Thresholds for initiation of a power law current response to ramped high voltage in silicone oil

Abstract

A technique has been developed in recent years with which accurately reproducible current characteristics are obtainable in silicone oil as a function of voltage when this is raised and lowered steadily at a uniform ramp rate [l]. Analysis of the data has revealed that the average of the recorded current response to the rising and falling voltage will display on a log-log scale two linear portions which are sharply divided [2J. This current response occurs in the high field regime indicating that it obeys power laws of the form I = ΚvSu, 1 in each sub-regime where the log-log slopes are su and s1 for the upper and lower of these respectively. It is the purpose here to report the results of an investigation into the influence upon this current response of the application of voltage with a variable ramp rate. This has been carried out with hemispherically tipped copper electrodes over the gap range from 1.55–6.20mm and dimethyl siloxane fluids (Dow Corning 200) of viscosities 5, 350 and 1000 c.s. The experiments moreover were repeated with a thin (25 micron) film of untreated polypropylene (075 gauge, Hercules EK 500) extended across the cathode and in contact with it. Voltage ramp rates employed in the investigation ranged from 3.0 to 7.0 kV. S−1 up to 70 kV. As in all previous investigations of this kind, the current response displayed directly as a V-I tracing on an X-Y recorder apparently showed hysteresis which has been attributed to the fact that the rising part of the characteristic is due to the sum of the current response and the displacement current while on the falling part this is much reduced because of the reversal of the ramp rate (Fig.1). Thus the strictly voltage sensitive response has been obtained simply by obtaining the mean value Im of the two observed current readings at each value of applied voltage. The part of the current which is sensitive to the sign of the ramp rate can similarly be obtained by deriving from the V-I characteristic one half of the difference between the current recorded when V is rising and falling. These two currents will be referred to as the mean and differential currents Im and Id to be precise with the nomenclature.