64 PHOTO-TELEGRAPHY 



shaft is cut with a thread, about 20 threads per 

 centimetre ; W represents the cog wheel of the 

 motive arrangement, so that as the shaft turns the 

 nut N slides along and so imparts to the stylus S a 

 lateral motion, hence the stylus traces a spiral path 

 over the surface of the picture, which is fixed round 

 the revolving drum D. 



The motive power is again a motor with two 

 slip rings fitted to the armature, so that alternating 

 current is generated for the actuation of a frequency 

 meter. The motor turns at 3,000 revolutions per 

 minute, and the speed is geared down so that the 

 cylinder revolves once in two seconds. As the 

 receiving apparatus is placed on the same table as 

 the transmitter, and contains as its essential part 

 the string or Einthoven galvanometer, it was found 

 necessary to mount the motor on a separate stand 

 and to connect it with flexible shafting. In the 

 most recent apparatus the motor and gear box are 

 mounted together on a stand fixed rigidly to the 

 wall, and flexible shafting connects the gear box 

 with the cog wheel that engages with the wheel W 

 shown in the diagram. 



The dimensions of the cylinders used at London 

 and Manchester are 63*5 mm. diameter and 

 130 mm. length. In the apparatus first installed 

 by Professor Korn at Berlin and Paris he employed 

 a small cylinder the same size as the receiving 

 drum, but it is a considerable advantage to have 



