Temperature upon Photo-electric Effects in Hiyli Vacuum. 199 



escape from the metal other than that due to the positive 

 potential assumed by the latter. The existence of such a 

 second type of retarding force Lenard's experiments upon 

 platinum, aluminium, and carbon seem clearly to demonstrate*. 

 but they also appear to indicate that this retardation is not 

 due, in measurable degree, at least, to the specific attraction 

 of the metal for the electron, but rather merely to the 

 attraction for the escaping electron of the excess of positive 

 electricity which the electron, by virtue of its escape, leaves 

 behind upon the atom, and which, for an instant at least, 

 must be assumed to remain at the particular point upon the 

 surface of the metal at which the electron escaped. 



If then this second type of force which opposes the escape 

 of the electrons is wholly electrical in nature, it would of 

 necessity be the same for all temperatures and probably also 

 for all metals, so that measurements of the effect of temperature 

 upon the positive potentials assumed in a vacuum by different 

 metals under the influence of ultra-violet light should give us 



■mm m a . C> & 



immediate information regarding the effect of temperature 

 upon the velocities of the electrons within the atoms of these 

 metals. The object of the experiments of the present sections is 

 then to determine the effect of temperature upon this velocity. 

 It might perhaps be inferred, without special experiment, 

 merely from the fact that the quantity of discharge was shown 

 in Tables I. and II. to be independent of temperature, that the 

 velocity of the electron within the atom would also of neces>ity 

 be independent of temperature, for the quantity of discharge 

 measures the stability of the atom in the presence of a given type 

 of disturbance ; and if this stability is independent of tempera- 

 ture, then, since an increase in the kinetic energy of the 

 electron within the atom might be expected to be attended 

 by a decrease in atomic stability, it would follow that inde- 

 pendence of stability upon temperature would mean also 

 independence of electronic velocity. ^Nevertheless, the direct 

 experimental proof of this independence is not wholly super- 

 fluous, especially in view of the fact that experiments upon 

 the positive potentials acquired can be made even more 

 accurately than experiments upon the rates of discharge, and 

 in view of the further fact that the only experiments of this 

 sort which have been previously recorded f, namely, those of 



* Lenard, Ann. tie Phys. viii. p. 186 (1902). 



t Since the above was written an article has appeared in the Annalen 

 der Phi/sik, vol. xxi. p. 281, by A. Lienhop, which establishes the 

 independence of the positive potentials acquired by and the rates of dis- 

 charge from carbon andiron upon temperature between room- temperatures 

 and that of liquid air. These experiments upon other substances and in 

 other temperature ranges coniirin in a most satisfactory manner the 

 results presented and the conclusions drawn in this paper. 



