Experiments on the Scattering of Electrons by Ionized Mercury Vapor

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Abstract
Further experimental studies were made of a phenomenon first described by Langmuir. He found that if electrons from a hot filament were projected into an ionized gas with a uniform velocity, they did not retain this uniform velocity but some were retarded and some accelerated as though a random velocity distribution had been impressed upon the uniformally moving stream. The pressure of gas was sufficiently low that only a small percentage of the electrons scattered had collided with gas atoms. Experiments were carried out to determine the exact nature of this scattering and its dependence upon the variables of the tube. The experiments on the variation with distance showed that the scattering all took place within a limited region surrounding the filament, the size of this region being directly proportional to the original velocity of the electrons. With this experimental basis a theory to account for the scattering is indicated. Fluctuations in potential distribution rapid enough to be comparable in period with the time taken by the electrons to pass across the space between the filament and the electrode used to study the velocity distribution are shown to be sufficient to account qualitatively for all the phenomena observed. No direct evidence that these fluctuations are actually present could be obtained. Reasons are given to show that they might be expected. Other possible explanations given by Langmuir have involved new ideas concerning the interchange of energy between either radiation or excited atoms and electrons which seem improbable and which are not required in the explanation here given.