MAGNETIC ALLOYS 127 



Initial and maximum permeabilities of 78.5 permalloy are improved 

 by elimination of impurities and also by special care in the quenching 

 process. As stated earlier, the rates of cooling required to develop 

 the highest initial permeability differ from those for the highest 

 maximum. 



Chromium Permalloy and Molybdenum Permalloy 

 When other metals are added to permalloys their resistivities, in 

 general, are increased. In research work at the Bell laboratories 

 chromium and molybdenum mostly were used. It was found that with 

 these elements a desirable combination of high resistivity and high 

 initial permeability could be obtained. The variation in resistivity, 

 keeping the nickel content constant at 78.5 per cent, is shown in Fig. 9. 

 Chromium increases the resistivities somewhat more than molybdenum 

 for a given addition, but the difference is not very large. The 3.8-78.5 

 Cr-permalloy has a resistivity of 65 microhms-centimeter, as compared 

 with 55 for the 3.8-78.5 Mo-permalloy. 



Figure 9 also illustrates the manner in which additions of these 

 metals affect the initial permeability and the sensitivity of the perme- 

 ability to rate of cooling. The solid line curves are for the annealed 

 and the broken-line curves for the quenched specimens. For the 

 quenched alloys the highest permeabilities are obtained when the 

 added chromium and molybdenum are 2.4 per cent and 1.6 per cent, 

 respectively. For this cooling rate the chromium permalloy seems 

 to develop a slightly higher initial permeability. The difference, 

 however, is small, and a greater spread between different samples 

 has been observed. For the annealed alloys the largest value of 

 initial permeability is obtained with molybdenum permalloy. F'or 

 3.8 Mo-permalloy an initial permeability of 20,000 is obtained. With 

 the same heat-treatment the initial permeability of the corresponding 

 chromium alloy is 12,000. It is surprising to note that small additions 

 of ' these non-magnetic metals increase the initial permeability 

 to values considerably higher than that for quenched 78.5 permalloy. 

 Beyond 5 per cent this improvement ceases. All additions decrease 

 the saturation induction values and the maximum permeabilities. 



Several of these alloys have been developed for commercial use. 

 Of these the most important are 2-80 Cr-permalloy, 3.8-78.5 Cr- 

 permalloy, and 3.8-78.5 Mo-permalloy. 



Perminvar 



The distinctive magnetic properties of the perminvars are constancy 

 of permeability at low flux densities, a low hysteresis loss in the same 



