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Mica is useful in microwave tubes to provide vacuum tight rf output seals and to provide vacuum seals in tunable resonant circuits to permit moving tuner parts to be external to the vacuum. With these applications in mind an experimental investigation was undertaken to determine the maximum continuous electric field strength which mica will withstand at 10,000 megacycles without failing due to dielectric heating. An examination of the previously published data on the dielectric proper­ties of mica brings one to the conclusion that rather specialized test equipment is required to create fields sufficiently high at 10,000 megacycles to cause failure of thin mica specimens due to dielectric heating.