Survey of Magnetic Materials and Applications 

 in the Telephone System 



By V. E. LEGG 



The great diversity of magnetic characteristics demanded by 

 telephone apparatus, and the large number of available magnetic 

 materials propose intricate problems in the choice of materials and 

 design of apparatus to attain greatest efficiency and economy. 

 The present paper undertakes to evaluate magnetic materials in 

 relation to apparatus needs. After a review of the earlier develop- 

 ments, the materials now available are listed, together with data 

 on technical characteristics and raw materials costs. The advan- 

 tages of various materials for specific applications are described. 

 The scope of possible further improvements in magnetic materials 

 is surveyed. 



Historical 



'TpVVENTY years ago, the telephone system used primarily iron, to- 

 -*■ gether with a small amount of silicon iron, for applications requir- 

 ing soft magnetic materials, and carbon, tungsten or chromium steel for 

 permanent magnet applications. The permalloys ^ were already fairly 

 thoroughly developed by 1920 in what is now the Bell Telephone 

 Laboratories, and 78.5 permalloy ^ shortly attained commercial recog- 

 nition for its utility as a continuous loading material for submarine 

 telegraph cables.' This and other nickel-iron alloys were soon serving 

 in many types. of telephone relays, and in various coils where the 

 designs could be profitably modified to adapt them to the new ma- 

 terials. Upon the development of commercial means for embrittling 

 and pulverizing permalloy, this material was soon in extensive use 

 because it offered improved characteristics over the compressed pow- 

 dered iron core material previously in use. Redesigns of filter and 

 loading coils have introduced such economies that practically all these 

 coils made by the Western Electric Company have until recently 

 employed compressed powdered permalloy cores.^ 



A desire to reduce the losses in a-c. apparatus arising from eddy cur- 

 rents in magnetic parts led to the development of permalloys of higher 



1 H. D. Arnold & G. W. Elmen, Jour. Frank. Inst. 195, 621 (1923). 



2 The approximate chemical compositions of the various materials herein discussed 

 are given in Tables I and II. 



3 0. E. Buckley, Jour. A. I.E. E. 44, 821 (1925). 



* W. J. Shackelton & I. G. Barber, Trans. A. I. E. E. 47, 429 (1928). 



438 



