Electric strength of moving electrodes in a still and in a moving gas

The influence on the sparking voltage across uniform field electrodes moving in a still gas, air, is considered quantitatively by the development of the effective-gap concept, assuming laminar flow. When the electrodes move in the direction of the ionising electron stream, the d.c. electric strength is reduced, and, when the electrodes move in the opposite direction, the d.c. electric strength increases, provided, in both cases, that the velocity of electrode movement is comparable in magnitude with the electron-drift velocity (under a.c. conditions, a reduction in strength always occurs). The argument is then extended to take into account the case of moving electrodes in a moving gas, and a similar effect is found to be superimposed on that obtained previously for a moving gas. Possible relevance to electrically stressed aerials moving at speed through the atmosphere is briefly considered.