140 PHOTO-TELEGRAPHY 



laterally in a very powerful magnetic field, and a 

 current of io~ 8 amperes will displace it to a con- 

 siderable extent. 



Light from a powerful but steady arc L passes 

 through the condensing lens C so as to form a 

 shadow of the wire AB upon a fine metal slit H. 

 When a current passes through the wire and causes 

 it to shift, the slit is uncovered, and light passes 

 through and illuminates a very sensitive selenium 

 cell SS. A weak current passes through this cell 

 from the battery EI into a sensitive relay R, which 

 also has a high speed of working. When the sele- 

 nium cell is feebly illuminated, its resistance drops, 

 and the current is sufficiently increased to actuate 

 the relay. The local circuit of the relay includes a 

 battery of about 20 volts and the telectrograph 

 receiver O, a contrary current being passed through a 

 resistance into it also, as a shunt in the manner 

 indicated in the diagram. 



As the image is visible, being received by the 

 electrolytic method, it is not necessary to have any 

 synchronising gear on the receiver. A datum line 

 is drawn right across the picture being transmitted, 

 and near the commencement. The reproduction of 

 this line is carefully watched during the reception, 

 and the line is made to lie close against a line 

 drawn across the electrolytic paper. If the line 

 being received diverges from the drawn line then it 

 is known that the receiving drum is travelling either 



