Intelligence and Miscellaneous Articles. 79 



irradiated, without excluding the action of the luminous rays on 

 it. Prom former experiments* this latter phenomenon was to 

 be expected, and was confirmed by the present experiments. The 

 luminous arc was at a distance of 45 centim. from the plate, the 

 rays coming from it passed through a diaphragm 6 centim. in 

 diameter before entering the metal case. 



The case was of rusty iron, so that the contact potential agaiust 

 the plates to be suspended was always negative. In this case only 

 could the rises of potential indicated by the electrometer be indu- 

 bitably explained. For if the plate had been negative to the case, 

 a rise of potential would have occurred due to the transport of 

 negative electricity by the illumination. If the plate, however, 

 was positive towards the case, and therefore possessed a positive 

 charge, an increase of potential could only be due to the fact that 

 positive electricity was produced on the plate by irradiation. The 

 determination of a difference of potential due to contact by a 

 method formerly describedt, gave for rusty iron towards brightly 

 polished zinc 1*2 volts. 



If, now, the aperture in the diaphragm through which the rays of 

 light could fall on the plate was closed by a mica plate, the electro- 

 meter was at rest when the arc was produced. When the mica 

 was replaced by a far thicker plate of selenite, the galvanometer 

 gave a gradually increasing deflection indicating positive electricity. 

 This at once ceased to increase when the mica again replaced the 

 selenite. The rise of potential cannot therefore be due to an 

 inductive action, nor can it be referred to the action of heat. 

 The complete absorption of the action by mica, and the great 

 transmissibility of selenite are here as prominent as in the action 

 of light on electrostatically charged bodies, so that the excitation of 

 electricity is probably due to the same rays as that phenomenon. 



The metals which have been used for the experiments just de- 

 scribed were zinc, brass, and aluminium. In all three positive 

 electricity occurred on irradiation with brightly polished surfaces. 

 Old surfaces no longer show the phenomenon. The radiation 

 itself lowers the potential to which the plates can be electrified, so 

 that with any succeeding experiment made with the same surface, 

 the potential obtained is lower, while the rise to it takes place 

 more rapidly, and the decrease is greater than when for the same 

 interval of time between the experiments the plate was not illu- 

 mined. The maximum potential with zinc amounts to over a volt, 

 with brass to about one volt, and with aluminium to 0*5 volt. These 

 values are the highest which have as yet been obtained, as the 

 maximum values change with the condition of the surface. 



As I have had to interrupt these experiments, I have ventured 

 meanwhile to communicate the chief results. Whether the exci- 

 tation of electricity by radiation with electric light is in direct 

 connexion with the loss of electricity by electrostatically charged 



* Loc. est. p. 305. 



t Wiedemann's Annahn, vol. xxix. p. 7 (1886). 



