492 



Dr. H. S. Allen on Electr 



onic 



that the factor n may assume values which differ from an 

 integer by ^. 



In the paper by Souder already referred to a curve 

 is given for freshly-cut potassium from which it appears 

 that the limiting wave-length is about 545 fi/m. This gives 

 Ni/ = 3x3'48xl0 15 . 



Mention must also be made of a recent investigation by 

 W. Wilson* on the complete photoelectric emission from 

 the liquid alloy of sodium and potassium, in which the work 

 done to remove an electron from the hot metal is found 

 to be 4*02 x 10 ~ 12 erg. This corresponds to a threshold 

 frequency v = 6*13 x 10 14 sec. -1 , which may possibly be 

 assigned to sodium, and we then find Nv = 2x 3*37 x 10 15 . 



From his theoretical work Richardson concluded that the 

 frequency corresponding to maximum activity should stand 

 to the limiting frequency in the ratio 3 to 2. The results 

 recorded above give agreement with this conclusion in some 

 instances, but in others a different ratio is found between the 

 frequencies. 



Table IV. 



Element. 



N. 



»mai. 



» . 



Katio. 



Li 



3 



11 



12 

 13 



19 



1 



3 



b 



4 



i 



2 



2* 



lit 



3 



3 



3 



2/1 



3/2 



2/1 • 



3/2 



5/3 



4/3 



Na 



Mg 



Al 



K 





* Richardson and Oompton. 



t Millikan. 



§ 4. Ionization Potentials. 



The minimum potential required for the ionization of a 

 oas is probably connected with v , the least frequency of 

 radiation which can ionize the gas photoelectrically |. The 

 equation of Einstein is 



eY =hv , 



where V denotes the ionization potential. 



If then v is subject to the relation Nv = nv E , we should 

 expect to find NV = nV B , where Y E denotes a constant 



* W. Wilson, Proc. Roy. Soc. A, vol. xciii. p. 359 (1917). 



t Einstein, Ann. d. Physik, vol. xvii. p. 132 (1905) ; Richardson, 

 Phil. Mae. vol. xxiv. p. 570 (1912) ; K. T. Compton, Phys. Rev. vol. viii. 

 pp. 386,412(1916). 



