with the Brush Discharge . 51 



6. Action of the Brush-Discharge upon Photographic Plates, 



The productions of chemical actions by the brush-dis- 

 charges immediately gave one the desire to try the action 

 upon photographic plates. The first experiments of this kind 

 were made shortly after the discovery by Rontgen ; but as 

 they did not lead to any results which were of such general 

 interest as the latter, the study was discontinued for a time 

 but taken up again recently. 



In these experiments, as in the former, the brush from the 

 machine gives better results than that from the coil, but the 

 positive seems to be more effective than the negative. In 

 the latter case a longer exposure is necessary, and sometimes 

 less definite results are obtained. Films of different kinds 

 were used, but because of the greater ease in development, 

 ordinary " slow " plates were most frequently employed 

 ("Ilford Ordinary"). 



Action of Brush-Discharge upon a Sensitive Film. 



If a photographic plate be placed on the table in a dark 

 room with the uncovered film upwards, while the positive or 

 negative brush from a machine or coil be arranged at some 

 distance (say 4 feet) above it and the point turned towards it ; 

 the plate after development will be found to be " fogged," 

 showing that a decomposition of the silver salts has been 

 brought about, similar to that produced by exposure to light. 

 With the brush from the positive terminal the reduction of the 

 silver compound is fairly uniform all over the plate, but when 

 the negative is used there are " blotches " in several places, 

 showing that the reducing action has taken place in some 

 spots more than in others. There is therefore produced at 

 the point an emanation of some kind, whether it be an undu- 

 latory movement or a stream of particles, which possesses the 

 power of reducing silver salts. 



If between the point and the sensitive film a solid object 

 be placed, a shadow of the object will be thrown upon the 

 plate. This shadow is sharp if the object is close to the 

 plate, but its edges are ill-defined if it be at some distance 

 from it, in this particular exactly resembling light. PI. I. 

 (figs. 1 and 2) shows this, the object being a piece of sheet- 

 zinc cut into the shape of the letter H and placed firstly at 

 1*5 centim. away from the plate, and secondly at 10 centims.^the 

 point being about 30 centims. away and the potential about 

 35,000 volts. By measuring the size of the object and images, 

 and the distance between them, it is easy to find the position 

 of the source from which the rays emanate. When this is 

 done it is found that the rays proceed from the extreme end 



E 2 



