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



region and of the fact that our source of light gave a line 

 spectrum. 



It seems, therefore, fairly certain that part of the increased 

 value of h deduced from (20) must be due to w Q being too 

 large. We think it improbable that w is appreciably larger 

 than what it pretends to be, namely, the part of the latent 

 heat of evaporation of the thermionically emitted electrons 

 which is independent of 6. It may be, however, that w is 

 slightly smaller for the electrons emitted photoelectrically by 

 the complete radiation, than it is for the electrons which are 

 emitted thermionically. If this could be established it would 

 be important as showing that thermionic emission is some- 

 thing different from the photoelectric emission arising from 

 the complete radiation characteristic of the temperature of 

 the hot body *. 



The values of v for different substances are closely related 

 to the contact differences of potential. This is shown by the 

 numbers in the following table, which represent the contact 

 differences of potential between platinum and the different 

 metals investigated : — 



Metal. 

 I. 



Contact P.D. with Platinum. (Volts.) 



Calculated. II. 



Observed. III. 



Calculated. IT. 



Ou 



•37 

 •77 

 •77 

 104 

 117 

 1-62 

 269 



•13 

 •35 

 •62 

 •90 



1-05 



1-2 



2-4 



•30 

 •64 

 •64 



•85 

 1-00 

 1-31 



2-2 



Bi 



Sn 



Zn 



Mff 



Al 



Na 





The observed values in column III. are the usual values, 

 except in the case of Mg, where the observed value was 

 •25 volt less than that given in the standard tables. The 

 calculated values in columns II. and IV. are derived from 

 the corresponding values of w in different ways. The 

 differences of w are taken to be equal to the differences of 



eY, where V is the corresponding intrinsic potential, 

 is in accordance with the formula 



This 



w, 



= *(V, 



V P -^(V, 



v,)), 



(21) 



* Cf. O. W. Richardson, Phil. Mag. vol. xxiii. p. 620 (1912). 



