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BELL SYSTEM TECHNICAL JOURNAL 



ony, where the requirements generally are very exacting, and where 

 other advances have imposed rigid demands on the magnetic materials. 

 In telephone circuits, standards of transmission efficiency require 

 that the magnetic materials used as circuit elements shall produce 

 maximum magnetic effect with minimum energy loss and distortion of 

 the transmitted currents. Translated into magnetic characteristics, 

 this means that at low magnetizing forces the material shall have high 

 permeability in combination with low hysteresis loss, and, in many 

 situations, constancy of permeability over the operating range. In 

 circuits for voice and carrier currents it is often necessary to reduce 

 the intrinsic permeability of the material to obtain the required con- 

 stancy and low losses in the apparatus. Furthermore, to minimize 

 eddy currents, a high resistivity is required and the material must be 

 structurally suitable for fabrication into thin laminations. For other 

 uses, such as for signaling and switching mechanisms, the magnetic 

 properties at medium and high flux densities determine the suitability 

 of the material. High permeability and low coercive force make for 

 improved sensitivity and speed of operation. Low coercive force is of 

 special interest in marginal apparatus where the difference between 

 the operating and releasing currents is small. 



Preparation and Composition of the Alloys 



A great many factors contribute to the final properties of an alloy. 

 Among the most important of these are the purity of the elements used 

 in the alloy, their preparation, and the heat-treatment. The mag- 

 netic properties attainable can be completely masked by the intrusion 

 of small quantities of certain impurities or by improper heat-treatment. 

 For iron the magnetic properties can be improved materially by 

 removing extremely small quantities of carbon and other non-metallic 

 elements through heat-treating * in an atmosphere of hydrogen and at 

 temperatures close to the melting point. This method of purification 

 also improves the magnetic properties for alloys of iron, nickel, and 

 cobalt. For communication purposes, it has not been found expedient 

 as yet to introduce this method of refinement in the commercial pro- 

 duction of these alloys. The purity of the constituents is controlled 

 by ordinary methods of chemical analysis, by methods of melting, and 

 by annealing processes which do not increase the amounts of important 



* There is a rapidly growing technical literature relating to the effects of very 

 small percentages of impurities on magnetic properties and the methods for their 

 removal, with notable contributions by T. D. Yensen of the Westinghouse Electric 

 and Manufacturing Company, W. E. Ruder of the General Electric Company, and 

 P. P. Ciofifi of the Bell Telephone Laboratories. 



