34 BELL SYSTEM TECHNICAL JOURNAL 



teristics are developed depends on the rate of cooling through the 

 critical temperature range, and that baking at 425° C. gave the most 

 characteristic results. The manner in which the permeability of the 

 45 per cent nickel, 25 per cent cobalt and 30 per cent iron composition 

 changes in this temperature range is illustrated in Fig. 8. The 

 temperature of an annealed ring was increased from that of the room 

 to 450° C. where it was held constant for twenty hours. It was 

 then raised to 500° C. and held for four hours, then lowered to 400° C. 

 where it was held for twenty hours, and finally cooled to room tempera- 

 ture. Permeability measurements were made at these temperatures 

 with an a.c. magnetizing force of .02 gauss. 



Inspection of these curves shows that in the range 400°-500° C, 

 the permeability lags behind the temperature, and that the time 

 required for the permeability to reach a constant value increases 

 very rapidly below 450° C. The changes in final permeabilities with 

 temperature decrease also rapidly below 450° C. In fact, when the 

 difference in permeability caused by the temperature coefficient is 

 corrected for, the permeability of the alloy after heating at 400° C. 

 is not very different from what it is after heating at 450° C. Other 

 experiments show that the critical temperature range extends below 

 400° C, but, as would be expected, the decrease in permeability is 

 very small. The range also extends above 500° C. for this alloy and 

 some experiments indicate that the upper limit is the magnetic trans- 

 formation temperature which for this alloy is 725° C. 



Effects of Variation of Compositions 



The composition range within which the magnetic properties charac- 

 teristic of perminvar are developed pronouncedly by annealing, 

 is represented by the area enclosed by the curve in the triangular 

 composition diagram Fig. 9. Magnetic properties for a few of the 

 compositions in this area are plotted in Figs. 10 and 11. Table 2 

 gives their chemical analyses, initial permeability (mo), the maximum 

 permeability (yUmax.), the magnetizing forces (//) and the flux densities 

 {B) in gauss to which the alloys may be brought with a permeability 

 variation not over 1 per cent, also the {B — H) values for a magnetizing 

 force of 1,500 gauss for some of the alloys, and the resistivity in 

 microhms-cm. The hysteresis losses for a number of flux densities 

 are given in Table 3. 



The area enclosed in Fig. 9 shows that approximately one third of 

 the alloys in the Ni-Fe-Co series show some of the characteristic 

 perminvar properties in the annealed condition. The proportions of 

 nickel and cobalt may be varied through a wide range. A great deal 



