446 BELL SYSTEM TECHNICAL JOURNAL 



improvements for iron and 2 V-permendur are shown in Fig. 2. Im- 

 provements in 4—79 Mo-permalloy and in 45 permalloy are similarly 

 possible/ A hydrogen purified alloy having very nearly the composi- 

 tion of the latter material is produced commercially under the name 

 of Hipernik.^^ A material similar to hydrogen purified 4r-79 Mo- 

 permalloy has been produced under the trade name "1040" Alloy. ^'^ 



Adaptations for A-C. Uses 



For a-c. applications, eddy current, hysteresis and residual losses 

 are generally important, in addition to the permeability. The ma- 

 terials for these applications have to be laminated or used in powdered 

 form to limit eddy current losses. Such modifications generally affect 

 the magnetic qualities, often adversely, through the introduction of 

 impurities or stresses, or through the concentration of magnetic flux 

 in parts of the material adjacent to air-gaps. 



Many a-c. applications involve flux densities so low that the per- 

 meability does not rise to more than perhaps 10 per cent above mo- 

 Similarly, many applications involve operation at frequencies low 

 enough to insure that the a-c. permeability is approximately equal 

 to the permeability obtained by d-c. methods. (At higher frequencies, 

 eddy current shielding reduces the a-c. permeability below the d-c. 

 value, as discussed in Sec. 4b.) In these ordinary cases, the a-c. 

 losses in a material working at a flux density Bm and frequency /, are 

 conveniently described in terms of the increased resistance R per unit 

 inductance L of the winding encircling the material. ^^ These resist- 

 ance increments are: 



1. Hysteresis, i?fe/L = a^m/x/. where a is the hysteresis loop constant. 

 At these low flux densities, the permeability coefficient 



X = (m — fJ,o)ffJioBm- 



It is related to the hysteresis coefficient to a good approximation by the 

 equation X = Sa/x/S. 



2. Residual, Rr/L = c/j./, where c is the residual loss constant. 



3. Eddy current, Re/L = ep.p, where e is the eddy current loss con- 

 stant. It is proportional to the square of the laminar thickness, or 

 particle diameter, and inversely proportional to the resistivity of the 

 material. 



Table IV gives easily attainable values of the constants of im- 

 portance in a-c. applications for several typical materials. Data on 



13 T. D. Yensen, Jour. Frank. Inst. 199, iii (1925). 



" H. Neumann, Arch. f. tech. Messen 4, 42T, 168 (1934). 



15 V. E. Legg, B. S. T. J. 15, 39 (1936). 



