A magnetic comparator for measuring the thickness of high conductivity, ferromagnetic films is described. The instrument compares the reluctance of a standard sample to that of a film of unknown thickness at the same value of magnetic field intensity to insure that the permeability is the same for both. The comparator utilizes an ac Hartshorn, mutual‐inductance bridge configuration and is designed to operate at low frequency and low magnetic field intensity to maximize the skin depth &dgr; and thus the measurable film thickness. The output of the comparator is linear in thickness for films less than &dgr;/2 and is capable of measuring films of thickness greater than &dgr; with some reduction in accuracy. The effects of the skin depth and magnetic properties of the film on the accuracy of the comparator are discussed.