MAGNETIC PROPERTIES OF PERMINVAR 



39 



that of the homogeneous material to one in which two branches con- 

 verge at the origin into a single line. This constriction of the hysteresis 

 loop is also illustrated for a parallel bimetallic magnetic circuit in 

 Fig. 12 where loops a and h are traced for a perminvar core and a bi- 

 metallic rod, respectively. The rod was 15 in. long and consisted of 

 a core of .04 in. diameter unannealed piano wire and a .006 in. wall 

 permalloy tube, heat treated to give high permeability, and fitting 

 closely to the wire. Though the magnetic circuit condition for the 

 perminvar core is not the same as for the bi-metallic rod, the similarity 

 of the two loops is marked and supports the theory that the con- 

 stricted loop of the perminvar core is caused by segregation in the 

 alloy. 



Fig. 12 — Hysteresis loops: a, Perminvar; &, Bi-metallic rod. Loops traced 

 with a cathode ray oscillograph 



The electrical resistance also is affected by the slow cooling. An 

 air-quenched alloy of the 45 per cent nickel, 25 per cent cobalt and 

 30 per cent iron composition was 10 per cent lower in resistivity after 

 it had been baked at 425° C. This change is also in line with the 

 idea that segregation takes place when the perminvar alloys are 

 cooled slowly. 



While these considerations point to a satisfactory explanation for 

 the constriction of the hysteresis loops they do not explain the ex- 

 tremely low hysteresis losses of the alloys at low flux densities. This 

 characteristic of perminvar suggests that one of the constituents 

 which is segregated by the heat treatment is itself a material of much 

 lower hysteresis loss than any previously known material and that 



