114 



THE ELECTRIC TELEGRAPH. 



In order to avoid the derangement incidental to the dis- 

 charge currents passing through the relay, the plan Fig. 60 

 is generally adopted. 



When the line L is brought, by means of the switch I, 

 into contact with the point s x , the relay G is in circuit for 

 receiving despatches. When I is in contact with the point 

 s a , the key D is put into circuit for forwarding despatches, 

 and the relay cut off from the line and prevented receiving 

 the discharge currents. 



If, in this position, the key D is pressed down on the 

 front-contact 3, a positive current is sent from the main 

 battery -f K ; but whilst the key rests upon contact 2, a 

 negative current is sent from the counteracting battery K 

 into the line. The discharge currents are thus made use of 



Fig. 61. 



in signalling. Lastly, in order to prevent the final discharge 

 passing through the relay when communication is re-esta- 

 blished with s u a contact is made by L with s 3 and earth in 

 passing by, so as to discharge the line. 



The mechanical construction of the key is shown in the 

 plan Fig. 61. The bearings b, b, in which the lever c works, 

 are cast on a base, b lt movable horizontally on an axis in 

 the board , in permanent contact with the terminal 1 and 

 the line wire, and also with the spring i. When the key is 



