464 BELL SYSTEM TECHNICAL JOURNAL 



stresses, or because of large eddy current shielding. Higher maximum 

 permeabilities than now attainable do not promise great utility. 

 However, the low values of coercive force and hysteresis loss found 

 with materials having high maximum permeability may be sufficiently 

 desirable, regardless of permeability needs. 



Another important objective of magnetics research has been to 

 reduce energy losses. Hysteresis loss at low flux densities, as indicated 

 by the loop area coefficient a, should be decreased to cut down har- 

 monic generation and modulation in magnetic core coils. Perminvar 

 has shown desirably reduced losses, but it is sensitive to magnetic and 

 mechanical conditions. Eddy currents are controlled by resistivity 

 and degree of subdivision of the magnetic core. The resistivity of 

 magnetic alloys can be increased to around 100 microhm-cms. by 

 alloying with large enough quantities of chromium, molybdenum, etc., 

 but at a serious sacrifice of magnetic quality for resistivities above 

 about 60. Eddy current suppression by laminating or pulverizing 

 the magnetic material thus offers a greater range of control than re- 

 sistivity adjustment. 



Permanent magnetic materials have also reached a very successful 

 stage, from the magnetic point of view. The greatest handicap of 

 the better materials is extreme hardness, which hampers fabricating 

 processes. 



