444 BELL SYSTEM TECHNICAL JOURNAL 



The saturation flux density for the soft magnetic materials can be read 

 from these curves, and it is Hsted in the adjacent Table III in the 

 column headed 47r/oo. The pre-eminence of 2 \^-permendur is note- 

 worthy. 



Further important magnetic properties are obtained from the 

 hysteresis loop. This gives the B, II relationship for a material which 

 has already been magnetized up to a peak value //,„. The flux density 

 remaining after the removal of a very large magnetizing force is the 

 residual By, and the reverse magnetizing force necessary to bring the 

 flux to zero is the coercive force He. The area of the B, H loop 

 when the peak magnetizing force is very large gives the maximum 

 energy W^ dissipated by hysteresis in the material when it is carried 

 from positive magnetic saturation to negative, and back again. Table 

 III gives values of W^, Br and //«. The low values of these properties 

 for 4—79 Mo-permalloy are noteworthy. Among soft magnetic ma- 

 terials, iron and 2 V-permendur have high residual and coercive force, 

 properties which are occasionally useful. 



Permanent magnet materials should have large values of Br and He, 

 although a sacrifice of residual can be more or less compensated by an 

 increase in coercive force. A more fundamental criterion of perma- 

 nent magnet quality is the peak energy product (5-//)max. obtained 

 from the demagnetizing section of the hysteresis loop." Values of 

 this product are given for several magnet steels in Table III. Mishima 

 steel is seen to be foremost in this regard, with remalloy nearly as good. 



The last column in Table III gives the resistivity p of each material. 

 A high resistivity such as obtained with molybdenum additions to 

 permalloy and perminvar is desirable in suppressing eddy current losses 

 for alternating current applications. Eddy current losses can also be 

 suppressed by proper subdivision of the material, but this method 

 becomes costly with fine subdivision. 



For apparatus depending upon the tractive force of a magnet, the 

 high flux density properties of materials are most important. Since 

 these do not show up clearly in Fig. 1, an accompanying Fig. 2 has 

 been prepared in which the {B — H) scale is quadratic, and thus pro- 

 portional to tractive force. The relative merits of various materials 

 for such applications are seen by inspection of these curves. 2 V- 

 permendur is outstanding at high flux densities, iron and 45 permalloy 

 at intermediate, and 4-79 Mo-permalloy at low flux densities. 



The use of a purifying hydrogen anneal has been shown by Cioffi '- to 



increase the ease of saturating iron and most magnetic alloys. The 



"S. Evershed, /. /. E. E., London, 58, 780 (1920); 63, 725 (1925). 

 12 P. P. Cioffi, Phys. Rev. 39, 363 (1932). 



