Compressed Powdered Molybdenum Permalloy for High 

 Quality Inductance Coils * 



By V. E. LEGG and F. J. GIVEN 



Molybdenum-Permalloy is now produced in the fprm of com- 

 pressed powdered cores for inductance coils. Its high permeability 

 and low losses make possible improved coil quality, or decreased size 

 without sacrificing coil performance. Its low hysteresis loss reduces 

 modulation enough to permit application where large air core coils 

 would otherwise be required. 



Introduction 



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.^ With the development of telephone repeaters and the ex- 

 tension of circuits to transcontinental length some twenty-five 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 secured ^ 

 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 iron ' 

 and later permalloy powder.^ This type of core is mechanically 

 stable; it intioduces in an evenly distributed fashion the requisite 

 air-gaps, while avoiding undesirable leakage fields ; and it sub-divides 

 the magnetic material so as to reduce eddy-current losses. Although 

 other means have been suggested,^' ^ 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. 



Loading coil cores made from electrolytic iron powder generally 

 satisfied the stability requirements for long lines, but on account of 

 their low magnetic permeability they were large and costly. The 

 search for materials with higher permeability and lower hysteresis loss 



* Presented at Winter Convention of A.I.E.E., New York, N. Y., January 22-26, 

 1940. 



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