Compressed powdered molybdenum permalloy for high-quality inductance coils

THE introduction of loading coils in the telephone system at about the turn of the century brought special demands on magnetic and electrical properties of core materials, and set in motion investigations which have had wide influence on the theoretical and practical aspects of ferromagnetism. The first step in this development led to cores of iron wire, which sufficed for loading coils on circuits of moderate length.¹ With the development of telephone repeaters and the extension of circuits to transcontinental length some 25 years ago, there arose need not only for loading coils, but also for network coils, which would have high stability with time, temperature, and accidental magnetization. Magnetic stability was at first secured² by employing iron-wire cores provided with several air gaps. Later, commercial and technical considerations led to a core structure made from compressed insulated powdered material, first electrolytic iron³ and later Permalloy powder.⁴ This type of core is mechanically stable; it introduces in an evenly distributed fashion the requisite air gaps, while avoiding undesirable leakage fields; and it subdivides the magnetic material so as to reduce eddy-current losses. Although other means have been suggested,^5,6 no way has yet been devised which provides these features so well and at so low a cost as the compressed powdered type of core.