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A re-entrant cylindrical cavity has been adapted to measure the dielectric constant and power factor of small disk samples of insulating materials. The methods of measurement, mechanical details, and electrical coupling and detecting circuits are all described. A single cavity can be utilized only over a frequency range of about 1.5:1 ratio from lowest to highest frequency. Therefore, several different sized cavities would be required to cover a range of frequency from 50 to 1000 megacycles. An accuracy of ±0.00005 in tan δ and ±1 per cent in dielectric constant may be obtained in routine measurements. Because the cavity has a very high Q (>2000), it is much more sensitive to low power-factor dielectric samples than any conventional coil-and-capacitor resonant circuit. The chief advantages of this method are that the operation of the apparatus is simple, very rapid, and similar to the susceptance-variation technique now used at lower radio frequencies. Also, the involved computations usually found in other methods operative in this frequency range are eliminated.