Abstract
The motion of the cathode spot in a direction opposite to that predicted by Ampère's law depends on the arc current, gas pressure, kind of gas, and magnetic field strength. Studies of the effects of these variables have been made, including measurements of velocity and the critical pressure at which reversal of motion occurs. The phenomena observed have not been clearly explained by any of the pictures presented to date. The existence of the retrograde motion indicates very strongly that the positive‐ion space charge outside the cathode is all important in determining the mechanism of current transfer.

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