study of Magnetic Losses at Low Flux Densities in 

 Permalloy Sheet* 



By W. B. ELLWOOD and V. E. LEGG 



Energy losses in ferromagnetic materials subject to alternating 

 fields have long been considered as due solely to hysteresis and 

 eddy currents. However, at the low flux densities encountered in 

 certain communication apparatus, a further loss is observed which 

 has been variously termed "residual," "magnetic viscosity," or 

 "square law hysteresis." The search for an explanation of this loss 

 has led to precise measurements of hysteresis loops with a vacuum 

 ballistic galvanometer, and of a.-c. losses with inductance bridges. 

 From these results, it appears that that part of the a.-c. effective 

 resistance of a coil on a ferromagnetic core which is proportional to 

 the coil current is strictly identified with the hysteresis loop area as 

 measured by a ballistic galvanometer, or as indicated by harmonic 

 generation in the coil. The hysteresis loop can now be constructed 

 in detail as to size and skewness on the basis of a.-c. bridge measure- 

 ments. This conclusion was reached previously on a compressed 

 iron powder core, and is now confirmed on an annealed laminated 35 

 per cent nickel-iron core. Observed eddy current losses for this core 

 exceed those calculated from classical theory by 20 per cent. This 

 excess is ascribed to the presence of low permeability surface layers 

 on the sheet magnetic material. The a.-c. residual loss per cycle 

 (nominally independent of frequency, like hysteresis) is not ob- 

 served by ballistic galvanometer measurements, although it indi- 

 cates an energy loss some eight times the hysteresis loss for the 

 smallest loop measured {Bm = 1.3 gauss). Analysis of the residual 

 loss shows that it increases with frequency up to about 500 cycles, 

 and remains constant at higher frequencies (to 10,000 cycles per 

 second). Concurrently with the increase of residual loss, the per- 

 meability of the alloy is observed to decline with increasing fre- 

 quency about 1 per cent below the value predicted from eddy 

 current shielding. This effect is most noticeable at frequencies 

 below 1000 cycles. 



THE search for an explanation of the excessive magnetic losses 

 observed at low flux densities by alternating current bridge 

 measurement as compared with theoretical indications based on direct- 

 current measurements has led to a more accurate review of both types 

 of measurement.^' ^' ^ The a.-c. energy loss per cycle which has re- 



* To be published in May 1937 issue of Jour, of Applied Physics. 



1 W. B. Ellwood, Physics, 6, 215 (1935). 



2 V. E. Legg, Bell SysL Tech. Jour., 15, 39 (1936). 



3 W. B. Ellwood, Rev. Sci. Inst., 5, 301 (1934). 



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