590 Prof. 0. W. Richardson and Mr. K. T. Compton on 



that the metals for which the values of k m show the nearest 

 approach to the theoretical value are platinum and sodium. 

 The former is notoriously averse to oxidation and the surfaces 

 of the latter were cut in situ in the best attainable vacuum. 



The Values of v and\ . 



According to the theory which underlies formulae (9) to (13) 



w = hv , ....... (16) 



where w is the latent heat of evaporation of electrons out of 

 the material at the absolute zero of temperature. The value 

 of w can be obtained from thermoelectric data combined 

 with the temperature variation of the rate of emission of 

 electrons from the material when heated. The metal which 

 has received the most extended study in this respect is 

 platinum, and it is probable that a fairly reliable estimate of 

 w can be obtained for this material. If a is the specific 

 heat of electricity, one of the writers * has shown that 



_1 f R 



" e iy-1 



a* J" 



(17) 



when e is the charge on an electron, R is the gas constant 

 for one molecule, y is the ratio of the two specific heats for 

 the electrons, and w is their latent heat of evaporation, per 

 electron, at temperature 0. According to Berg f the 

 Thomson coefficient for platinum is practically constant 

 between - 50° G. and + 100° C. and equal to -9*2 X 10 2 erg 

 per e.m.u. per ° C. Thus from (17) w is a linear function of 

 0. If n is the total number of electrons emitted thermioni- 

 cally at 0, when the coefficient of reflexion of the electrons 

 is neglected 



« = A#Jw^, (18) 



where A 1 is independent of 0. Substituting the experi- 

 mental numbers given by the Thomson effect, this becomes 



n=A 2 2,11 *-"*' Bfl , (19) 



where A 2 is independent of and iv is the value of w when 

 = 0. 



We have applied the formula to determine w from the 

 experimental measurements of the saturation electronic 



* O. W. Richardson, Phil. Mag. vol. xxiii, p. 605 (1912). 

 t Cf. Baedeker, Elektrische Erscheinungen in Metallischen Leitern, 

 p. 76. 



