A Comparison of the Thermionic and Photo-Electric Work Function for Platinum

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Abstract
Variation of Photo-electric Effect with Temperature, to 420°.—(1) Pt filaments coated with oxides of Ba and Sr were mounted in an evacuated tube with a quartz window and could be heated electrically to any desired temperature. The photo-electric current received by a copper oxide coated Faraday cylinder when a strip was illuminated with light from a quartz arc was measured with a sensitive electrometer, and was found to increase rapidly with the temperature reaching a value, on first heating to 420°, about 68 times as great as at 20°. The long wave-length limit was found to shift with increasing temperature from 2860 to about 3800 Å, the corresponding work function changing from 4.31 to 3.24 volts. On cooling, however, the sensitiveness and wave-length limit did not return to their original values, except very gradually, showing marked time-lag effects, due probably to changes in the oxide surface. (2) Pt strips, uncoated, were tested in the same manner. After preliminary treatment by repeated heating to 900°, the current was found to be independent of the temperature to 500°, except for a slight decrease due to the magnetic effect of the heating current, and the long wave-length limit remained constant within one per cent. at 2570 Å. Hence the work function or work necessary to detach an electron photoelectrically from Pt comes out 4.80 volts, which is close to the values found for the corresponding thermionic work function.