The paper describes apparatus devised for the measurement of the permittivity and power factor of insulating materials over a wide range of radio frequencies. The range covered is from 10 kc to 100 Mc, and the same calibration of the apparatus holds good over the entire range without the use of any troublesome corrections. The only changes necessary in order to proceed from one part of the range to another are changes of tuning coils.The method is that of capacitance-variation in a tuned circuit, with a thermionic voltmeter as a detector of resonance. The adjustments are made by means of two micrometer condensers, one being a plate condenser in which the sample is inserted, and the other a cylindrical condenser of linear law and of very small range, which serves to measure the sharpness of resonance. Both permittivity and power factor may be obtained as the ratio of capacitance readings: frequency is not involved in their calculation, and it is this property which gives the apparatus its very large frequency-range.The errors due to residual inductance and resistance at the highest frequencies are discussed, and it is shown that these errors, together with those associated with imperfect contact of electrodes, may be eliminated, partly by very careful attention to details in the construction of the apparatus, and partly by the procedure followed in making the measurements.The apparatus is also applicable to measurements on resistors over the same range of frequencies.