VII. On the emission velocities of photo-electrons

Abstract

Since the discovery of the photo-electric effect by Hertz, many experiments have been made on the emission of negative electricity from metallic surfaces when illuminated by light. Yet with regard to many important points the results are often indefinite and contradictory. Most theories of the photo-electric effect indicate definite relations between the velocity of emission of the electrons and ( a ) the nature of the metal from which they are emitted, and ( b ) the wave-length of the incident light. Up to the present, however, the experimental evidence as to these two relations must be regarded as quite inadequate to afford any decisive test between rival theories. This research was undertaken to obtain, among other things, quantitative evidence on these two relations. 2. Previous Work .— Ladenburg made some valuable experiments on the velocity with which electrons are emitted from metals when illuminated by ultra-violet light. He concluded that the maximum emission velocity was inversely proportional to the wave-length. The velocity varied from metal to metal; thus, for light of wave­ length λ 2010, the maximum emission velocity (measured in volts) for platinum was 1·86 volts, and for zinc 1·12 volts. The source of light used by Ladenburg was a mercury arc. His metals were polished with emery and oil, and were exposed to the atmosphere for some time before the apparatus for measuring the velocity could be exhausted.