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.