438 Profs. J. Elster and H. Geitel on the Alleged 



If, in fact, this action of light can be shown to take place, 

 then it is no longer possible to believe that we have here to do 

 with a specific phenomenon of the kathode, and, moreover, 

 the view that the photoelectric process depends upon the 

 discharge of the one (the gaseous) coating of an electric 

 double layer which is continually renewed at the surface of 

 contact between the conductor and the gas must be put to 

 experimental proof, since the nature of the electricity escaping 

 in light must always be the same as that which the gas in 

 contact with the conductor itself takes. From this point of 

 view, therefore, it would not be intelligible that one and the 

 same conductor in the same atmosphere should give off both 

 electricities more easily in light than in darkness. Now 

 experiment shows that the illumination of a negatively 

 charged surface, with proper choice of light and of substance 

 illuminated, causes an active discharge of electricity into the 

 surrounding gas, whilst the corresponding phenomenon for 

 positive electricity — if it takes place at all — must be much 

 more insignificant. Thus Hrn. Stoletow and Righi have not 

 Jbeen able certainly to recognize the action of ultra-violet 

 light upon positively charged surfaces, and we ourselves have 

 so far not been able to observe any loss of positive electricity 

 in light which was not sufficiently well accounted for by the 

 usual loss of electricity or by the sources of error to be more 

 definitely spoken of in what follows. 



A paper by Herr E. Branly has recently appeared*, in 

 which the acceleration of the electric discharge by ultra-violet 

 light is maintained to hold good also for positive electricity. 

 On account of the importance of the subject we have repeated 

 the experiments described in this paper, and with the arrange- 

 ments which seemed to us best suited to exclude sources of 

 error, and following the method of Herr Branly as closely as 

 possible in essential points. After we had failed in obtaining 

 the same result as Herr Branly, we tried whether the alkali 

 metals, which are so sensitive to ordinary light with negative 

 electrification, would show a photoelectric discharge also with 

 positive electricity. In what follows we venture to report 

 upon the results obtained in these experiments. 



The most obvious method of observing the scattering of 

 electricity in light, which method was also employed by Herr 

 Branly, is to connect the electrified surface to be examined 

 with an electroscope, and to judge of the loss of electricity 

 produced by the light in a given time from the decrease in the 

 divergence of the leaves. This method has the disadvantage 

 that, on account of the high tension employed, the whole of the 

 * Compt. Bend. cxx. p. 829 (1895). 



