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XIV. On the Velocities of the Electrons emitted in the Normal 

 and Selective Photo- Electric Effects. By A. Ll. Hughes, 

 D.Sc, P. A., Assistant Professor of Physics at the Pice 

 Institute, Houston, Texas *. 



THERE is a marked difference between the photo-electric 

 effect in the alkali metals and that in other metals. 

 When polarized light falls upon a polished metal, such as 

 platinum or copper, the number of electrons emitted per unit 

 energy of the incident light increases rapidly as the wave- 

 length decreases. The emission of electrons is somewhat 

 greater when the light is polarized in the Eli plane than 

 when it is polarized in the Ej_ plane f. This, however, is 

 fully accounted for by the fact that when light is polarized 

 in the Ell plane, more light is absorbed than when the light 

 is polarized in the E± plane {. So, if we measure the number 

 of electrons emitted per unit energy of absorbed light, the 

 photo-electric effect does not depend upon the state of 

 polarization of the light. With the alkali metals, however, 

 the case is very different. Many more electrons are emitted 

 when the light is polarized in the Ell plane than when it is 

 polarized in the E_L plane. The typical relation between the 

 photo-electric current from an alkali metal illuminated by 

 polarized monochromatic light and the state of polarization 

 is shown in fig. 1. Pohl and Pringsheim § distinguish 

 between two kinds of photo-electric effect, the fct normal 

 effect ' J and the " selective effect/'' The selective effect is 

 shown only by the alkali metals, and then only when the 

 light is polarized in the Ell plane and limited to a certain 

 range of wave-lengths. The selective effect is the abnormally 

 great photo-electric effect obtained with light (limited to a 

 certain range of wave-lengths) polarized in the Ell plane. 

 The number of electrons emitted with light of the wave- 

 length at which the selective effect is a maximum may be 

 from 20 to 300 times as great as the normal effect for the 

 same wave-length. With unpolarized light of this wave- 

 length, the photo-electric current is mainly due to the 

 electrons emitted in the selective effect. Pohl and Pringsheim 



* Communicated by the Author. 



t We shall refer to the plane of polarization parallel to the plane of 

 incidence as the Ell plane, and to the plane of polarization at right 

 angles to this as the EJ. plane. In terms of the Electromagnetic Theory 

 of Light, E denotes the electric force and the sign after it denotes the 

 relation of the force to the plane of incidence. 



t Pohl, Verh. d. JDeutsch. Phys. Ges. x. pp. 339, 609, 715 (1909). 



§ Pohl and Pringsheim,J Verh. d. Deutsch. Phys. Ges. xii. p. 215 (1910). 



