68 PHOTO-TELEGRAPHY 



were always illuminated by a constant amount of 

 light, a sensitive photographic film wrapped round 

 the drum DI would receive a long, thin, spiral line of 

 exposure . 



Now, however, imagine a line drawn from the 

 centre of the filament of the Nernst lamp N to the 

 slit, this line produced coinciding with the optic 

 axis of the small sliding lens in the dark box. A 

 condensing lens C projects the light from N upon 

 the ebonite screen in which the slit is fitted. 



The light passes through holes bored in the pole- 

 pieces of a powerful electro -magnet M, across the 

 field of which is stretched a single flat silver wire 

 WW, and the shadow of this wire is thrown sharply 

 over the slit, the adjustment being carried out by 

 means of the sliding lens L. The magnet is excited 

 by 1 1 o volts i ampere from a battery of secondary 

 cells, and the current from the connecting telephone 

 line is passed through WW. 



When current flows, this wire is laterally dis- 

 placed, and consequently its shadow rises above the 

 slit and allows the light from N to reach the 

 sensitive film on DI. Should the current received 

 be more than is required just to uncover the slit, 

 the necessary amount of extra resistance may be 

 inserted in series with the line. 



A small battery opposing the line current may be 

 inserted at E as shown, and a regulating resistance 

 R shunted across the galvanometer. By means of 



