Electrical breakdown in vacuum

Direct and impulse voltages were applied to an electrode system in a vacuum of the order of 10¿5 mm Hg. Of particular interest were the results obtained with a needle-and-sphere system, the breakdown voltage of which was considerably higher than that of a normal sphere-gap with the same electrode spacing, especially when a very fine needle was used. Oscillograms were taken of the current during a breakdown in vacuum, and of the collapse of voltage across the gap. With the fastest time-sweep available the current rise and voltage collapse were found to take place in 0.1 microsec. A longer sweep demonstrated that there is a minimum stable current below which a continuous breakdown will not occur; if circuit conditions limit the current to below this value, it will flow only intermittently. This may account for the difficulty experienced by some workers in making measurements of breakdown in vacuum with direct voltages.