Permeability Changes in Ferromagnetic Materials Heat Treated in Magnetic Fields

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Abstract
Very high maximum magnetic permeability has been obtained in certain ferromagnetic alloys by slowly cooling them in a magnetic field. In permalloy with 78.5 percent nickel it has thus been raised above 140,000 that is, to the same order as is obtainable by rapid cooling without an applied field. In perminvar with 45 percent nickel, 30 percent iron and 25 percent cobalt this magnetic heat treatment is also found to be effective. It is also shown that the maximum permeability is highest if the testing field is in the same direction as that of the field applied during cooling. In directions at right angles to this, the maximum permeability is decreased. The ratio of the two permeabilities in one instance was over 14 for permalloy, and in another instance nearly 70 for perminvar.

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