The kinetic energy of electrons emitted from a hot tungsten filament

The first measurements of the kinetic energy of the electrons emitted from hot bodies were made by Prof. Richardson and Dr. F. C. Brown in 1907-1909. These experiments showed that the velocity distribution among the emitted electrons was in close agreement with Maxwell’s law of distribution for a gas, of molecular weight equal to that of the electrons, in thermal equilibrium at the temperature of the source. In the simple unidimensional case where the cathode and anode form parallel planes of indefinite extent, the current which flows against a retarding potential, V, depends only on the normal velocity component and with Maxwell’s distribution is given byi =i 0 e -2hℯ V, whereh = 1/2k T,k being Boltzmann’s constant, T the absolute temperature of the source, and e is the electronic charge. The mean kinetic energy of the electrons in the stream is given by the quantity 2k T. These experiments showed that with platinum, which was the only metal tried, the exponential equation was very accurately obeyed, and the average of eight determinations ofk agreed with the theoretical value to within a fraction of 1 per cent. although the individual determinations differed from the average by almost ±20 per cent. These experiments were,- however, subject to a number of defects, the most important of these being due to the presence of electric and magnetic fields caused by the electric currents used in heating the source. Schottky carried out some experiments in 1914, and he used the case of a filament surrounded by a concentric cylindrical anode. The effects of the magnetic and electric fields of the current used to heat the filament were avoided by an interrupted current method due to v. Baeyer. Schottky’s experiments were made with carbon and tungsten, and the data were in good agreement with the requirements of Maxwell’s law, except that the average energy of the emitted electrons was in every case in excess of the value calculated from the temperature of the source. This, however, was estimated from the value of the saturation current using the emission constants given by other authors. This makes his temperature determinations very uncertain, because of the known large effects on the emission of traces of certain contaminants.