Frequency and Atomic Number. 



491 



constant. A theory of photoelectric action has been deve- 

 loped by Richardson* which leads to a result of this form. 

 The quantity hv is a measure of the work that must be done 

 to detach an electron from the metal. In consequence of 

 variations in the surface conditions the determination of v 

 is a matter of great difficulty; in fact some experimenters 

 have concluded that it is by no means constant. 



Richardson and Compton have deduced two sets of values 

 for the limiting frequency, one from the maximum energy of 

 the electrons emitted, the other from the mean energy. They 

 attach more weight to the values from the Tnean energy. Both 

 sets have been employed in the construction of Table III. 



Table III. 



Element. 



N. 



NvoXlO" 15 . 



Maximum Energy 1 Mean Energy. 



Na 



A\ 



11 

 13 

 12 

 30 

 50 

 83 

 29 

 78 



2x2-83 



3x2-73 



3x3-14 



8x3-00 



13x3-19 



23x3-28 



9x3-22 



25x324 



2x2-86 



3X3-1G 



3x3-20 



8x3-15 



13x3-42 



23x321 



9x3-13 



25X321 



Mg 



Zn 



Sn 



Bi 



Ou 



Pt 



With the exception of the values for sodium, and one value 

 for aluminium, the agreement with the relation Nv = wv E is 

 as good as could be expected in view of the uncertainty of the 

 observed values. 



Millikan f has published values for the limiting frequency 

 obtained from surfaces freshly cut in a very high vacuum. 

 In the case of lithium the values observed in two separate 

 experiments were v — 57 "0 x 10 13 and v = 597 X 10 13 . 

 Taking N=3, the mean value of ~Nv G 



= l-75xl0 15 =ix3'50xl0 15 . 



For sodium the value given for v is 43' 9 X 10 13 , and the 

 resulting value of Nv is l^x3'22x 10 15 . Hence there is 

 a strong presumption that a rule similar to that already 

 found holds in these cases ; but it is necessary to suppose 



* Richardson, Phil. Mag. vol. xxiii. p. 615 (1912) ; vol. xxiv. p. 570 

 (1912). 



t Millikan, Phys. Rev. vol. vii. pp. 18, 355 (1916). 



