Photoelectric Fatigue of Metals. 569 



Table VI. 

 Zinc plate in air o£ room. Fatigue period, 16 minutes. 



I 



100 



Gauze 4-ve 



100 



Zinc +ve 



100 

 Gauze +Te 



100 

 Zinc — ve 



II. to VII.... 



VIII 



50 



49 



51 



51 



IX 



45 



46 



47 



47 



When the zinc plate was under examination in the brass 

 testing vessel and was charged positively, a small effect was 

 sometimes observed on reversing the direction of the field for 

 the purpose of taking a reading of the photoelectric current. 

 The effect in question corresponded to a small increase in the 

 observed current, the increase amounting to about ten per 

 cent, of the anticipated value of the current. The increase 

 only persisted for a few minutes after the direction of the 

 field was reversed. This effect is comparable with that 

 observed by Campbell * in the case of the leak from hot 

 bodies, the reading taken immediately after reversal being 

 greater than the normal. 



3. Fatigue of various Metals in Air and in 

 Hydrogen. 



Zinc. — In my earlier papers (loc. cit.) I have shown that 

 the fatigue of zinc proceeds in such a way that the activity 

 for some hours after polishing can be represented as the sum 

 of two exponential terms. This would indicate that after a 

 prolonged period the activity would approach asymptotically 

 a zero value. It has been found, however, that the zinc plate 

 retains a small sensibility after several days (in one case after 

 26 days) ; a result which points to the existence of a state in 

 which the zinc plate would show a small residual activity. 

 This would mean, provided the atmospheric conditions 

 remained invariable, the addition of a small constant term to 

 the two exponential terms. 



Experiments were made with the zinc plate when the 

 testing vessel was filled with hydrogen, prepared by the 

 action of pure hydrochloric acid on pure zinc and dried by 



* N. E. Campbell, Phil. Mag. ix. p. 549 (1905). 



