136 



PHOTO-TELEGRAPHY 



two dry cells, and the local circuit of this second 

 relay included the receiving circuit of the telec- 

 trograph, as shown in the diagram. 



The use of a contrary current running through 

 the receiver was necessary to render the marks clear 

 and short. 



A local shunt circuit was also used from the first 

 relay to actuate the decoherer, which consisted of a 



To Aerial 



To Earth 



FIG. 62. 



very lightly built electro -magnetic striker which 

 tapped the coherer. This decohesion was later 

 effected in a much simpler manner, the act of 

 striking the coherer closing a local circuit which 

 produced an electrolytic mark on the paper of 

 the receiving drum. 



In the figure,, EE is an electro -magnet, the 

 windings in series with the battery Y, and the 



