An Optical Study of the Formation Stages of Spark Breakdown

Abstract

With the electrooptical shutter described in the preceding paper a visual and photographic study has been made of the manner in which a static breakdown occurs in an air gap with an approximately homogeneous initial field. The study covered a range of pressures $P$ from 20 to 76 cm of Hg and gap lengths $\delta$ from 1.25 to 10 mm. The manner of breakdown for any condition can be told by the product ($P^{\frac{1}{3}}\delta$): for values less than 11.9 the breakdown starts at the cathode; for values greater than 11.9 there are two initial breakdown regions, one at the cathode and one out in the gap. The exact manner in which the breakdown proceeds is shown and discussed in detail. The effect of space charges on the time required for the gap to completely breakdown is also shown and a possible arrangement of space charges is suggested which would explain the phenomena observed. Data on the rate of voltage fall of the gap are given.