Prof. Minchin's Experiments in Photoelectricity. 227 



great resistance, S, of the same order of magnitude as the 

 resistance, R, of the photoelectric cell itself-—/, e., several 

 megohms. This resistance S is composed of lead lines traced 

 carefully on glass and then covered with shellac, and is, in 

 my experiments, something like 10 megohms. The poles, 

 A, B, of the cell being connected with the electrometer, if e 

 is the (disturbing) E.M.F. of the cell in the dark (which 

 may be zero or very small), and E that of the Daniell, we 

 shall obtain a deflexion, A, given by the expression 



the signs + being taken according as e produces a deflexion 

 in the same sense as E or in the opposite sense. This is on 

 the assumption that e is not modified by E, which is possibly 

 false, but not material to the result. Whether e is or is not 

 modified by E, it is clear that if S is very small compared 

 with R, the deflexion on the scale will simply indicate E, no 

 matter how great e may be ; and hence if light is allowed to 

 fall on the cell with this arrangement, there will he no indica- 

 tion of its effect on the scale. But taking S of the same order 

 of magnitude as R, we obtain, when the cell is in the dark, a 

 deflexion of, say, half the amount produced by the Daniell 

 alone. When the connexions are those indicated in fig. 4, 

 i. e. when the Cu pole is connected with the sensitive plate, 

 if light is allowed to fall on the cell a very large deflexion of 

 the spot (of course in the direction opposed to A) is produced. 

 If after this we reverse the connexions, i. e. connect the Zn 

 pole with the sensitive plate, and allow the spot to settle to 

 its position of rest in the dark, and then let the light fall on 

 the cell, the deflexion produced by light is very ranch smaller 

 titan before. To quote a particular case — when the Cu pole 

 was connected with the sensitive plate and the spot came to 

 rest, the spot was deflected from this point through 260 

 divisions on the scale ; and wdien the connexions were 

 reversed and the spot again allowed to come to rest, it was 

 deflected from the point of rest through only 50 divisions. 



From the above expression it is obvious that, in the first 

 mode of connexion with the Daniell, the deflexion from the 



zero produced by light is e—\+^ (E — e + X), where X is 



ivt o 



the E.M.F. due to light, on the supposition that R is con- 

 stant, so that the observed deflexion on the scale from the 

 point of rest due to the Daniell is 



M 1— ., ^ V or 



H&> 



R+JS 



