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Dielectric specimens in the shape of a frustum of a cone were evaluated for surface flashover strength in vacuum of 10-5 torr. Data were obtained for positive and negative pulses of several hundred kilovolts amplitude and with rise times in the nanosecond and microsecond region. The results indicate that most materials exhibit a greater resistance to breakdown if the base of the cone is at the cathode and that the strength is greatly affected by the angle that the dielectric makes with the applied field. Lucite, polystyrene, polyethylene, cast Nylon, and 7740 glass are shown to be relatively resistant to flashover in comparison with the ceramics and molded plastics. It is also observed that the surface strength in vacuum is a small fraction (approximately 1/10 or less) of the volume strength and consequently breakdown through the bulk seldom occurs. Data are given for several common insulating materials.
O. Milton (Wed,) studied this question.