138 E. H. Mall — Effect of Magnetic Force on the 



On Jan. 7th, 1888, Nos. 5, 6 and 8, were tested in a field of 

 about 3800. The relative sizes of the R. P's in this field are 

 represented by the following numbers, no one of which is 

 probably in error more than 5 per cent of itself : — 



For No. 5 100 



6___ 158 



8 436 



It will be seen that the relative standing of Nos. 5 and 6 is 

 not seriously changed, although No. 6 gains a little as was to 

 be expected. No. 8 makes a large gain. 



If in the results of these tests, we look for something more 

 than the general influence of shape upon transverse effect, and 

 inquire whether the relative intensity of magnetization in each 

 cross corresponds accurately to the relative magnitude of the 

 R. P. therein, we find it impossible to determine exactly the 

 intensity of magnetization in the several crosses, as they do 

 not closely resemble any of the forms for which accurate cal- 

 culation has been made. It may be worth while, however, to 

 note what are the results of calculation in the case of certain 

 typical forms. 



We find in Maxwell, Art. 400, that in general 



B = (1+4ttk) H (1) 



where B is the intensity of " magnetic induction through the 

 the body," II the "magnetic force" ("polar definition") 

 within the body and x is Neumann's coefficient, a quantity 

 which Maxwell supposed to be about 30 for soft iron, but 

 which Rowland has found to be at times much greater than 

 that in Norway iron. 



Letting H represent the magnetic force directly due to the 

 poles of the electro-magnet, we find from Maxwell, Art. 438, 

 that in a cylinder placed between these poles so as to become 

 magnetized transversely this relation holds 



H=.YL-<lnkB\ or H= ,** (2) 



From (1) and (2) we find 



1 + 4^ 



1+27TK V ' 



For ordinary values of x we find approximately for this cylinder 



£=2H. 

 In case of a sphere we find 



B — approximately 3 H. 



