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LX. The Photoelectric Fatigue of Metals. By H. Stanley 

 Allen, M.A.,D.Sc, Senior Lecturer in Physics at University 

 of London, King's College*. 



THE recorded facts relating to the diminution of the 

 photoelectric activity o£ metal surfaces with time are 

 somewhat confusing and contradictory. The difficulties have 

 been in part removed by the researches of Hallwachs and 

 his fellow workers f. Hallwachs maintains that the photo- 

 electric " fatigue " is not primarily due to illumination, and 

 that the size of the vessel in which the plate is kept affects 

 to a marked degree the rate at which the fatigue takes 

 place. Ignorance of the latter result goes far to explain the 

 contradictions amongst the earlier experiments. 



My investigations, which have been in progress for some 

 years past, have led me to the same conclusions; and in view 

 of the importance of these conclusions in explaining the 

 changes involved in fatigue, it seems desirable to put on 

 record a short account of my results. 



In the following paragraphs it is shown that in the case of 

 zinc, (1) light is not the primary cause of fatigue, (2) the 

 fatigue is practically independent of the electric field, (3) the 

 fatigue takes place in an atmosphere of hydrogen as in 

 ordinary air, (4) the fatigue proceeds more slowly when the 

 plate is kept in a small vessel. 



To explain the last result we are forced to the conclusion 

 that the fatigue must be due to some substance (ozone, Hall- 

 wachs j in the case of zinc, ozone, water vapour, Ullman) 

 present in small quantity in the atmosphere surrounding the 

 plate. The fatigue must be associated with the condition of 

 the gaseous films on the surface of the plate or with the gas 

 occluded in the metal. 



The foregoing remarks apply to fatigue in gases at ordinary 

 pressures ; in a vacuum other sources of fatigue may possibly 

 be present t, though recent results tend to show that with a 

 perfectly clean metal surface in a very high vacuum there 

 would be no fatigue §. 



* Communicated bv the Author. 



t W. Hallwachs, Phys. Zeit. v. p. 489 (1904) ; Ber. d. math. -phys. 

 Klasse d. Kgl. Sachs. Gesellsch. d. Wissensch. zu Leipzig, lviii. p. 841 

 (1906) ; Ann. d. Phys. xxiii. p. 459 (1907) ; Abh. d. natur wissensch. 

 Gesellsch. Isis in Dresden, i. p. 65 (1909). H. Beil, Ann. d. Phys. xxxi. 

 p. 849 (1910). E. Ullmann, Ann. d. Phys. xxxii. p. 1 (1910). 



\ As, for example, changes in pressure due to absorption of gas by the 

 metal (Dember, Phys. Zeit. ix. p. 188, 1908). A change in pressure due 

 to gradual absorption of gas by charcoal at the temperature of liquid air 

 may have been the cause of the apparent fatigue of zinc in a vacuum 

 recorded in my first paper (§ 12). 



§ Millikan and Winchester, Phys. Rev. xxix. p. 85 (1909). 



