MAGNETIC LOSSES AT LOW FLUX DENSITIES 



223 



subtracting the value at /i from that at fi and dividing by the frequency 

 interval fi — /i to give the eddy current coefficient e of equation (1).' 

 Figure 6 gives e thus derived, as a function of /, showing a value ap- 

 proximately 20 per cent higher than calculated from the relation 

 e = 4:Tr^t^/3p at frequencies above 500 cycles, and progressively higher as 

 the frequency approaches zero. 



The fact that e is larger than predictable from classical theory has 

 been ascribed to the presence of a low permeability surface skin on 



500 



75 



25 



200 400 600 

 FREQUENCY 



e,FROM SLOPE OF - — g 



154.5 X 10" 



1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000 11,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 6 — Intercepts of Fig. 5 vs. frequency. Slope of the curve gives eddy current 

 coefficient e. Residual loss coefficient c varies with frequency near/=0. 



practically all materials which have been reduced to sheet or wire form 

 by mechanical deformation.^ But since the eddy current coefficient 

 depends only on the resistivity and effective thickness of the material, 

 any apparent variation of e with frequency can only be interpreted as 

 an indication that the residual loss is varying with frequency. 



Taking the value of e = 154.5 X 10~^ characteristic of the higher 

 frequencies, the eddy current loss per cycle has been calculated, and is 

 indicated in Fig. 6. The amount by which the observed loss exceeds 



^ Correction terms must be included at higher frequencies to take account of eddy 

 current shielding as noted in ref. 2. 



8E. Peterson and L. R. Wrathall, /. R. E. Proc, 24, 275 (1936). 



