Review of Past Work on Liquid Breakdown

The electrical breakdown of liquid dielectrics is a complicated process and there is no single theory that explains all the experimental results. Specific experimental conditions will be the determining factor as to which of the several mechanisms will be operative. Because of poor control of liquid purity prior to 1950 the large scatter of breakdown measurements had precluded a determination of factors controlling the intrinsic dielectric strength. During the fifties it became possible to measure a breakdown strength which depends upon a particular variable, other variables being held constant. This is done by giving special attention to chemical and physical purification (filtering) of the sample, close control of the electrode surface polish and geometry, and the use of pulsed voltages. Under these antiseptic laboratory conditions, the extrinsic mechanisms (which usually result in low values of breakdown) can be more or less avoided and one can measure a relatively high strength. These values can be reproduced by different investigators and serve as a basis for evaluation of theories of breakdown. More recently new techniques have been developed to investigate transient phenomena prior to, and in the early stages, of breakdown. These have supplied additional evidence concerning the mechanisms by which ionization leads to a conducting path between the electrodes. On the one hand, there are the mechanisms in which ionization occurs in the liquid phase while on the other, those in which it occurs in a gas (low density) phase produced prior to breakdown (bubble or cavitation theory).