436 BELL SYSTEM TECHNICAL JOURNAL 



supplied, its magnetic properties were not even as good as iron in a 

 similar magnetic condition, but when heat treated it was superior to 

 the iron at low field strengths, the region in which I was especially 

 interested. 



This alloy differed from iron in another important characteristic. 

 Experience with iron had shown that the best magnetic quality was 

 obtained when the material was heated to a high temperature, and 

 then cooled slowly to room temperature. It was considered par- 

 ticularly important to cool slowly in order to give the iron time to 

 pass through its transformations and to allow it to build up a large 

 grain structure. When the nickel-iron alloy was heat treated in this 

 manner, it was found to have lower permeability than it has when 

 cooled fairly rapidly. 



The discovery that rapid cooling was required for this alloy to give 

 the best magnetic quality was one of the major contributions from 

 our early work. It showed us that one of the important factors in 

 developing the magnetic properties of new alloys was the determination 

 of the rate of cooling for the best magnetic quality for each alloy. 



Another difference between this alloy and iron relates to the energy 

 loss caused by the hysteresis at low flux densities. In this range 

 iron has lower hysteresis loss when it is in a mechanically hard con- 

 dition than when it is well annealed. As the flux density increases, 

 however, the hysteresis loss of the hard material increases more 

 rapidly than does that of the annealed and at medium and high flux 

 density, the mechanically hard iron is much inferior. Tests on the 

 nickel-iron alloy show that both for high and low flux densities the 

 hysteresis loss was a great deal higher for the mechanically hard 

 material than for the one heat treated to give the best magnetic 

 quality. 



The discovery of the unusual magnetic properties of this 70-30 

 per cent nickel-iron alloy gave us the lead that we were looking for, 

 and started our investigation of the magnetic properties of the whole 

 series of nickel-iron alloys. We found, as we had reason to expect, 

 that the 70-30 per cent alloy was one of a large group of alloys in the 

 same series which had similar magnetic properties. In fact sub- 

 stantially all of the alloys containing more than 30 per cent nickel 

 had similar characteristics except that some of them were not as 

 sensitive to heat treatment as the first alloy we had tested. 



Because of the technical possibilities of the nickel-iron alloys, we 

 spent several years in their investigation and their commercial appli- 

 cation. We were especially concerned with increasing the resistivity^ 

 of a number of these alloys, and with this in view we added other 



