Compressed powdered molybdenum permalloy for high-quality inductance coils

William Fondiller (Bell Telephone Laboratories, Inc., New York, N. Y.): The paper by Messrs. Legg and Given, adds another interesting chapter in the development of new magnetic materials. In the past 25 years we have seen the cores of loading coils change successively from hard-drawn four-mil iron wire to compressed powdered electrolytic iron; then to powdered Permalloy, and finally to molybdenum Permalloy. As a result of these developments, the effective permeability of the magnetic cores has been raised from approximately 35 to 125. We are dealing here with the initial permeability, that is, the permeability at very low magnetizing forces, and with flux densities of the order of ten gausses. In this same period developments in connection with sheet Permalloy have made available material having an initial permeability of about 2,000. Looking at the matter superficially one might question whether it was worth 25 years' effort to raise the permeability of compressed powdered cores to 125, with this high permeability sheet material ready at hand. I think that a study of the curves of core loss and stability with superposed magnetization just shown by Mr. Given will demonstrate that no continuous sheet-metal core, no matter how thinly rolled the metal, could be made to meet all of the requirements as satisfactorily, considering cost and quality.