Intrinsic Electric Strength and Conductivity of Varnish Films and their Variation with Temperature

Recent advances in the theory of the intrinsic electric strength of solids are briefly discussed. Methods for the preparation and testing of samples of baking insulating varnishes and shellac-varnish films for the determination of their intrinsic electric strength are described and the results of such determinations, made on a thickness range of 0.02 mm. to 0.1 mm. and a temperature range of ¿196° C. are reported. The investigation has also included subsidiary tests, such as conductivity measurements, effect of different ambient conditions (immersion media and applied pressure), time/voltage tests, and the effect of centrifuging and filtering and comparison tests on varnish-impregnated paper films, which were made to establish the validity of the method of test and to assist interpretation of the results. The large and continuous increase of intrinsic electric strength with decrease of temperature down to ¿196° C. which is found, disposes of the assumption hitherto made by some investigators that intrinsic electric breakdown is independent of temperature. Consideration of the results in relation to the recent theory proposed by Fröhlich to account for a temperature-dependence of the character obtained, indicates that this theory in its present form is not applicable, but an alternative explanation is suggested.