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XL VI I. The Magnetization of Iron in Strong Fields. 

 By Shelf'okd Bid well, M.A., F.R.S.* 



N the Philosophical Magazine for April 1890, p. 

 . Mr. Du Bois takes exception to the equation 



W# = 27rI 2 + HI, 





293, 



which I employed (Proc. Roy. Soc. no. 245, 1886, p. 486) 

 in calculating the curve of magnetization from observations 

 of the pull between two halves of a divided ring-electro- 



H 2 



magnet. He objects that a term £— ought to have been added 



for the mutual attraction of the two half coils of wire. 



In point of fact this attraction was found to be quite in- 

 significant, and was purposely neglected. Within the limits 

 of the magnetizing forces used it would have no effect what- 

 ever upon the calculated values of I as given in my table. 

 The greatest magnetizing force employed was 585C.G.S. units, 

 when the weight supported was 15,905 grammes per sq. centim. 

 Substituting these values for H and W, and putting # = 981 

 and 7r=3'14, we get from my equation 



1=1530-4. 



H* 



If we add the term — the solution is 



07T 



1 = 1530-2. 



The value given in my table (in which decimals are dis- 

 carded) is 



1=1530. 



For the greatest value of H the error is therefore only 2 in 

 the first place of decimals : for smaller values it would of 

 course be less ; and it is clear that, whatever the results of my 

 experiments may be worth, they are not affected by the 

 omission to which Mr. Du Bois calls attention. 



I regret that, through a mistake, I found fault with Row- 



B 2 



land's expression W#= t— . It would certainly have afforded 



a better and more direct_method than my own of arriving at 

 the values of B. 



Communicated by the Author, 



