i 3 4 PHOTO-TELEGRAPHY 



apparatus shown diagrammatically in Fig. 60. 

 The left-hand side shows the transmitter, the right 

 the receiving arrangement. 



D is the drum of a telectrograph as described in 

 Chapter IV., the stylus tracing over a sketch drawn 

 in insulating ink on a sheet of lead foil ; D and 

 the style had a condenser shunted across to prevent 

 sparking. The current from the battery A was 

 interrupted by the lines of the picture, the mag- 

 netism in M being thus intermittent. The relay at 

 M broke the contact of the battery B in circuit with 

 the primary P of an induction coil, S being the 

 secondary, electrical oscillations being set up in the 

 manner already described. 



A negative print was used on the drum D, so that 

 sparking between the balls took place only when a 

 " line " in the picture was in contact with the stylus. 

 The capacity K and inductance J in the oscillatory 

 circuit could be adjusted, and for long distances 

 the aerial and earth would be connected inductively. 



A short wave of about 40 metres was em- 

 ployed, this being determined by the expression 



- = , ) where v = 3 X io 10 cm. 



* v capacity inductance 



= the velocity of electro-magnetic oscillations, and 

 A is the wave-length. 



Turning now to the receiving circuit the aerial 



and earth were connected to the primary of a 



' jigger," the current being transformed down by 



