MAGNETIC ALLOYS 



121 



A 



2000 



4000 6000 8000 10,000 12,000 14,000 



INDUCTION (b) in gausses 



Fig. 5 — Permeability curves for permalloys. 



The permeability for alternating current of small constant amplitude 

 as a function of superposed d.-c. magnetizing force is shown in Fig. 7 

 for some of the alloys. In most apparatus where both alternating and 

 direct current are involved, this "butterfly curve" must be relatively 

 flat over the expected range of d.-c. excitation. The important 

 magnetic constants for these alloys are given in table II. 



Permalloys 



In Fig. 8 the initial and maximum permeabilities and the coercive 

 force and resistivities are plotted for quenched alloys of the iron-nickel 

 series. These curves show the remarkable variations in magnetic 

 properties with composition in this series of alloys. The permalloy 

 region includes alloys between 30 and 95 per cent nickel, as indicated in 

 Fig. 1. Some of the alloys in this region, particularly from 50 to 85 

 per cent nickel, require rapid cooling to develop the magnetic proper- 

 ties indicated in the curves. If they are merely annealed, both the 

 maximum and the initial permeabilities are much lower. The greatest 

 effect in reducing the permeabilities by slow cooling appears to be for 



