﻿2C>2 On Photo-Electric Fatigue. 



sncsess, to establish some relation between photo-electric 

 fatigue and contact potential *. Lenard f came to the con- 

 clusion some time ago that such a surface-layer must exist. 

 The following experiments seem to argue in favour of the 

 existence of such a layer. 



It was found that the final positive potential taken up by 

 the zinc electrode under the action of light was by no means 

 constant. Starting with it at the same potential as the metal 

 cylinder surrounding it, i. e. earthed, its potential as registered 

 by the deflexion of the electrometer-needle rose rapidly at 

 first, after the light was put on, and then rose much more 

 slowly (fig. 2). The typical form of the curve can be divided 

 into these two parts, of rapid rise and of slow rise. In all 

 curves, the first part was always of the same form, but the 

 second part varied very much, especially if the light was 

 allowed to fall on the metal for a long time. Sometimes it 

 remained parallel to the axis of time for some minutes and 



then commenced to rise : at other times it increased gradually 

 "... ~ . 



all the time, and sometimes it increased very slowly at first 



and then suddenly began to rise rapidly. It might be men- 

 tioned that this was also found to be the case when mono- 

 chromatic light was used, so that the light was of small 

 intensity. 



A satisfactory explanation of this has not been attempted, 

 but very probably it has something to do with a double layer 

 at the surface of the metal. 



The conclusions to be drawn from this paper are entirely 

 different from Allen's. Here a real photo-electric fatigue has 

 been observed. The point to be considered in connexion with 

 Allen's results is whether the main thing he investigated was 

 photo-electric fatigue, or whether the fatigue he observed 

 was merely of secondary importance, occurring as a result of 

 some other effect. Very possibly the surfaces of his electrodes 

 were not in a stable condition just after polishing, and some 

 sort of readjustment of the surface sets in immediately. This 

 would occur quite independently of the action of light or of 

 the electric field. A similar phenomenon has been observed 

 by the present writer. When quartz-fibres are made con- 

 ducting by depositing a metal on them from a cathode in a 

 discharge-tube, the electrical resistance of the fibre varies 

 very much after coming into the atmosphere. When the 

 fibre is coated with platinum the resistance increases with 

 the time, and when it is coated with copper it may become 



* Loc. til. 



t Ann. der Phys. xii. p. 449 (1903). 



