﻿i The Contact Differencce of Potential of Distilled Metals. 62?) 



to the observations of Compton and Richardson, the relative 

 magnitude of the two humps is determined by the amount of 

 photoelectric fatigue which has occurred, The data of both 

 pairs of observers can be harmonized on the following 

 assumptions : — 



(1) That Pohl and Pringsheim's typical curves correspond 

 to a state of less advanced photoelectric fatigue. This in- 

 volves the assumption of the attainment of better vacuum 

 conditions in their experiments, which seems to be borne out 

 by an examination of the general character of their results. 



(2) That the first hump (the " resonance " hump (a)) is 

 present from the beginning, or at least is formed very 

 quickly, and does not suffer much alteration in the earlier 

 stages of the photoelectric fatigue. 



(3) That the second hump is small initially, and increases 

 to a maximum value during the early stages of fatigue. 

 According to Compton and Richardson's experiments, this 

 initial increase is followed by a decay, which is slower 

 than that of the first hump, as the fatigue progresses. 



It is not claimed that this is the only possible explanation 

 of the observed differences ; it is put forward as a possible, 

 and, on the whole, rather probable one. 



LXIV. The Contact Difference of Potential of Distilled 

 Metals. By Fernando Sanfoed*. 



IT has seemed to the present writer that some of the results 

 of the experiments published under the above caption in 

 the September number of this Journal, may perhaps be 

 •accounted for in a manner not taken into consideration bv 

 Mr. Hughes. I refer especially to the observation that after 

 a film of zinc or bismuth had been condensed from the metallic 

 vapour upon a very thin film of platinum on a glass plate in 

 a high vacuum, the condensed metallic film was at first much 

 less electropositive to the platinum film than it became after 

 standing for some time, and that the change to the more 

 electropositive condition was hastened by admitting a very 

 small quantity of air to the vacuum. 



In some experiments which I have made for another 

 purpose I have observed that when a section of a glass tube is 

 heated to a temperature of 100 degrees, or even less, it becomes 

 plainly electronegative to the colder parts of the same tube. 

 The change is necessarily slow, since, on account of the low 

 conductivity of glass, the electrons require considerable time 

 to gather in the heated parts of the glass. It would seem that 



* Communicated by the Author. 



