Dielectric breakdown of insulating liquids

Lichtenberg figures in insulating oil are employed in order to make clear the breakdown mechanism of a dielectric liquid. The thin figure, which may be explained as the trace of the pure electronic process in the liquid, is not affected by the variation of atmospheric pressure above the oil surface; but the thick figure, which may show the discharge in the gas phase produced by the rupture of the bonding of the oil molecules by electron bombardment, is greatly broadened by reduction of pressure. Even when an initial discharge takes place in the gas phase and breakdown occurs at lower field strength, the pure electronic process in liquid remains unchanged. These results are in agreement with those of spectrographic observation of impulse corona in liquids.