CHAP. III.] 



ELECTRIC TELEGRAPHY. 



553 



the same direction, and then loses itself in the earth. As we shall see 

 further on, the earth plays the part of the return wire, so that the 

 negative pole of the battery of the sending apparatus completes the 

 circuit by the intermediate pieces T K b m K" N. 



When, on the contrary, the handle is turned to the left, the direc- 

 tion of the current is reversed, on account of the position of the 

 points b and #', of which the first presses against the spring K 7 and the 

 second against the spring u, which is separated at the same time from 



FIG. 352. Two needle telegraph. 



the piece M. The path followed by the current is then indicated 

 by the series of letters N K" m b K z' G z L, the line, and then T u I' 

 n R p. The current circulating in the opposite direction, deflects 

 the needle of the sender to the left, at the same time as that of 

 the receiver. 



We see then that the galvanic current, in this system, traverses at 

 the same time, and in the same direction, the galvanometers of the 

 two extreme telegraphic stations. It is interrupted simultaneously 



