An Effect of Light on the Electron Emission from Hot Filaments

Photo-electric currents from hot, oxide-coated platinum filaments.---The increase in the electron emission from hot, oxide-coated platinum filaments when illuminated by the full radiation from a water-cooled quartz mercury arc was measured, for different filament temperatures, as a function of potential between filament and anode. The photo-electron currents were large enough to be readily measurable with a galvanometer. Measurements on one filament showed the photo-electron current increasing with potential to a maximum at about 14 volts. For another filament, twice as thick, the maxima were between 2 and 4 volts. In the case of both filaments increasing the temperature shifted the maximum to lower values of potential. No photo-electric fatigue was observed. By the use of various absorbing screens and different sources of light it was found that the photo-currents are, for the most part, due to radiations of wave-lengths shorter than {λ}3000; but that the long wave-length limit is somewhat above that value.Photo-electron currents from other filaments.---In all, four types of filaments were tried: (a) oxide-coated platinum; (b) plain platinum; (c) oxide-coated tungsten; (d) plain tungsten. Of these, only the oxide-coated filaments gave an appreciable emission due to the light.The possible source of the photo-electrons is discussed and calculations are made as regards their velocity distribution. Results suggest that the action of the light is to free electrons from a thin film of metal of which the work-function varies with temperature of the filament.