MAGNETIC MATERIALS IN TELEPHONE SYSTEM 439 



resistivity, containing several per cent chromium or molybdenum.^ 

 The problems of embrittlement and pulverization of molybdenum 

 permalloy were also successfully solved. This material has been 

 recently adopted for manufacture of filter and loading coil cores,^ in 

 which material of higher resistivity is especially advantageous. 



Attempts to decrease the losses due to hysteresis led to the discovery 

 of the nickel-iron-cobalt alloys — the perminvars. A molybdenum- 

 perminvar was perfected for use in the continuous loading of submarine 

 telephone cable.'' 



The large economic advantages promised by improvements in soft 

 magnetic materials confined much of the earlier work to this field. 

 However, within the last 20 years new permanent magnet materials 

 have been discovered here and abroad which offered radical improve- 

 ments in this direction. Such materials have been introduced in 

 telephone apparatus wherever found advantageous. 



Characteristics of Available Materials 



The number of different magnetic materials in use is quite large 

 on account of the various combinations of properties required for 

 special applications and on account of a multiplicity of trade names. 

 For the present purpose, an abbreviated listing is given of typical 

 materials covering the whole range of magnetic properties, particu- 

 larly those of interest to the telephone system. A compilation of 

 representative data is given in Tables I and II. 



The fundamental property which distinguishes a ferromagnetic ma- 

 terial is that when it is subjected to a magnetic field it develops 

 magnetic flux considerably larger than similarly attained in air. The 

 magnetizing forces of interest in telephone apparatus range from less 

 than 10~^ to upwards of 10^ oersteds, and the flux densities from less 

 than 1 to 30,000 gausses or more. The relation of flux density B to 

 magnetizing force H for important materials as first magnetized is 

 given on a logarithmic scale in Fig. 1. The ratio of 5 to ^ is the 

 permeability, which can be read on the diagonal scale ^ in the figure. 

 It is evident that the initial permeability mo and the maximum permea- 

 bility Mm vary over a wide range from the hard magnet steels to the 

 softest magnetic alloys. For commercial materials, 4-79 Mo-perm- 

 alloy gives the largest initial permeability — around 22,000, and 78.5 

 permalloy gives the largest maximum permeability — ^about 105,000. 



5G. W. Elmen, Jour. Frank. Inst. 207, 583 (1929). 

 "O. E. Buckley, Jojir. Applied Phvs. 8, 40 (1937). 

 ' G. W. Elmen, Elec. Engg. 54, 1292 (1935). 

 8 Scale due to Aiken; Jour. Applied Phys. 8, 470 (1937). 



