326 MODERN ELECTRICAL INVENTIONS 



sounder. For example, the relay is so arranged that current 

 from the main line runs through it exactly as it runs through 

 M in Figure 217. When current is made, the relay attracts 

 an armature, which thereby closes a circuit in a local battery 

 and thus causes a click of the sounder. When the current 

 in the main line is broken, the relay loses its magnetic attrac- 

 tion, its armature springs back, connection is broken in the 

 local circuit, and the sounder responds by allowing its arma- 

 ture to spring back with a sharp sound. 



302. The Earth an Important Part of a Telegraphic System. 

 We learned in Section 299 that electricity could flow through 

 many different substances, one of which was the earth. In 

 all ordinary telegraph and telephone lines, advantage is taken 

 of this fact to utilize the earth as a conductor and to dispense 

 with one wire. Originally two wires were used, as in Figure 217; 

 then it was found that a railroad track could be substituted 

 for one wire, and later that the earth itself served equally 

 well for a return wire. The present arrangement is shown 

 in Figure 220, where there is but one wire, the circuit being 

 completed by the earth. No fact in electricity seems more 

 marvelous than that the thousands of messages flashing along 

 the wires overhead are likewise traveling through the ground 

 beneath. If it were not for this use of the earth as an un- 

 failing conductor, the network of overhead wires in our city 

 streets would be even more complex than it now is. 



303. Advances in Telegraphy. The mechanical improve- 

 ments in telegraphy have been so rapid that at present a 

 single operator can easily send or receive forty words a 

 minute. He can telegraph more quickly than the average 

 person can write ; and with a combination of the latest im- 

 provements the speed can be enormously increased. Re- 

 cently, 1500 words were flashed from New York to Boston 

 over a single wire in one second. , 



