Effect caused hy Incident and Emergent Light, 333 



Method of Experimenting \ 



The saturation current from the illuminated plates to the 

 cylinders could be measured by the following differential 

 method. The keys Gr, H were opened simultaneously. A 

 beam of light was then thrown on the plates until a 

 measurable deflexion of the electrometer was attained. This 

 deflexion was equal to the difference in the saturation currents 

 from A and B. A was then grounded by connecting Gr to 

 earth ; the resulting- deflexion giving the saturation current 

 from B. The difference between these two readings gave 

 the corresponding reading for A. 



The experiment consisted in measuring for various thick- 

 nesses of metal films, the saturation current for two successive 

 positions of the plates (fig. 2). 



Position (1), A and B so placed that their film side faced 

 the " 



Light. 



Fig. 



3 



H 



Direction 



POSITION (1) 



POSITION iZ) 



Position (2), A with the film side towards the light, and 

 B with the film side away from the light. 



Readings were taken alternately for Positions (1) and (2). 

 Thus in every experiment two similar plates with equal 

 thickness of metal films were used, and the ratios of the satu- 

 ration currents A/B for Positions (1) and (2) successively 

 determined. Hence it is seen that the plate B is always 

 compared for each of the two Positions with plate A. Thus by 

 always referring the measurements to A, the standard plate, 

 the otherwise troublesome variations of intensity of the arc 

 were rendered harmless. Unless the films were very thick 

 it was always found that A/B for Position (1) gave rise to a 

 relatively greater photoelectric effect than the ratio A/B in 

 Position (2), although in the latter case the emergent beam 

 of light was obliged to pass through the quartz plate before 

 affecting the film. 



The object of the investigation is the comparison of the 

 ionization due to incident and emergent beams of light of 

 the same intensity. A legitimate way of measuring this 

 intensity is by means of the photoelectric effect such beams 



