Surface and volume dielectric losses

A RELIABLE and convenient system has been devised for measuring large numbers of specimens to detect changes in dielectric constant and dissipation factor due to exposure to controlled conditions of temperature and humidity. A group of four plastic materials consisting of a molded mica-filled phenolic (MFE), a glass silicone laminate (GSG), polytetrafluoroethylene (PF) and polyethylene (PE), were selected for study over the frequency range from 60 cycles per second (cps) to 100 megacycles per second (mc). Measurements of alternating current surface and volume conduction losses, and d-c surface and volume conductivity were made, along with measurements of dielectric strength and flashover strength.1 The effect of humidity and temperature on these electrical quantities was a major objective in an extensive program. Earlier work at The Johns Hopkins University had greatly stimulated the present interest in the measurement of surface conductivity.2,3 Although the earlier efforts were made with direct current only, it was generally considered that the work could be extended to the a-c case through the use of a high-voltage Schering bridge. This has been accomplished, data having been obtained at 60 and 1,000 cps. Such measurements are possible with a high-voltage Schering bridge up to 100,000 cps. Using other types of networks, the surface measurements were also taken at 2, 18, and 100 mc. The volume determinations of dielectric constant and dissipation factor were made at the same frequencies. Perhaps the most stringent requirement met in these tasks was that all specimens had to be mounted in holders and measured on location without being disturbed during their 6-month exposures to various conditions of temperature and humidity.