MAGNETIC ALLOYS OF IRON, NICKEL, AND COBALT 455 



Fig. 18 illustrates the manner in which the permeability at a low 

 magnetizing force for a rapidly cooled 78.5 permalloy composition is 

 affected by passing it through a temperature cycle between room 

 temperature and 650° C. The permeability was measured for a 

 constant magnetizing force of .003 gauss as the temperature was passed 

 through this cycle. The rate at which the sample was heated and 



Fig. 16 — Hysteresis loops for 78.5 permalloy. 



cooled was rather slow, requiring upwards of two hours to complete 

 the heat run. 



The permeability of the air-quenched sample was 7,000 at the start, 

 increasing rapidly up to about 315° C. With further increase in 

 temperature there is a rapid drop in permeability until 500° C. is 

 reached. With further increase in temperature the permeability rises 

 very rapidly to a sharp peak at about 530° C. and then decreases, the 

 alloy becoming non-magnetic at 590° C. With decreasing temperature 

 the alloy again becomes magnetic at about the same temperature, at 



