268 



MAGNETISM 



If I is the intensity of magnetisation of the rod and TTQ* is its 

 cross-section, its pole strength is -rra 2 !. Let QQ', Fig. 205, repre- -n t 

 the two poles of the rod,o being the magnetometer, the hori/ontai 

 component of the field in which it is placed being perpendicular to 

 the plane of the figure, and being of inten-itv !'. 



If is the deflection of the needle, then 



-i J) 



and l = d* F tan 0/ira* (l - |^ 



This gives I in terms of measurable quantities. Since 

 B = /zH = (l+4)H = H + 4x1, if ire know H and I 

 we find B and /x. The value of H' due to the mag- 

 netising current i> not, however. the actual value of 

 the field, for the poles tunned in the xprcinien produce 

 Fio. 20t. an opposing field, and SOUR- assumption is needed to 

 allow for the reduction due to this. If the, specimen 

 is very thin with diameter, say, 1/400 of the length, the reduction 



-3-2-101*3 



to a 12 a i+ u 16 n it 19 : 



FIG. 206. 



is very small. Swing supposes that such a specimen may be 

 treated as an elongated ellipsoid and finds that the actual field is 



H = H'-0 00045 I. 



