318 



Mr. A. K. Chapman on the 

 Fig. 5. 









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3600 



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Fig. 6 is a plot of the values of m against H; m is 

 directly proportional to the circular current. It is seen 

 that, in one region, the circular current actually decreases 

 for an increasing field ; it rises quickly to 5"1 and falls to 

 3 # 7. Beyond 1179 the curve is a straight line. 



It is important to observe that the curve for m/H remains 

 practically of the same form when the internal field is 

 introduced instead of the external field. We have 



H,=H-NI, 

 where N represents the demagnetizing factor ; H; the 

 internal field ; H the external field ; and I the intensity of 

 magnetization. For a disk of the above form N comes out 

 12*25, which is so near 47r that the external field and the 

 induction are practically the same. We may write 

 above equation for H i} 



H i= xr/.. in = n^Ata.. nearl y> 



the 



1+ 



NQk-1) 0-97476/* 

 4tt 



where /jl is the permeability. 



Since, for this particular specimen of iron, no values of jjl 

 were at hand, the numbers given in the Recueil de Constantes 

 Physiques par Abraham et Sacerdote (1913) were used. 

 From this H was determined and 2m/H plotted against H 

 as in fig. 7 ; this curve is plotted on a much larger scale 

 than the previous ones. 



