Velocities of Electrons emitted in Photo-Electric Effects. 101 



believe that the selective and normal effects have their origin 

 in two independent processes. I£ this were so, one would 

 expect that the electrons emitted in the selective effect and 

 in the normal effect would have quite different velocities and 



Fig-, l. 



Photo -E/ec trie 

 Current*. 



X2000 



distributions of velocities. The indirect evidence bearing on 

 this point, however, suggests that there is practically no 

 difference between the velocities of emission in the two 

 effects. Certain definite laws have been established as to 

 the relation between the maximum emission velocity and 

 distribution of velocities on the one hand, and the wave- 

 length on the other. These laws have been found from 

 experiments on metals outside the alkali group. But 

 Richardson and Compton *, Kadesch f, and Millikan % have 

 included in their work experiments on sodium, and it is 

 significant that there is nothing abnormal in the results for 

 sodium. It has already been mentioned that, with unpolarized 

 light, the majority of the electrons emitted from an alkali 

 metal are those associated with the selective effect. The 

 evidence from these experiments, therefore, does not point 

 to any appreciable difference between the two effects so far 

 as we can judge from velocities. It should be mentioned 

 that Elster and Greitel § record an observation in the course 



* Richardson and Compton, Phil. Mag. xxiv. p. 576 (1912). 



t Kadesch, Phvs. Rev. iii. p. 367 (1914). 



X Millikan, PhVs. Rev. iv. p. 73 (1914). 



§ Elster and Geitel, Phys. Zeits. . 457 (1909). 



