338 Prof. A. LI. Hughes on the Contact 



-and therefore proportional to its energy, n the frequency, 

 and k and V are constants. V varies from metal to metal, 

 while k is almost independent of the nature of the metal. 

 In a theoretical discussion * of the photo-electric effect, 

 Richardson has identified V with the " intrinsic potential " 

 of the metal. The meaning of this may be made clear in 

 the following way. If Y (Cu) be the value of V for copper 

 and V (Zn) be that for zinc, then V (Zn)— V (Cu) obtained 

 from photo-electric experiments should be identical with the 

 contact difference of potential between the metals. 



Quite recently, some very interesting results, which may 

 .lead to a more definite knowledge of the nature of photo- 

 electricity, have been obtained by a number of investigators. 

 Kiistner f found that no photo-electric effect is shown by 

 zinc when it has been scraped in a vacuum after extraordinary 

 precautions have been made to exclude gases, particularly 

 active ones. (The shortest wave-length available in these 

 experiments was probably X 1850.) Wiedmann and Hall- 

 wachs X found that the removal of occluded gases from 

 potassium by repeated distillation in a very high vacuum 

 caused its photo-electric effect to disappear completely. 

 (Since the light had to pass through glass, the shortest wave- 

 length was probably about X 3400.) These results were 

 confirmed by Fredenhagen §. On the other hand, Pohl and 

 Pringsheim || found that potassium behaved in practically the 

 same way whether it was freed from gases by repeated 

 distillation in a very good vacuum or prepared in the usual 

 way for photo-electric cells. We cannot therefore regard 

 the suppression of the photo-electric effect by the removal of 

 gases from the metallic surfaces as established beyond dispute. 

 Nevertheless, the results are of great interest. It is implied 

 in the papers of Wiedmann and Hallwachs, Fredenhagen, 

 and Kiistner, that the photo-electric effect does not exist when 

 occluded gases are completely removed from the metal. A 

 less violent departure from current views would be to regard 

 the experiments as indicating that V in equation (1) is 

 increased to such a value that light of the wave-lengths 

 available in their experiments is no longer capable of exciting 

 the photo-electric effect. The law expressed by equation (1) 

 holds not only for the electrons released by light from metallic 



* Richardson, Phil. Mag. xxiv. p. 570 (1912); Eichardson and 

 Compton, Phil. Mag. xxvi. p. 550 (1913). 



t Kiistner, Phys. Zeits. p. 68 (1914). 



X Wiedmann and Hallwachs, Verh. d. Deutsch. Phys. Ges. p. 107 

 (1914). 



§ Fredenhagen, Verh. d. Deutsch. Phys. Ges, p. 201 (1914). 



|| Pohl and Pringsheim, Verh. d. Deutsch. Phys. Ges. p. 336 (1914). 



