Magnetic Properties of Perminvar ^ 



By G. W. ELMEN 



Synopsis: This paper describes the magnetic properties of a group of 

 iron-nickel-cobalt alloys, named "perminvar." With certain heat treat- 

 ments these alloys have unusual constancy of permeability and extremely 

 small hysteresis losses at low flux densities, and peculiarly shaped hysteresis 

 loops constricted in the middle as the maximum flux densities of the loops 

 are increased. Methods of preparing and heat treating the alloys are 

 described, limits of composition, and changes in the magnetic properties with 

 composition and with difi^erent heat treatments are illustrated. A theory of 

 constitutional changes effected by heat treatment and responsible for the 

 unusual magnetic properties is suggested. 



IN 1921 the writer was Investigating the magnetic properties of a 

 series of permalloys ^ to which a few per cent of a third metal was 

 added to the nickel and iron. One of these alloys contained cobalt. 

 Magnetic measurements indicated that up to moderate field strengths 

 the permeability of this nickel-cobalt-iron alloy was remarkably 

 constant. The constancy was materially better than for soft iron, 

 notwithstanding the fact that the initial permeability was several 

 times higher. This was unusual, as small permeability variation 

 ordinarily is found only in materials with low permeability. Measure- 

 ments of other magnetic properties were equally surprising. When 

 the hysteresis loop was traced for a cycle which carried the flux density 

 up to a few thousand gauss, it was found to have an extraordinary 

 form in that it was sharply constricted in the middle. These and 

 other differences which were observed indicated that this alloy was a 

 new type of magnetic material in which the magnetic properties were 

 fundamentally difi"erent from those of previously known materials. 



This discovery aroused a great deal of interest for it was recognized 

 that magnetic materials possessing these properties were of great 

 scientific and technical importance. In order to develop the possi- 

 bilities which this alloy suggested, an exploration of the whole field 

 of the iron-nickel-cobalt series was undertaken. For it was, of course, 

 apparent that the alloy which had aroused our interest must be one 

 of a group of compositions which possessed similar properties in a 

 greater or less degree. In this survey, alloys varying in 10 per cent 

 steps in composition and including the whole range of the ternary 



1 Reprinted from The Journal of the Franklin Institute, Vol. 206, No. 3, September, 

 1928. 



2 Arnold & Elmen, Jour, of Frank. Inst., May, 1923, p. 621. 



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