On the Photoelectric Fatigue of Metals, 565 



Method of experimenting. — The apparatus used in the 

 present research was identical with that described in my 

 earlier papers referred to below. I desire again to express 

 my thanks to the Government Grant Committee of the 

 Royal Society and to the Council o£ King's College for 

 defraying the cost of the greater part of this apparatus. 

 The mercury- vapour lamp of fused quartz supplied with 

 current from a special set of accumulators was used as a 

 source of ultra-violet light throughout this investigation. 

 Provided sufficient time (from 20 to 30 minutes) is allowed 

 for it to assume a steady state, this gives a sufficiently constant 

 stream of radiation. 



The photoelectric current between the positively charged 

 wire gauze and the metal plate was measured by means of a 

 Dolezalek electrometer in connexion with a suitable condenser. 

 Readings of the rate of leak were usually taken at intervals 

 of two minutes. Most of the results recorded w r ere obtained 

 with a zinc plate polished with fine emery-paper; both the 

 initial activity and the rate of fatigue showed considerable 

 variations from day to day, probably in consequence of the 

 atmospheric conditions, but the results obtained on any 

 particular day were usually concordant. In most cases the 

 figures given represent the mean of two or three concordant 

 determinations. 



(1) Light is not the primary cause of fatigue. 



In my earlier investigations I found that the rate at which 

 fatigue takes place is not much affected by the intensity of 

 the illumination * or by the character of the source of light 

 (mercury-vapour lamp or Nernst lamp t) • Later experiments 

 carried out to determine the influence of light on fatigue 

 confirm the conclusions of Hallwachs. The photoelectric 

 fatigue of zinc proceeds in darkness almost at the same rate 

 as when the metal is continuously exposed to light. 



The results of these experiments are embodied in the 

 following tables (I., II., and III.). The tests in Table I. 

 were made with a zinc plate polished with fine emery and 

 rouge, tested in the air of the room, readings of the activity 

 being taken at intervals of two minutes. The plate was 

 exposed continuously to the mercury-vapour lamp except 

 where an asterisk appears in the table. In the latter case 

 the plate was shielded from ultra-violet light but not from 

 the dim light of the room. The activity is expressed as a 



* H. S. Allen, Proc. Roy. Soc. (A) lxxviii. § 7, p. 489 (1907). 

 t H. S. Allen, Proc. Roy. Soc. (A) lxxxii. § 5, p. 164 (1909). 



