Abstract :
A method for the direct measurement of the quantities involved in the a.c. permeability of a specimen, namely Bmax. and Hmax., is described. It is applied in an investigation of the dependence of the a.c. permeability of commercial sheet materials on frequency and wave-shape at power and telephonic frequencies. In agreement with other observers, the effect of frequency is found to be neglible up to 500 cycles per sec. apart from a reduction, attributable to eddy currents, at magnetizing forces for which the permeability is high. Tests on a range of specimens with magnetizations giving wave-shape conditions varying between sinusoidal voltage and sinusoidal current, failed to show the increased permeability with approximation to a sinusoidal current wave which was observed by Lombardi and McLachlan. Within the accuracy of the measurements, the permeability appeared independent of wave-shape between these two practical limiting conditions. In the second part of the paper a comparison between the results of loss measurements on magnetic meterials by bridge and wattmeter methods is described. The discrepancies observed are shown to be entirely attributable to the errors in the bridge results due to the presence of harmonics, and the conditions under which the bridge method may be used satisfactorily for iron-testing, at both power and telephonic frequencies, are deduced.
