Prof. Minchin's Experiments in Photoelectricity. 221 



Dispersion of the Residual Effect. — On the withdrawal of the 

 light, the fall of E.M.F. in the cell is usually much slower 

 than the rise of E.M.F. on exposure ; and this fact would 

 constitute a grave inconvenience if there were no speedy 

 remedy. The effect of the light can, however, be quickly and 

 satisfactorily overcome by connecting the exposed plate with 

 the copper, and the unexposed with the zinc pole of a Daniell 

 cell for a few seconds — the time of connexion being longer as 

 the time of exposure of the plate to light was longer. In fact, 

 a series of three or four impulsive contacts with the poles of 

 the Daniell, followed by a few seconds' short-circuiting, will 

 suffice to remove the residual effect of light, and to leave the 

 spot on the electrometer-scale at the point from which it 

 started. 



This result is important, because when feeble light, such as 

 that of a candle, falls on the cell, the maximum E.M.F. takes 

 some minutes to develop, and the return of the spot on the 

 scale would occupy a long time. 



Variation of the Effect with the Distance of the Source of 

 Light. — Six cells connected in series were placed on circles of 

 varying radius, and a candle was in each case placed at the 

 centre of the circle. The E.M.F. developed by the light of the 

 candle was, with fair accuracy, found to be inversely propor- 

 tional to the distance of the candle from the cells. As the 

 intensity of the light varies inversely as the square of the 

 distance, it follows that the square of the electromotive fore 

 is proportional to the intensity of the light. 



Curve of Rise of E.M.F. — The law of increase of E.M.F. 

 during exposure was studied by placing a " standard " candle 

 at a distance of 6 inches from 6 cells connected in series, the 

 poles of the series being connected with a Thomson quadrant- 

 electrometer. The deflexions on the scale were noted every 

 quarter minute, and a curve w T as traced having the deflexions 

 for ordinates and the times for abscissae. The maximum 

 E.M.F. attained was *566 of that of a Minotto cell, giving for 

 each cell '094 of this amount. 



If we denote by A the maximum E.M.F. developed, and by 

 7] the E.M.F. at any time t, it would appear to be legitimate 

 to assume the equation 



J-4CA-,), 



where k is a constant. This gives, by integration, 



9; = A + Be ? 

 where B is another constant. The curves actually traced in 



