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V. 



THE EFFECT OF A LOW POTENTIAL ELECTRIC CURRENT 

 ON PHOTOGRAPHIC PLATES. 



By REV. H. V. GILL, S.J., B.A. (Cantab.), Belvedere College, Dublin. 



(Plates I. and II.) 



[Read May 20. l^ublished July 17, 1913.] 



In this paper I propose to give an account of some experiments made during 

 the course of last year, which seem likely to help towards the solution of 

 certain problems presented by the obscure phenomena connected with the 

 blackening of a photographic plate. I do not at tliis stage propose to enter 

 into a detailed examination or discussion of the results I have obtained, but 

 will content myself with describing the conditions under which the 

 experiments were made and the results obtained. These experiments were 

 carried out during irregular intervals which occurred in the midst of other 

 work, and do not pretend to be exhaustive, but it seems better to publish 

 them such as they are, rather than postpone doing so indefinitely. 



A considerable amount of experimental work has been done concerning 

 the nature of the electric discharge, by causing sparks to pass over the 

 sensitive surface of dry photograpliio plates. The question naturally arises 

 whether the record obtained on development of the plate is to be attributed 

 to some electric reaction, or simply to the luminosity of the discharge. 



It seemed worth while making some experiments with currents which 

 certainly were not luminous, to determine the part played by the purely 

 electric elements of the discharge. Experiments were made in the first 

 instance on dry photographic plates, the source of electricity being a ' water 

 battery ' consisting of 200 small Zu-Cu cells which could be used in 

 multiples of 20. As the potential difference between Zn and Cu may be 

 taken at 0'9 volt, the complete series gave a potential difference of about 180 

 volts. As tested by a gold-leaf electrometer, the voltage of the battery 

 remained constant for many months. The strength of the current from such 

 a battery is very minute, and no attempt was made to measure it. 

 Exposures of from twelve to twenty-four hours were made with this battery 



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