The paper presents a description of sensitive and accurate apparatus for measuring the power factor of dielectrics in the frequency range, 60–7,200,000 c.p.s. A Wien bridge with Wagner ground for the direct measurement of both power factor and frequency in the range, 60–10,000 c.p.s. is described. Double shielding of the series resistance arm eliminates errors otherwise inherent in this bridge. The operation of the resonance‐substitution method is discussed for the frequency range, 10,000–7,200,000 c.p.s. Errors due to eddy current losses are eliminated through measurements on loss free liquids. A measuring cell for use with these circuits and also with d.c. measuring circuits is described. Some experimental power factor‐frequency curves illustrating the results obtainable with this apparatus are presented. It is shown that power factor‐frequency data furnish a delicate method of tracing out dispersion phenomena and are useful where the concentration or electric moment of material producing dispersion is small.