The Cathode Region in the Glow Discharge

Abstract

The present study of the cathode region in the glow discharge is divided into three sections: I. The range of the electrons in the negative glow. II. Positive ion formation in the Crookes dark space. III. Energy of the electron entering the negative glow. I. The length of the negative glow and the drop in potential across the Crookes dark space have been measured simultaneously for various gases over a wide current and pressure range. The observed lengths correspond exactly to the range of electrons as determined by Lehmann for voltages equivalent to the cathode potential drop. From this it is concluded that the energy of the electrons entering the negative glow must correspond to the entire difference in potential across the dark space. II. The length of the Crookes dark space has been measured under various conditions and the values obtained compared with the mean free path between ionizing collisions as given by Tate and Smith. The number of positive ions found in the dark space per electron of current has been computed by making use of Aston's equation for the potential distribution and Tate's values for the efficiency of ionization as a function of voltage. The number of positive ions so computed is, in general, materially less than one per electron; this contrasts sharply with from 50 to 100 estimated by some investigators. III. The energy distribution of the electron entering the negative glow has been measured by the deflection method, a nonhomogeneity of velocity in the electron beam being discernible by an elongation of the fluorescent spot on a Willemite screw. No change in the shape of the spot was detected for the electrons leaving the dark space although an elongation of it was observed for electrons in the negative glow. The experimental arrangement was such that fluctuations in voltage of more than 10 percent could have been observed. The general conclusion to be drawn from these studies is that the energy of the electrons entering the negative glow corresponds closely to the entire cathode potential drop, and that only a small fraction of the energy of the electrons leaving the cathode is expended in the Crookes dark space. A possible mechanism for the cathode region is briefly outlined.