Influence of Temperature upon Photo-electric Effects. 189 



temperature, and, if so, to what extent. This was the main 

 purpose of this investigation. 



2. Historical Summary. — Many investigations have already 

 been made upon the effect of temperature upon photo-electric 

 discharge, but the results have not been at all concordant, 

 and none of them seem to be conclusive. Hoor *, working 

 in air, found a decrease in the discharge from zinc between 

 18° C. and 25° C. Stoletow | states that a rise in .tempera- 

 ture increases photo-electric effects. Zeleny % found that 

 the effect was more than doubled in the case of platinum 

 when the temperature rose from 0° 0. to 650° C, while the dis- 

 charge from iron increased forty-fold between 0°0. and 700°C. 

 All of these experiments, however, were made in air. 



Elster and Greitel § alone record measurements in a good 

 vacuum, the metal upon which they worked being potassium. 

 In raising the temperature from 20 o, 3 C. to 50 o, 3 C. they 

 found that the discharge changed from 27*9 to 44*2 scale- 

 divisions, an increase of about 60 per cent. 



Relying chiefly upon the experiments of Elster & Geitel and 

 Zeleny, J. J. Thomson concludes that " the photo-electric 

 effects of metals are greater at a high temperature than at a 

 low one " || . He mentions, but does not record, some results 

 of his own on the alkali metals which are in accord with this 

 conclusion. He inclines to the view that the cause of this 

 phemonenon is to be found in the absorption by the free 

 electrons of the metal of the energy of the ultra-violet 

 light and the consequent acquisition by these corpuscles of 

 sufficient kinetic energy to enable them to escape. This view 

 follows naturally from the fundamental assumption of the 

 electron theory of conduction as elaborated by Riecke ^[, 

 Drude **, and J. J. Thomson ft> an( l applied with so much 

 success to the explanation of the relation between the thermal 

 and the electrical conductivities of metals. For according to 

 this assumption, there exist at all times within conductors 

 free or " metallic " corpuscles, which, in accordance with the 

 Maxwell-Boltzmann law, possess a kinetic energy of agitation 



* Hoor, Wien. Berichte, xcvii. p. 719 (1888). 



t Stoletow, Journ. de Phys. ix. p. 536 (1890). 



X Zeleny, Phys. Keview, xii. p. 321 (1901). 



§ Elster & Geitel, Wied. Ann. xlviii. p. 625 (1893). 



|| Conduction of Electricity through Gases,' pp. 237 & 241. This 

 sentence is not found in the new edition of this book which has appeared 

 since the present article was written. Nevertheless the view that photo- 

 electric effects vary with temperature is still retained. 



5] Reicke, Wied". Ann. lxvi. p. 353, & p. 545 (1898). Also Ann. d. 

 Phys. ii. p. 835 (1900). 

 ** Drude, Ann. d. Phys. i. p. 566, & iii. p. 369 (1900). 

 ft J. J. Thomson, Rapports au Cony res International, de Physique, iii. 

 p. 138 (1900). 



