Measurements of Pre-Breakdown Currents in Dielectrics With a Cathode-Ray Tube

1. A method has been developed for photographing repeating traces on a cathode-ray tube which measure current in liquid and solid dielectrics with slowly increasing unidirectional voltage up to within two milliseconds of breakdown. The range of current sensitivity is from 10^¿8 amperes up. Current values at time intervals closer to the breakdown point could be measured by increasing the speed of motion of the film and decreasing the sweep interval. The probability of catching fast transient of the order of one microsecond duration is only 2 per cent (ratio of sweep time to sweep interval). 2. In liquids, the evidence so far obtained leads to the tentative conclusion that the final breakdown current increases from a substantially steady value to failure in fractions of a microsecond. This indicates that the final breakdown mechanism is electronic. 3. In liquids, it appears also that the pre- breakdown ionic current may be unrelated to the breakdown electronic current since the breakdown gradient appears to be affected more by the electrode surface conditions than by the ionic conductivity of the liquid. 4. In polystyrene and polyvinyl formal only one record was obtained showing any pre-breakdown current greater than 10^¿8 amperes up to within two milliseconds of failure at breakdown gradients of the order of 2.5 × 10⁶ volts per centimeter.