Effect of electrode polarity and additives on the direct breakdown voltage of silicon oil under highly non-uniform fields

The direct breakdown voltage versus gap length (50 to 900 mu m) characteristics were obtained using point-sphere electrodes for degassed silicone oil, oxygen-saturated oil and oil containing varying concentrations (0.005 to 1.0 molar) of 1-methylnaphthalene as additive. The characteristics of degassed and oxygen-saturated methylnaphthalene were also obtained. The shape of the characteristics for both polarities of the point depended on the material of the sphere electrode, the presence or absence of oxygen, and the nature of the liquid itself. In silicon oil the crossover gap length, at which the breakdown voltage is the same for both polarities of the point electrode, was increased greatly by the presence of oxygen, whereas oxygen greatly reduces this gap in hydrocarbon liquids. In both degassed and oxygen-saturated oil the presence of the additive caused a shift in the crossover gap to smaller lengths.<>