21 I Prof. Minchin's Experiments in Pliotoelectricity. 



but which was in the state in which it was obtained from 

 the manufacturers, were fixed on plates of glass and placed 

 in the cell. One of them being screened from light and the 

 other exposed, a very strong current was the result. When 

 the tin plates were replaced by copper, a current, but of less 

 strength, was also produced. Gold plates gave no current 

 at all. It was found, however, that the fluorescine was 

 unnecessary, and that when common tap-water was used in 

 the cell, the currents produced by light seemed to be no 

 weaker than before. If the hand or any other screen were 

 moved rapidly in front of the exposed plate, the spot on the 

 galvanometer-scale moved correspondingly. A telephone 

 was put into the circuit with a view to the production of 

 sound by variations in the incident light, but the alterations 

 were not sufficiently rapid to produce this result. A battery 

 of three cells in series was then formed, but no sound was 

 produced in the telephone, and, moreover, the current 

 indicated by the galvanometer was no greater than when 

 only one cell was used. This latter fact seemed most extra- 

 ordinary ; but, on examining the cells separately, I found 

 that one of the cells was producing a current in a sense 

 opposed to that of the current given by the other two. Of 

 this more presently. 



When the liquid in the cell was distilled water, currents 

 were still produced by the action of light on the tin. When 

 a drop of sulphuric acid was added to the water, the currents 

 ceased. 



In nearly every cell that I used with tin plates — whose 

 surfaces, as stated above, had not been treated in any way — 

 the exposed plate was positive to the unexposed ; but, after a 

 time varying from a few minutes to a few hours, it was 

 found that this positive current died out and was replaced 

 by an apparently stronger current, in which the exposed 

 plate was negative. Thus there was a change in the sign of 

 the E.M.F. produced by the continuous action of light. 

 This again reminds us of M. Becquerel's observation about 

 the thickuess of sensitive layers. There seemed to be almost 

 no exception to the rule that the exposed plate begins by 

 being positive and ends by being negative, the negative 

 regime lasting for many days of prolonged exposure to 

 light. 



It was also found that when the chloride of any substance 

 was dissolved in the distilled water of the cell, all photo- 

 electric action ceased. Again, if the tin foil is cleaned by 

 immersion in sodic hydrate and then in hydrochloric acid, or 

 by immersion in a solution of either of the chlorides of tin, 



