22 BELL SYSTEM TECHNICAL JOURNAL 



series of these metals were made up and their magnetic properties 

 measured. 



These measurements showed the range of compositions which 

 shared in such unusual magnetic properties, and indicated that heat 

 treatment was an important factor in the development of these 

 properties. A large number of alloys have been made up in this 

 range, for which the variations in composition were evenly distributed 

 but much smaller than for the initial survey. From these alloys a 

 few were selected which appeared to be specially suited for magnetic 

 uses in electrical communication circuits. Our experience with these 

 alloys has been that when good grades of commercial materials are 

 used, the castings are readily reduced mechanically to the desired 

 dimensions, and the magnetic properties from different castings of 

 the same composition are quite uniform. 



We felt that these alloys were so unique as regards magnetic quality 

 that they should be grouped in a class under a common name wiich 

 should readily distinguish them from other materials. We have 

 chosen "perminvar" as the name for alloys in the iron-cobalt-nickel 

 series, which are characterized, when properly heat treated, by con- 

 stancy of permeability for a considerable range of the lower part of 

 the magnetization curve, by small hysteresis loss throughout the 

 same range of flux densities, and by a hysteresis loop constricted at 

 the origin for medium flux densities. 



This paper describes the magnetic properties of the perminvar 

 group of alloys. Results are given for several alloys selected to show 

 the variation in magnetic properties when the proportions of the 

 constituent metals are varied over a wide range. Detailed measure- 

 ments under a variety of magnetic conditions and heat treatments 

 are recorded for the composition 45 per cent nickel, 25 per cent cobalt 

 and 30 per cent iron. This composition is a typical one and was 

 chosen early in our experimental work as specially suitable for com- 

 mercial uses, for it had, in addition to the unusual properties in which 

 we were most interested, a fairly high initial permeability. 



Preparation of Alloys 



The alloys were cast from the best available commercial materials. 

 Armco iron, electrolytic nickel and commercial cobalt were melted 

 together in the desired proportions in a silica crucible in a high fre- 

 quency induction furnace. Before pouring, one half of one per cent 

 of metallic manganese was added to the molten metal. Part of this 

 manganese deoxidized the metal and went into the slag, and the 

 remainder, usually about one half of the added amount, remained in 



