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XVIII. The Normal and the Selective Photoelectric Effect. 

 By R. Pohl, Ph.D., and P. Pmngsheim, Ph.D.* 



T^HE researches made by different authors on the photo- 

 electric effect lead to some results which seem to be in 

 striking contradiction to each other. 



1. E. Ladenburg f and Mohlin % have shown that the 

 number o£ electrons produced by the unit o£ ultraviolet 

 light increases with decreasing wave-lengths. 



2. Elster and Geitel § found qualitatively that, for the 

 alkali metals, this number has a maximum in the visible part 

 of the spectrum, and T. Braun || proved this to be right 

 quantitatively for Rb and K> 



3. On the contrary, HallwachsIF denied the existence of 

 these maxima, and from new measurements he concluded 

 that Ladenburg's and Mohlin's law is true also for the alkali 

 metals. 



Contradictions of a similar kind are found in the researches 

 on the effects of polarized light. 



4. According to Pohl** the orientation of the vectors of 

 ultraviolet light has an influence upon the number of 

 liberated electrons only so far as the absorption of light 

 depends upon it ; this is true for solid surfaces of Cu and 

 Ptftj as we ll as £° r liquid Hgtt* 



5. But Elster and Geitel §§ had found previously for the 

 visible part of the spectrum, that with alkali metals the 

 photoelectric current is proportional to the absorbed energy 

 of light at different angles of incidence only if the electric 

 vector swings at light angles to the plane of incidence : if 

 the electric vector swings in this plane there is only an 

 approximate proportionality between the component of the 

 electric vector normal to the surface, and the photoelectric 

 current due to this component. In this second case the 



* Communicated by the Authors. 



+ E. Ladenburg, Verh. d. D. Phys. Ges. ix. p. 504 (1907). 



% H. Mohlin, Akad. Abhandl., Upsala, 3907. 



§ T. Elster and H. Geitel, Wied. Ann. Hi. p. 493 (1894). 



|| T. Braun, Dissertation, Bonn, 1905. 



11 W. Hallwachs, Verh. d. D. Phys. Ges. xi. p. 514 (1909). 



** R. Pohl, ibid. xi. p. 339 (1909). 



tt R. Pohl, ibid. xi. p. 339 (1909). 



XX R. Pohl, ibid. xi. p. 609 (1909). 



§§ T. Elster and H. Geitel, Wied. Ann. lxi. p. 445 (1897). 



