MAGNETIC ALLOYS 133 



considerably less than can be obtained under the best conditions of 

 heat-treatment and absence of strains. With this loading, the speed 

 of transmitting messages was increased fivefold.^ By the time a 

 second cable project was undertaken the chromium permalloys had 

 been developed, and 2-80 Cr-permalloy was selected. This alloy has 

 a resistivity of 45 microhms-centimeter, and the initial permeability 

 of the loading on the laid cable was in the neighborhood of 3,700. The 

 increase in permeability and in resistivity increased materially the 

 message carrying capacity.^ 



The largest use of permalloys in the telephone plant has been in 

 cores of loading coils,® where the alloy is used in the form of com- 

 pressed insulated dust. Iron dust cores had been standard for these 

 coils. ^ The lower magnetic losses of permalloy dust, however, per- 

 mitted utilizing higher core permeabilities. This has resulted in a 

 very material decrease in the size of loading coils. For a high grade 

 loading coil core made from iron dust the effective core permeability 

 at low flux densities had to be limited to ZZ. The first permalloy used 

 for loading coil cores was 80 permalloy. The insulated and compressed 

 core was designed for an eflfective permeability of 75 — more than 

 double that for the iron dust. Development work on an improved 

 compressed magnetic dust core in which molybdenum is used, is now 

 approaching completion. It is expected that the new material will 

 have a substantially higher permeability than that of the 80-permalloy 

 dust cores, and that it will have intrinsically superior eddy current 

 and hysteresis loss characteristics. By virtue of these properties, it 

 will be practicable to make a further substantial reduction in the size 

 of loading coils without sacrifice in service standards. The decrease 

 in the size of the cores with improvement in the core material is illus- 

 trated in Fig. 11. 



Fig. 11 — Equivalent cores for loading coils. Iron dust core (left); permalloy dust 

 core (center); molybdenum permalloy dust core (right). 



* For all numbered references see list at end of paper. 



