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 XVII. Intelligence and Miscellaneous Articles. 



ON THE DEPENDENCE OF THE PHOTOELECTRIC CURRENT ON THE 



POSITION OF THE PLANE OF POLARIZATION OF THE EXCITING 



LIGHT IN REFERENCE TO THE SURFACE OF THE KATHODE. 



BY DR. J. ELSTER AND H. GEITEL. 

 1 T is possible in many cases to lower the potential required for 

 ■*- the spontaneous discharge of an electric current in a gas by ex- 

 posing the kathode to the action of light. For the same difference 

 of potential the strength of the current depends on the nature of 

 the kathode, the gas, and the light. While kathodes of platinum, 

 mercury, copper, and several other metals require irradiation with 

 ultra-violet light of high intensity, kathodes of sodium, potassium, 

 rubidium, in an atmosphere of hydrogen of about 3 millim. 

 pressure, give currents which can be measured galvanometrically, 

 even with feeble light within the region of the visible rays. 



The question may be asked whether this action of light on the 

 electrical discharge depends on the orientation of the vibrations 

 with respect to the surface of the electrode struck. 



Experiments with polarized ultra-violet light of sufficient in- 

 tensity are difficult to make because it is absorbed by the ordinary 

 polarizing arrangements, such as Nicol's prism, tourmaline plates, 

 glass disks, &c, so that only polarization by reflexion remains ; 

 even with this method, however, great losses of light are un- 

 avoidable. Thus in an analogous phenomenon, the development 

 of electrical sparks by ultra-violet light discovered by Hertz, 

 M. Wanka did not succeed in establishing the influence of the 

 direction of vibration of the light, which he had supposed to exist. 



This difficulty disappears with kathodes of the alkaline metals, 

 since we can here work with ordinary polarized light ; but another 

 difficulty presents itself, that a polished and, if possible, a plane 

 surface of these substances must be produced in a vacuum. This 

 requirement is, however, satisfied by using as kathode the alloy of 

 potassium and sodium, which is liquid at ordinary temperatures. 

 "When placed in a roomy receiver a sufficient quantity sets plane 

 and horizontal in the central part of the surface. 



We have investigated the photoelectric action of polarized light 

 as follows. 



In the circuit of a voltaic battery of about 250 volts are a sensitive 

 galvanometer, a commutator, and a sensitive cell of the liquid 

 potassium and sodium alloy of the form shown in Wiedemann's 

 Annalen, vol. xlii. p. 564, so inserted that the negative-pole 

 wire leads to the surface of the alkali metal. In order that the 

 active luminous pencil should have a constant section, the cell was 

 coated with opaque varnish, with the exception of a small circle of 

 15 millim. diameter turned towards the source of light. The cell 

 is so arranged that the rays entering this aperture centrally and 

 parallel strike the centre of the metal surface under an angle of 

 about 65°. Between the source of light and the cell a lens was 

 introduced for the production of parallel light as well as a pola- 

 rizing arrangement (a Nicol's prism or a set of glass plates). 



