The Dielectric Strength of Benzene and Heptane

Abstract

The dielectric strengths at 25°C of benzene and of heptane in a uniform electric field produced with a sphere gap and of heptane in a non-uniform field between concentric cylinders, were measured on the pure filtered liquids and with rigorous control of the electrode surface. With any one experimental procedure, the breakdown gradients were reproducible to within 5 percent provided all variables were carefully controlled. The most important of these variables, the character of the electrode surface, is influenced by such factors as the degree of polish or etching, adsorbed gas or ions, and the gases dissolved in the electrodes. Adsorbed oxygen or hydrogen, for example, induces incipient discharges in either liquid with molybdenum electrodes, but not in heptane with nickel electrodes; in the latter case, the gas adsorbed on the electrodes slightly increases the breakdown gradient. Below one atmosphere, the dielectric strengths of benzene and heptane are independent of the pressure. In a non-uniform field, the dielectric strength of heptane is the same for a.c. and d.c. of either polarity. Although reproducible dielectric strengths were obtained by any one experimental procedure, the results from different procedures varied from 100 to 1700 kv (Peak)/cm. It appears, therefore, that the dielectric strengths of pure liquids depend primarily on the nature of the electrode surface which varies with the experimental procedure.