Photoelectric Fatigue of Metals. 



567 



and in the table has been taken as 100. In experiments 

 a, d, and e the plate was illuminated continuously by the 

 mercury-vapour lamp. In b and c it was in complete dark- 

 ness for 9 minutes, as indicated by the asterisks in the table. 

 During this period the testing vessel was closed with a light- 

 tight wooden cover. 



Table III. 

 Zinc plate in testing vessel. Fatigue period, 16 minutes. 



a. 



b. 



c. 



d. 



e. 



I 



100 



100 



100 



100 



100 



II 



91 



93 



92 



92 



89 



HI 



85 



88 



87 



83 



80 



IV 



79 



X 



x 



79 



78 



V 



78 



* 



¥t 



78 



75 



VI 



75 



•* 



* 



77 



71 



VII 



73 



* 



* 



72 



70 



viir 



71 



73 



75 



71 



69 



IX 



67 



73 



74 



71 



68 



It appears from the results in the table that the fatigue 

 takes place in complete darkness, though there is evidence of 

 a small increase in the rate of fatigue under the influence of 

 the ultra-violet light. 



We conclude that light cannot be the primary cause of 

 fatigue, though it may play a secondary part in accelerating 

 or retarding fatigue. These secondary actions are illustrated 

 in my earlier experiments on amalgamated zinc *, or on 

 polished zinc at different distances from the mercury- vapour 

 lamp f. 



Ullmann { arrives at similar conclusions, attributing the 

 secondary actions of light to the formation of ozone and to 

 the heating of the plate. 



I have also examined the action of Rontgen rays on the 

 plate and could detect no decisive change in the rate of 

 fatigue, at any rate for an exposure of one or two minutes to 

 the rays from a focus-tube at a distance of about 50 cms. 

 Hallwachs § records a similar result. 



* H. S. Allen, Proc. Roy. Soc. (A) lxxviii. § 11, p. 492 (1907). 

 t H. S. Allen, Proc. Roy. Soc. (A) lxxxii. § 2, p. 163 (1909). 

 % Ullmann, Ann. d. Phys. (4) xxxii. § 5, pp. 15-20 (1910). 

 § Hallwachs, Ann. d. Phys. (4) xxiii. p. 467 (1907). 



