386 BELL SYSTEM TECHNICAL JOURNAL 



led to permalloy ^ which, by 1925, had been produced in powdered 

 form and fabricated into cores. This development provided coils for 

 voice frequency applications (loading coils and filter coils) which were 

 cheaper and yet superior electrically to those made from electrolytic 

 iron. These coils became available at a time when the telephone 

 plant was undergoing a very large extension of loaded cables. As a 

 result, large economies in cost and space were realized in the more than 

 six million coils involved in this plant expansion. 



Iron powder, and later permalloy powder, ground to a finer size and 

 diluted to lower permeability than used in loading coils, also found 

 application in coils for oscillators, filters and networks of multiplex 

 carrier telephone and telegraph systems employing frequencies up to 

 30 kc* and in receivers for transoceanic radio telephone communication 

 employing frequencies up to approximately 60 kc' Permalloy powder 

 improved the electrical characteristics — particularly modulation — of 

 coils for use in high frequency circuits, because of its low hysteresis 

 losses. 



Continued research for a powdered material having still better 

 intrinsic properties has recently made available new compressed powder 

 cores which permit further important gains in coils for voice-frequency 

 circuits and in coils for high-frequency carrier system applications. 

 The latter take on considerable significance at this time because they 

 play an important part in making practical for commercial use the 

 new broad-band carrier telephone systems intended for use on existing 

 open-wire and cable lines and on new types of cable. Again, therefore, 

 the advent of a new core material is well-timed to be of assistance in 

 further growth of the telephone system. 



The development of this core material was based on the discovery ^° 

 that the addition of a small percentage of molybdenum to permalloy 

 increases its permeability and electrical resistivity, and decreases its 

 eddy current and hysteresis losses. Decreased losses are necessary for 

 improvements and economies for both voice and carrier frequency 

 operation. The increased permeability of this alloy is essential for the 

 improvement of voice-frequency coils. It is readily reduced to the 

 proper values for high-frequency coils by diluting the powdered mag- 

 netic material with insulating material before compressing into core 

 form. In this development many problems of alloy embrittlement, 

 pulverization, insulation and heat treatment had to be solved both on a 

 laboratory and factory scale. The alloy composition finally selected 

 as giving the best combination of desirable properties, contains ap- 

 proximately 2 per cent molybdenum, 81 per cent nickel, and 17 per cent 

 iron, and is designated as 2-81 molybdenum-permalloy. This new 



