﻿Photo-Electric Fatigue. 261 



the light is shut off it begins to recover its original activity 

 again, only becoming fresh again after a few minutes ; 

 (2) that the metal is fatigued only when it gives off* electrons 

 under the action of ultra-violet light. 



Thus there is a real photo-electric fatigue, i. e. one due to 

 the action of light, as opposed to that fatigue observed by 

 Allen. 



The result that the fatigue is only .observable when electrons 

 are emitted is worthy of some little consideration. Hallwachs* 

 considered a number of possible causes of photo-electric fatigue, 

 and came to the conclusion that the chief cause is the gas 

 occluded in the metal. The kind of fatigue that he investigated 

 was of a similar nature to that mentioned by Allen. A possible 

 explanation of the results of this paper can be obtained on 

 Hallwachs's view. When the electrons leave a metal there 

 will be a steady stream of them from a certain depth of the 

 metal, and it is fair to suppose that some of them will bring 

 molecules of the occluded gas to the surface, thus increasing 

 the amount of gas there for the time being. For these 

 electrons, giving rise to the photo-electric effect, which have 

 a small velocity, gas has a large absorption coefficient t; so 

 that as the gas accumulates at the surface fewer electrons can 

 get through, thus accounting for the decrease in the photo- 

 electric current. On the other hand, when the electrons 

 cannot leave the metal, by having the latter charged positively, 

 there will be no stream of electrons to the surface, and so no 

 accumulation of gas there. 



The explanation of the recovery of the photo-electric current 

 on this view is that the gas will diffuse from the surface 

 either back into the metal or out of the metal, when the light 

 is shut off, leaving the gas in the metal uniformly distributed 

 over it. 



Some support of the above view is given by the fact that 

 very thin electrodes which have been in a vacuum for some 

 time show no photo-electric fatigue. This has been shown 

 for thin aluminium leaf and for a very thin deposit of 

 platinum on quartz. 



A point which needs further consideration is why the 

 particles of gas when carried to the surface of the metal by 

 the electrons do not leave the metal altogether, instead of 

 accumulating there. One may consider that there is some 

 surface effect to account for this. Hallwachs has discussed 

 such a possibility, and tried experimentally, hut without 



* Ami. der Phys. \xiii. p. 459 (1907). 



f Lenard, Ann. der Phys, xii. p. 714 ( 1908) ; Robinson, Ann. der 

 x\xi. p. 809 (1910). 



