 

 means of these relays, telegrams may 

 be sent long distances, even across 

 the continent. The message may be 

 sent to one city, as from New York 

 to Chicago, and then the message can 

 be repeated at Chicago and sent to 

 San Francisco. 



Duplex telegraphy. When the telegraph was first used, 

 only one message could be sent at a time over the wire ; but 

 in a few years the system of duplex telegraphy was invented 

 by means of which two messages could be sent at the same 

 time in opposite directions. 

 Then, later, diplex telegraphy 

 made it possible to send two 

 messages in the same direc- 

 tion at the same time on one 

 wire. The next development 

 allowed two messages to be 

 sent from each end of the line 

 at the same time, or four in all. Now we have the multiplex 

 telegraphy by means of which many messages can be sent in 

 both directions at the same time. It has been found possible 

 to send thirty-six messages in each direction at the same 

 time. In actual practice, however, it has been found best 

 to limit this to six in each direction. For this work twelve 



FIG. 133. Telegraph relay. 



