Positive and Negative Point-to-Plane Corona in Air

The present work has involved a careful comparison of positive and negative corona and observations at reduced pressures in the range 756- to 210-mm Hg. Oscillographic observation of corona pulses with a time resolution of the order of ${10}^{-7\mathrm{}}$ second, visual and photographic observation of the corona, and current-voltage measurements have thrown new light on fundamental corona processes. Some of the significant results obtained were as follows: For a polished platinum point of 0.38-mm diameter of curvature, the duration of positive pre-onset streamers and negative Trichel pulses near onset was about 0.4 microsecond; for positive breakdown streamers, 0.8 microsecond. A comparison of positive and negative current-voltage characteristics showed that the intermittent onsets were approximately equal but showed a small pressure-dependent difference. Both curves showed linear Ohm's law regions, and the negative current was always the larger; these effects are due to space charge. The negative corona revealed the fluctuations and dependence on surface conditions noted by previous workers.