576 Prof. 0. W. Richardson and Mr. K. T. Compton on 



The main points wherein the present investigation differs 

 from the earlier work on this subject are as follows : — 

 (1) The contact difference of potential between the emitting 

 and receiving electrodes is allowed for *. (2) The photo- 

 electric cell is placed in position in an arm of a new type 

 of monochromatic ultra-violet illuminator, made by Adam 

 Hilger Ltd., of London, which is a distinct advance as a 

 method of producing accurate and powerful illumination by 

 monochromatic ultra-violet light. (3) The peculiar form of 

 the photoelectric cell practically eliminates electron reflexion 

 without the use of a screen and an auxiliary field, and at the 

 same time it enables the distribution of total kinetic energy 

 to be measured instead of simply the distribution of the 

 velocity component normal to the emitting electrode. 



The apparatus used is shown in fig. 1. The emitting 

 electrode consists of a strip S of metal foil about 1 mm. wide 

 and 5 mm. long placed at the centre of a glass bulb D of 



Fig. 1. 



7*5 cm. diameter. This bnlb is silvered on its inner surface. 

 It is connected with an electrometer having a sensibility of 

 936 divisions per volt, and acts as the receiving electrode. 

 The strip S is held in position by a thin platinum wire sealed 

 in a small glass tube, and is introduced through the ground- 



* See K. T. Compton, Phil. Mag. vol. xxiii. p. 579 (1912). 



