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Electrical insulating plate made of inorganic bonded mica splittings has a long satisfactory history as heater conductor insulation in applications requiring relatively little mechanical support for the insulation. Nevertheless, standard test methods for determining the quality of this material are singularly lacking, as are specifications prescribing acceptable quality levels for the material as such. The insulating heater plate is required, among other things, to resist in end use the passage of current through its volume and across its surface while exposed to elevated operating temperatures. The relative deleterious effect of the surface and volume leakage currents is determined by the specific design of the appliance. Electrical volume and surface resistivities at the operating temperatures are, therefore, two of the various properties of heater plate that are functionally related to the end use. The purpose of this paper is to describe a test method for the determination of the volume and surface resistivities, measured at the extreme temperature normally associated with the use of inorganic bonded muscovite heater plate. The test method is intended to provide a convenient means of measurement suitable for use as an acceptance test and for manufacturing control, with the degree of accuracy and precision necessary to distinguish between quality levels of heater plate and to produce satisfactory agreement between various laboratories as to the quality levels.
Mica Insulator Co., Schenectady, N. Y.