from a Zinc Plate by Ultra-Violet* Light. 299 



If the field o£ force is uniform, the number of ions which 

 reach the positive plate is given by 



n =■ 



w (a — (3)e(«-P )d 



where ?i is the number actually discharged from the plate, 

 a, /3 are constants depending on the electric force and 



the pressure, 

 d is the distance between the plates. 



The resultant positive charge is shown by the motion of 

 the electrometer-needle, but in practice it was found ex- 

 pedient to use an induction-balance, owing to the large range 

 of intensities used and to avoid the inherent defects of the 

 ordinary method of electrometer deflexions. 



The method used is shown diagrammatically in fig. 2. 



Hff. 2. 



TO ELECTROMETER 



TV EARTH 



TO EARTH 



1 1 \^-^^^^^^^^u^^^ 



CC is a parallel-plate condenser consisting of seven plates 

 set up in a brass case which was permanently earthed. The 

 plates c 1 c 2 c 3 were insulated from the other plates, and a 

 short connector from each plate terminated in an insulated 

 mercury cup placed outside the case. The four plates d were 

 connected together and insulated from the case, while some 

 of the plates c x c 2 c 3 were always connected to the insulated 

 quadrants of the electrometer. 



* Prof. J. S. Townsend, Phil. Mag. Nov. 1903. 

 X2 



