18 BELL SYSTEM TECHNICAL JOURNAL 



properties in that the flux density is higher. In addition, the electrical 

 resistivity of the alloy is approximately three times that of the 78 per 

 cent alloy. The electrical resistivity is important in alternating current 

 applications since the eddy current loss in sheet of the same physical 

 dimensions varies inversely with the electrical resistivity of the 

 material. 



To the metallurgist the effects of heat treatment in establishing the 

 magnetic properties of iron-nickel alloys possess a great fascination. 



16000 



14000 



4 5 6 



H IN OERSTEDS 



Fig. 7 — Magnetization curves for 78.5 permalloy after two heat treatments — Elmcn^'. 



The effects in question are illustrated by curves shown in Fig. 8. In 

 the region of 78 per cent nickel, the maximum magnetic softness is ex- 

 hibited after air-quenching, while increases in magnetic hardness are 

 obtained after annealing or baking for extended times at low tempera- 

 tures. The compositions near 50 per cent nickel are not as much af- 

 fected by heat treatment. 



Elmen ^"^ attributed the heat treating effect to the separation of the 

 homogeneous phase, stable at high temperatures, into two phases on 

 slow cooling. The two phase structure under these conditions would 

 be less magnetic. More recently in Germany, investigators ^* have 

 advanced the analysis by postulating that in the region of 76 per cent 

 nickel, where the ratio of iron to nickel atoms is 1 ; 3, the alloy can exist 



