The electrical properties of high permeability wires carrying alternating current

Abstract

It is well known that certain alloys of nickel and iron of the “perm­ alloy ” and “mumetal” group have abnormally high values of magnetic permeability in small fields, and the properties of these alloys have been the subject of a considerable amount of research during the past ten or twelve years. It seems to have escaped attention that their high magnetiz-ability at low field-strengths causes wires of these alloys to have many interesting properties when used as conductors of alternating current. Owing to the “skin effect”, the a. c. resistance of a wire bears to its d. c. resistance a definite ratio which is a function of the frequency of the current, and of the diameter, the electrical conductivity, and for ferromagnetics the permeability of the wire. With fine wires (diameter 0•5 mm. or less) of non-magnetic material such as copper, this ratio does not materially exceed unity until the frequency approaches 10 5 cycles per second. But with wires of ferro-magnetic alloys, whose permeability may be in the neighbourhood of 50,000, the ratio R AC /R DC is appreciably greater than unity even at the lowest audio-frequencies. Furthermore, a ferromagnetic wire possesses an internal self-inductance, which, with alternating current, is also a function of the frequency, diameter, con­ductivity, and permeability. Now permeability is a function of magnetiza­tion, and since a wire carrying a. c. is magnetized by the current as well as by the external field in which it may be situated, it may be expected that its a. c. resistance and inductance will show some sort of dependence on both current and external field.