A new instrument is described which enables nondestructive measurements to be made of film and overlay thicknesses. It utilizes the change in electrical impedance of a composite sample resulting from differences in the thickness of the material being measured to cut off a swept‐frequency oscillator at a frequency determined by that thickness. Operating frequencies from 200 kc to 20 Mc have been used with frequency‐sweep widths, or deviations, up to 10% to measure overlay thicknesses ranging from 0.00005 to 0.007 in. on composite samples in which the conductivity‐to‐permeability ratio of the base and overlay differ by at least a factor of two. Equations are developed to illustrate the conditions for which a reactance‐tube oscillator is employed in a unique manner to make the measurement automatic.