FACTORS AFFECTING MAGNETIC QUALITY 



269 



combination of properties of various kinds. The properties shown in the 

 figure are important in determining the best balance: the maximum per- 

 meabih'ty, ^im , only indirectly (it is a good measure of hysteresis loss and 

 maximum field necessary in use), and the Curie point, d, only in a minor 

 role. The saturation Bs , permeabiUty, and resistivity p, should all be as 

 high as possible. Bs , 6 and p are structure insensitive, and vary with com- 

 position in a characteristically smooth way, practically independent of 

 heat treatment; jum depends on heat treatment (strain), impurities and 

 crystal orientation. There are no phase changes to give sudden changes with 

 composition of properties measured at room temperatures. 



4x10^ 



20 40 60 80 



PER CENT NICKEL IN IRON 



Fig. 19 — Variation of saturation magnetostriction, Xs, and crystal anisotropy, A', with 

 the composition of iron-nickel alloys. 



Some of the properties of the iron-nickel alloys are given in Figs. 18 and 

 19. The change in phase from a to 7 at about 30 per cent nickel is responsible 

 for the breaks at this composition. The permeabilities, yuo and Hm , (Fig- 20) 

 show characteristically the effect of heat treatment. The maxima are closely 

 related to the points at which the saturation magnetostriction, X,. , and crys- 

 tal anisotropy. A', pass through zero (Fig. 19). 



Additions of molybdenum, chromium, copper and other elements are 

 made to enhance the desirable properties of the iron-nickel alloys. 



The iron-cobalt alloys, some properties of which are shown in Fig. 21, are 

 usually used when high inductions are advantageous. The unusual course of 

 the saturation induction curve, with a maximum greater than that for any 

 other material, is of obvious theoretical and practical importance. The sud- 



