Space Charge in Liquid Dielectrics

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Abstract
Studies were made of the unequal potential distribution commonly observed in insulating liquids under continuous potential difference. The potential distribution between parallel plates is measured in two insulating oils for over‐all voltages between 500 and 1500 volts, and at temperatures between 20 and 60°C. This distribution is produced by an accumulation of space charge near the electrodes which accumulation, however, extends measurably into the volume of the liquid. The amount of space charge as computed from the potential distribution increases with increase of applied voltage and decreases with increase in temperature. On reversal of the applied voltage, the initial over‐all conductivity is of approximately the same value as that preceding reversal. The initial current on reversal may remain constant over an appreciable interval, say from 1 to 4 sec. Following the initial constant value, the reverse current rises sharply to a maximum from 2 to 6 times the initial constant value, and at first rapidly and then slowly decreases to the same long‐time values as that of the preceding opposite potential. In this condition, a reversal of the foregoing pattern of potential distribution takes place. It is suggested that two types of ion are involved in the space charge, one of high mobility, and the other of low mobility, due to the accumulation about it of an aggregate of neutral molecules. Oscillographic studies of the variation of current on reversal offer means for approximate computations of the mobility of the ions involved. The mobility of the ions in a given liquid may take a wide range of values. As between different oils and for the same oil at different temperatures, mobilities vary inversely as the viscosity. In a low viscosity insulating oil a high value of 10−4 cm/sec./volt/cm has been observed at 22°C.

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