inductive loading for telephone facilities 189 



(11) Compressed Molybdenum-Permalloy Powder Core Loading 



Coils 



11.1 The Improved Core-Material 



The continuing search for still better core-materials culminated during the 

 middle 1930's in the development of the 125-permeability compressed molyb- 

 denum-permalloy material, described in an A.I.E.E. paper^^ by V. E. Legg 

 and F. J. Given, "Compressed Powdered Molybdenum-Permalloy for High- 

 Quality Inductance Coils." In its intrinsic magnetic and electrical properties 

 as used in voice-frequency loading coils, the improved permalloy-material 

 is nearly as much superior to the old standard 75-permeability permalloy 

 as this latter material was to the 35-permeability compressed powdered- 

 iron which it superseded as standard during the late 1920's. 



The improved permalloy owed its superior characteristics mainly to the 

 inclusion of molybdenum in the alloy, the principal constituents of which 

 are approximately 2% molybdenum, 81% nickel, and 17% iron. The molyb- 

 denum component substantially raises the permeability and the specific 

 resistance, and materially reduces hysteresis. The eddy-current losses are 

 much lower than in the 75-permeability permalloy material, because the 

 effect of the higher permeability in increasing these losses is more than 

 offset by the effect of the higher intrinsic resistance in reducing them. 



In developing the material, it was considered to be very important to go 

 as far as possible in improving the intrinsically favorable hysteresis prop- 

 erties. This was accomplished without adverse effects on permeability and 

 eddy-current losses and other important properties, by annealing the pressed 

 core rings in hydrogen at a much higher temperature than that previously 

 used with material under treatment while exposed to the atmosphere. Also 

 an improved type of particle insulation was developed. AU in all, a large 

 number of unusually difficult processing problems had to be solved in the 

 research and development stages. Also, some of the processes required 

 considerable additional attention during the manufacturing preparations 

 for quantity production. 



In the voice-frequency loading applications, the successful efforts to 

 reduce hysteresis to a minimum fitted in with the preliminary economic 

 design studies, which indicated it would be deskable to take advantage of 

 the superior intrinsic properties of the new magnetic material by using it in 

 smaller cores, so as to reduce loading-apparatus costs as much as possible 

 without appreciably degrading transmission performance. In this connection, 

 it is noteworthy that with any given magnetic-material the hysteresis losses 

 become greater as the core size is reduced, in consequence of the greater 

 intensity of magnetization. 



The 125-permeability molybdenum-permalloy core material under dis- 



