Three thousand tests on the dielectric strength of oil

Three thousand successive disruptive strength tests are herewith shown. These were made in six groups with 500 tests in each group under constant conditions of test. Three groups of tests were taken of standard insulating oil with three different shapes of the electrostatic field; small sphere gap, large sphere gap, and sphere-needle gap. One group of tests was made with commercial and another with chemically pure benzol. The last group was made with air as a dielectric. A brief review of results and conclusions follows. In measuring the voltage by a single sphere gap, set in air with reasonable care, the maximum error of test does not exceed 4 per cent and the average error is 1 per cent. In the average of six successive tests the maximum error decreases from 4 per cent to 2.9 per cent and the average error decreases from 1 per cent to 0.6 of 1 per cent. In contrast to air, the behavior of oil is very erratic. Successive observations of its disruptive voltage, made under most carefully controlled conditions, differed by a percentage many times greater than the accuracy of the test, — the minimum value was as low as 49 per cent of the maximum. This inconstancy of the disruptive strength of oil appears inherent to the material. Little of the variation is dependent on the shape and size of the electrostatic field. Much of the variation is due probably to the complex chemical and physical nature of the oil. Benzol gives far more consistent values of disruptive strength than oil, the more so the purer it is, but nevertheless benzol is much more erratic than air. Under successive tests oil, first slowly and then more rapidly, deteriorates by carbonization due to the disruption. Benzol deteriorates very rapidly at first, and then becomes fairly constant. Filtration restores the initial disruptive strength, but the filtered material seems to deteriorate more rapidly than new material. This information indicates that there is either an intermediate chemical state of the disintegrated product or an absorption.