Temperature upon Photo-electric Effects in High Vacuum. 207 



appear in the rate-of-discharge series. The absence of this 

 relation constitutes, then, further evidence in favour of 

 Lenard's view as to the mechanism of the discharge, for 

 atomic stability obviously bears no necessary relation whatever 

 to the velocity of the electron within the atom. 



A further conclusion which can be drawn from the order 

 of the iron and gold in Table III., as well as from that of 

 the aluminium and antimony which were also symmetrically 

 placed with reference to the field, is that the metals do not 

 arrange themselves > with respect to positive potentials 

 acquired in vacuo, in the order of the Volta contact series, as 

 Bichat and Swyngedauw * believe that they do in air, the 

 gold being at the top and the zinc at the bottom of the list. 



10. Fatigue Effects. — Our final results upon fatigue effects 

 are not yet ready for publication, but there are certain con- 

 siderations relating to these effects which it seems necessary 

 to present at this time, because of their connexion with the 

 preceding discussion. These so-called fatigue effects have 

 been observed by many previous experimenters, and consist 

 in a diminution, with continued exposure, in the rate of 

 discharge from a given metal under the influence of a given 

 source. Thus, Elster and Geitel f found that a freshly 

 polished surface of zinc lost half its photo-electric sensitive- 

 ness in five minutes. Similarly, Branly J found that a freshly 

 polished aluminium disk in air had the initial rate of discharge 

 in daylight of 27, but after remaining under yellow glass for 

 35 minutes it had a discharge rate of but 8, while after 

 exposure to the air for several days it had become almost 

 inactive. J. J. Thomson § is inclined to attribute all such 

 fatigue effects to oxidation. 



Buisson ||, however, finds that though they are due in part 

 to oxidation in the case of some metals, they are in some 

 cases at least due wholly to other causes ; for he observed 

 that an amalgamated zinc surface, after beino- fatigued by 

 exposure, regained entirely its original sensibility when left 

 for several hours in the dark. Such a recovery phenomenon 

 obviously precludes the oxidation theory. 



Lenard % refers fatigue phenomena to the absorption of the 

 discharged electrons by the air-molecules and the consequent 

 formation of an electrical double-layer which consists of the 



* Rapports au Cony res International de Physique, iii. p. 168 (1900). 



t Elster & Geitel, Wied. Ann. xxxviii. p. 503 (1889). 



X Branly, Jour. d. Phys. ser. 3, ii. p. 300 (1893). 



§ ' Conduction of Electricit}^ through Gases,' p. 224. (In the edition 

 which has appeared since the above was written the inadequacy of the 

 oxidation theory is recognized.) 



|| Buisson, Comptes Rendus, cxxx. p. 1298 (1900). 



«1 Lenard, Ann. d, Phys. viii. p. 196 (1902). 



