Abnormal Shot Effect of Ions of Tungstous and Tungstic Oxide

Abstract

Part I. The formation of positive ions of tungstous and tungstic oxide. It was found that when oxygen attacks a hot tungsten filament, positive ions are formed, and a mass spectrograph test has shown these ions to be a mixture of tungstous and tungstic oxide. The positive ion current is proportional to the pressure of the gas, as is the rate of formation of the oxide. The evaporation of an oxide layer following the pumping away of the oxygen, yields an ion current which falls away exponentially with the time. The ratio of the number of charged particles of the oxide, to the number of uncharged particles, is approximately one in twenty thousand, and this ratio does not vary greatly with the temperature of the tungsten filament.Abnormal shot effect of the currents of positive ions. When the positive ions are drawn to a collecting electrode, in series with which there is a tuned shot circuit, an abnormal shot voltage is developed. The hypothesis is advanced that this shot voltage arises from fluctuations in the inter-electrode capacitance of the tube, caused by variations in the thickness of the electron sheath surrounding the anode as the sheath is penetrated by the positive ions. On this basis, an expression for the resulting mean square shot voltage is presented. This expression was found to be correct as regards the variation of the shot voltage with the ion current, and it has yielded reasonable values for the change in the inter-electrode capacitance.Part II. Abnormal shot effect due to the trapping of the positive ions. When positive ions of tungstous and tungstic oxide were trapped in the minimum of potential surrounding a hot cathode, an abnormal shot voltage resulted. This abnormality was found to vary inversely with the square of the resonance frequency of the tuned shot circuit, analogously with the "flicker-effect." An expression for the mean square shot voltage has been developed, which permits the calculation of the average plate current increase due to the trapping of a single ion. The simultaneous measurement of the shot voltage and the total plate current increase has permitted the determination of the number of electrons released by each positive ion, and also the average time of trapping of the ions. From a knowledge of the trapping time, values of the electron space charge density in the minimum of potential were computed. These values were in agreement with those computed from a knowledge of the filament temperature, and the space current, and the two methods of arriving at the space charge density yielded magnitudes which varied in the same manner with the electron current through the tube.