MAGNETIC ALLOYS 129 



range, and, for medium flux densities, a characteristic constriction 

 in the middle of the hysteresis loop. In some alloys this constriction 

 is so extreme that the coercive force vanishes, making the two branches 

 of the loop coincide when the magnetizing force is reduced to zero, in 

 spite of the considerable hysteresis loss involved in the entire cycle. 

 At high flux densities this constriction disappears and the loops have 

 normal shapes. 



The degree to which these properties can be developed depends on 

 the composition and the heat-treatment. For the most typical alloys 

 slow cooling in the annealing process produces this effect to a certain 

 degree. Baking for 24 hours in the 400-500 degree (centigrade) 

 temperature range brings most alloys into a stable condition in which 

 no further baking materially will affect the magnetic properties. 



As indicated in Fig. 1, some of the binary alloys tend toward the 

 perminvar characteristics wath long baking. Of the permalloys a 

 considerable proportion of those that must be quenched to develop 

 the desirable magnetic properties show perminvar characteristics 

 when they are baked. 



45-25 Perminvar 



The perminvar characteristics have been developed most intensely 

 in 45-25 perminvar. The magnetization curve in Fig. 3, and the 

 permeability curve in Fig. 6, illustrate this fact. The constancy of 

 permeability at low magnetizing forces and the necessity of "baking" 

 to attain this condition are illustrated in one of the sections of Fig. 10, 

 where the permeabilities are plotted for the quenched and baked 

 conditions. The permeability of the quenched alloy begins to change 

 at very low magnetizing forces, but that of the baked alloy, though 

 lower, remains constant for magnetizing forces up to 3 oersteds. 



Hysteresis loops for this alloy in the two conditions are shown in 

 Fig. 10 for maximum flux densities of less than 1,000 and more than 

 5,000 gausses. For the baked alloy the hysteresis loops for maximum 

 flux densities less than 1,000 gausses cannot be measured by ordinary 

 ballistic methods, because the two sides of the loop coincide in a 

 straight line. For loops with higher maximum flux densities the area 

 begins to appear, but the two branches of the loop still meet at the 

 origin. Although the coercive force is sensibly zero for the baked 

 alloy until the maximum flux density exceeds 5,000 gausses, the hys- 

 teresis loss represented by the loop may become considerably greater 

 than that for the quenched alloy. 



7-45-25 Mo-Perminvar 

 The extremely low hysteresis loss and constancy of permeability at 

 low flux densities makes 45-25 perminvar a suitable material for 



