Abstract
The original theory of the origin of the electron emission from glowing solids, which is based on the electron theory of metallic conduction, has recently been subjected to criticism on account of the manner in which the emission can be reduced by continually removing impurities from the discharge tube. The critics maintain that the emission occurs as a result of chemical action between the hot cathode the hot cathode and the surrounding gas, or between the constituents of the cathode itself. In a recent paper the author has summarised the evidence in favour of this latter view, and has described experiments which show that the results which it leads us it expect do not always occur. In particular it has been shown that the chemical action theory of the origin of the activity of a Wehnelt cathode, as propounded by Fredenhagen and by Gehrts, cannot be accepted. The experiments described in the present paper were designed to test further the theory that the electron emission is due to chemical action. The experiments consisted in studying the ionisation produced by Nernst filaments heated in various gases, of different chemical affinities for the material of the cathode. The apparatus and method of experiment were similar to those described in the paper already referred to, and its temperature was determined by means of a specially standardised Féry optical pyrometer which was kindly lent to me by Prof. T. Mather, of the City and Guilds Engineering College, London. The anode consisted of two parallel platinum plates, fixed in the discharge tube at equal distance on opposite sides of the filaments, and connected together outside the apparatus. The potential difference applied to the ends of the filaments was measured by a Siemens alternating voltmeter, which was used in series with a resistance of equal magnitude, the total resistance in parallel with the filament being 4572 ohms. The junction of the voltmeter and series resistance was connected to earth, and thus the mid-point of the glowing filament was kept at zero potential. The platinum plates forming the anode were connected through delicate galvanometer to the positive pole of a high potential battery, the negative pole of which was earthed. A diagrammatic view if the arrangements is given in fig. 1, which for simplicity the discharge tube is not shown. The filaments experimented with were all of the same type and were intended for use on a 100-volt alternating circuit. The thermionic currents in air under similar conditions varied slightly for different filaments, but in every case the currents measured after heating for some time were remarkably constant. In this respect the electron emission from a Nernst filaments is very different from that obtained with a metal cathode, which usually decreases continuously with time. Measurements of the thermionic currents in air, nitrogen, oxygen, and hydrogen were made. In each case the variation of the current with the pressure of the gas under a constant applied potential difference with a constant gas pressure in the discharge tube. The observations were usually made with the filaments at a temperature of 1525° C., but measurements were also made at other temperatures.