The behaviour of ferromagnetic sheets in alternating electric and magnetic fields II. Measurements of the electrical resistance of iron strips from 0 to 30 Mc/s

Abstract

Resistance measurements made on iron strips of thicknesses ranging from 7.1 to 92-9pm over the frequency range 15 kc/s to 30 Mc/s are presented together with supplementary measurements of the permeability and loss angle on rings cut from the same strip. Both strips and rings had been subjected to the same thorough anneal. A special specimen geometry (Bishop 1962) was employed to ensure current uniformity across the strip width at all frequencies. These measurements were designed for comparison with the predictions of the classical uniform permeability theory and the domain eddy current theories developed in the preceding paper. The measured losses are found to be much higher at low frequencies and somewhat lower at high frequencies than those calculated by the classical theory, especially with the thinner specimens. In this respect these observations agree in broad outline with the predictions of the simplest domain theory (which assumes identical equally spaced domain walls normal to the current flow) but the transition from square law to square-root law dependence of the losses on frequency is found to take place over a much wider frequency range than either theory predicts. The breadth of this transition is however satisfactorily interpreted in terms of a more refined domain theory which takes into account the very wide spread of domain wall restoring forces which is to be expected in the light of contemporary knowledge of the domain structure of iron.