The Conductivity of Insulating Oils

The paper describes experiments on the charging current and other associated phenomena in high grade transformer oil. The charging currents remaining after the elimination of the initial transient were studied to within a few hundredths second of application of continuous voltage. Two samples of the same oil obtained at different times differed radically. In one case the charging current fell off from the start, while in the other it remained constant for an appreciable time or even temporarily increased. On reversal of polarity after a long charge the initial current with both specimens was the same as before reversal, but with one a large momentary increase occurred a few seconds later. Evidence is given on the time of recovery of the initial condition. The existence of space charges in the charged oil is shown, and the time of formation of these charges together with the resulting non-uniform distribution of voltage is measured. The importance of the charging current in its early stages is emphasized, since it is the initial conductivity which determines the a-c. loss. In an appendix a theoretical discussion is given on the influence of the space charges. It is shown that at least qualitatively these account for the observed phenomena.