THE ELECTRIC TELEGRAPH. 



Thus, by alternately moving the hands A" and A between the 

 contact pieces p and K, the current may be changed from one 

 direction to the other on the up wire as often and as rapidly as 

 may be desired. 



The same reversion may be made in exactly the same manner 

 on the down wire, if the hand A' be turned upon D. 



The reversion may be made with equal facility and rapidity if 

 the current be established along the entire line by merely inter- 

 changing the position of the hands directed upon P and N, as 

 described in 115. 



117. To suspend and transmit alternately the current during 

 any required intervals. Whether the current be established on 

 the up line or on the down line, or on both, this is easily accom- 

 plished by removing any one of the hands from the contact piece 

 on which it rests, and restoring it to its place after the required 

 intervals. When it is withdrawn, the current is suspended; 

 when restored, the current is re-established. The intervals of 

 such suspension and transmission may be as long or as short as 

 may be desired. They may be equal or unequal. They may 

 succeed each other with any degree of rapidity whatever. Thus 

 there may be ten thousand intervals of suspension and ten 

 thousand of transmission in a minute. The instantaneous cha- 

 racter of the propagation of the electric fluid already noticed will 

 sufficiently explain this. 



118. Having thus explained how the agent controls the current 

 in transmitting signals to a distant station, we shall now show 

 how he treats the current which arrives from a distant station, so 

 as to allow it to produce before him the intended signals. 



The current must arrive either by the up wire or by the down 

 wire, and therefore at either of the contact pieces, u or D. 



119. To make the arriving current give the alarm. When the 

 agent at a station is not engaged in transmitting signals, he must 

 always be prepared to receive them. A contrivance called an 

 alarum is provided, to give him notice when signals are about to 

 be transmitted. The alarum, which will be fully explained here- 

 after, is an apparatus so constructed, that whenever the current 

 passes through it, a bell is rung, by which the attention of the 

 agent is called. 



The contact piece B is here supposed to be connected with a 

 wire leading to such an apparatus. 



When not engaged in transmitting signals, the agent connects 

 both the up and down wires with his alarum. To accomplish 

 this, he turns A' upon u, and A upon B. The contact piece B 

 being supposed to be connected with the wire which enters the 

 alarum, the wire which issues from it is connected with B'. Two 

 182 



