Electron emission from langmuir probes and from the cathode of the glow discharge through gases

The current to the cathode of a direct current discharge through gases is made up of a flow of positive ions to the cathode surface together with a flow of electrons away from the electrode. It is of importance in the theory of the discharge to obtain an estimate of the relative proportions of the current carried by electrons and positive ions. Owing to the very small ionising efficiency of positive ions with the energies present in the discharge, it is necessary that the electron current should arise largely from the action of the positive ions on the surface of the cathode, and it is generally assumed that the electron emission should increase with the energy with which the ions strike the electrode. This paper presents the results of measurements made to determine the positive ion and electron currents at the surface of a simplified form of cathode, a negatively charged Langmuir probe in the positive column of an arc discharge through various gases. The theory of the cold probe developed originally by Langmuir and Mott-Smith assumes that the electron emission is zero, but it will be shown that evidence of electron emission is given both by observation of the current-voltage characteristic of such a probe and by measurement of the heating effect of the positive ions. The results will then be discussed from the point of view of the glow discharge.