Survey of Magnetic Materials and Applications in the Telephone System

' I ^ W E N T Y years ago, the telephone system used primarily iron, together with a small amount of silicon iron, for applications requiring soft magnetic materials, and carbon, tungsten or chromium steel for permanent magnet applications. The permalloys 1 were already fairly thoroughly developed by 1920 in what is now the Bell Telephone Laboratories, and 78.5 permalloy 2 shortly attained commercial recognition for its utility as a continuous loading material for submarine telegraph cables.3 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 materials. 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 powdered 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.4 A desire to reduce the losses in a-c. apparatus arising from eddy currents in magnetic parts led to the development of permalloys of higher H. D. Arnold & G. W. Elmen, Jour. Frank. Inst. 195, 621 (1923). The approximate chemical compositions of the various materials herein discussed are given in Tables I and I I . 3 0 . E. Buckley, Jour. A. I. E. E. 44, 821 (1925). 4 W.