40 PHOTO-TELEGRAPHY 



selenium circuit flows through the line to the 

 receiver. When it is displaced and connects with 

 Q, the current from the battery has merely to pass 

 through a small fixed resistance, about one -tenth 

 that of the selenium, so that the current is some ten 

 times as great. As soon as the cylinder has turned 

 a little more,, the spring returns into contact with 

 P again. 



Now at the receiving machine we have a similar 

 pin which throws over a spring N from contact with 

 R into contact with S. Whereas R leads to the 

 galvanometer, S leads to one coil of a relay as 

 shown, the other coil of the relay being con- 

 nected with the other unit of the line, together with 

 the other end of the galvanometer. 



What happens then is that every time the pin on 

 the sending cylinder throws the spring against 

 the contact Q, a strong current is transmitted 

 through the line. If at that instant the corre- 

 sponding spring N of the receiver be also thrown 

 against S, the current received passes into the 

 relay instead of into the string galvanometer. 



By making the drum D turn quicker than the 

 cylinder C it will obviously reach the end of its 

 revolution first. It is then suddenly stopped by 

 the check T, which strikes against a movable 

 steel check F. When the slower turning send- 

 ing cylinder has finished its revolution, the 

 strong current is sent, as we have seen, into 



