The electrical resistivity of sheet mica, a ground coat porcelain enamel, two alkali-free ceramic coatings, a silicone varnish, and a coating consisting of a silicone varnish mixed with ground mica was measured in the temperature range 100 to 500 C and at an applied direct current potential of 200 v. A plot of the logarithm of the resistivity versus the reciprocal of the absolute temperature for each of these materials showed that marked deviations from the Rasch-Hinrichsen law occurred only with silicone and silicone-mica coatings. The electrical resistivity of all coatings tested was found to be independent of coating thickness and of duration of aging at 200 C. The measured resistivities in ohm-cm of the materials when tested at 400 C were (a) silicone-mica coating, 1400 × 10⁸, (b) mica, 320 × 10⁸, (c) silicone varnish coating, 280 × 10⁸, (d) alkali-free ceramic coating, 1.6 × 10⁸, and (e) ground coat enamel, 0.0025 × 10⁸.