Influence of insulating films on conduction

Abstract
This paper gives the results of conduction-current measurements on air-saturated transformer oil using direct voltages and both bare and coated aluminium electrodes. The coated electrodes were covered with a thin plastic insulating film and the variations studied were: (a) both electrodes bare, (b) both electrodes coated, (c) the cathode coated and the anode bare and (d) the anode coated and the cathode bare. It was noted that the magnitude of the conduction current was considerably reduced by coating either one of the electrodes or both.With bare electrodes, a region of rapid current increase with increasing voltage was always present. When only one of the electrodes was coated, the magnitude of the current was reduced by two orders of magnitude and the current/voltage characteristics showed different trends dependent on whether the cathode or the anode was the coated electrode. With a coated anode, provided that sufficient voltage was applied across the gap, the current showed a region of rapid rise with voltage, but, with a coated cathode, the current increase with voltage remained small up to the highest voltages used. An unexpected result was that, with both the electrodes coated, the region of rapid current increase reappeared. These results indicate the importance of the conditions at the electrode-liquid interfaces on the processes of generation and neutralisation of charge carriers and should contribute towards a better understanding of the conduction and breakdown mechanisms in liquid insulants.The paper concludes with a discussion observations of a mechanism which could be responsible for the experimental observations.

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