CONDUCTIVITY OF FILMS 13 



photographic negative. Let S be a stylus travel- 

 ling over the film ; now consider any points 

 PQR, it being supposed that the film has been 

 coated 'on metal foil instead of glass or celluloid. 

 If one terminal of a battery be connected to S, the 

 other to the metal foil, current will flow from S to 

 P, in one instance the reduced silver grains form- 

 ing the image being represented by dots. Now 

 suppose the stylus to be at S', where, owing to a 

 light part of the picture, there is much less deposit 

 of silver. Assuming trie film to be of gelatine (in 

 a moist condition), 

 less current will , s - s ' 



ir ,>, 



flow from S' to R **p~'~"'' : ' "'-' :: ?" ' : '^ 



than from S to "p q ^ 



P, as between S FIG. 2. 



and P there are 



many more granules of silver to render the film 

 more conductive. Hence, if the photograph be 

 rotated on a cylinder, and the stylus trace a spiral 

 path over its surface, the current flowing through it 

 to a receiver should vary in accordance with the 

 depth of silver deposit. I also tried using a relief 

 carbon image on copper foil, the gelatine being 

 saturated with a badly conducting medium, so that 

 the current passing from style to copper base would 

 vary inversely as the thickness of the film ; some 

 fair results were obtained in this -way, but the 

 method would be always very uncertain, as the 



