Corona in Oil

Experiments were made to determine certain effects of high local voltage stresses in transformer oil. The resistivity of the oil was measured by a special method immediately following a period of voltage stress. The results were: 1. A greatly reduced resistivity when the stressing voltage was greater than that producing visible corona. 2. A gradual increase in resistivity following removal of stress. The ``rapidly applied'' breakdown voltage was found for samples of oil which had been previously subjected to high voltage stresses. The dielectric strength varied in a manner similar to the resistivity, but showed an actual improvement in dielectric strength when the oil was given a rest period of 15 minutes. Definite conclusions are given at the end of the paper.