Aspect of electric strength of a moving gas

The influence on the prebreakdown processes and the sparking voltage of a gas moving at an angle to an electric field across a uniformly stressed gap is considered quantitatively by invoking the equivalent-pressure concept. The velocity with which a moving gas displaces the charged particles taking part in the processes leading to a spark is found, and the variation of the gap strength with the gas flow is calculated. With a pure cross gas flow, no change in the electric strength is predicted, and this is in agreement with experiment where gas speeds up to 2 ×103m/s have been attained. With axial gas flow, the moving gas can either increase or decrease the gap strength depending on the direction of the gas flow relative to the field due to the direct applied voltage, and provided the gas speed is comparable in magnitude with the electron drift velocity (under a.c. conditions, a reduction in strength always occurs). A novel method for obtaining the electron drift velocity, and hence the electron-molecule-collision frequency, by the use of a moving gas is evolved. The analogy between a moving gas and an applied crossed magnetic field is pointed out. This leads to a second method for obtaining the electron drift velocity by the simultaneous application of a moving gas and a crossed magnetic field. Possible relevance to compressed-air-blast circuit breakers is considered.