Propagation of Potential in Discharge Tubes

Abstract

The speed of propagation of potential waves in long discharge tubes containing dry air has been measured as a function of pressure in three tubes with internal diameters of 1.7, 5 and 18 mm. Positive and negative impulsive potentials of approximately 125 kv were used. In general, in the low pressure range the speed increases with increasing pressure. From 0.02 to 0.2 mm the curves are steep; above this range there is a decided flattening. At relatively high pressures the speed decreases and the wave shape is greatly distorted. No appreciable difference between the speeds in dry air, C${\mathrm{O}}_2$ or ${\mathrm{H}}_2$, could be detected in the 5 mm tube. The speed at constant pressure in dry air is approximately a linear function of the applied voltage (75-180 kv-5 mm tube). The initial wave which traverses the tube from high voltage end to the grounded end is followed immediately by a return discharge wave starting at the grounded end. The speed of this wave is about ${10}^{10}$ cm/sec. and is apparently independent of tube diameter. There is a slight increase in speed with pressure. Maximum currents in the initial wave were measured as a function of pressure. Voltage attenuation and some knowledge of the field in the wave fronts were obtained. The speed is found to obey the principle of similarity. Speeds in the 18 and 5 mm tubes are the same at pressures in the small tube 3.6 times those in the large tube. Speeds in the 1.7 mm tube are higher than they should be according to this principle. This is probably due to the high current density (4000 amp./${\mathrm{cm}}^2$).