MAGNETISM 



on Ss tends to lessen the positive lines of force through the shell, that 

 is, Ss tends to move upwards when H is as drawn. If we represent 

 current by C, field by H, then force is in the direction in which a 

 screw would move if its head were turned round from C to H, where 

 C and H both act towards or both form the point of intersection. 

 The rule may be remembered bv noting the alphabetical order of 

 CtoH. 



Field due to a uniformly magnetised sphere. Let us 

 suppose that a sphere consists of small parallel bars or fibres of 

 equal cross-section a and with equal poles la. Then it is said 

 to be uniformly magnetised with intensity I in the direction of the 

 fibres. 



Let PQ, Fig. 173, represent the end of one of the fibres, and let the 

 radius OP make with the direction of magnetisation. 



The polarity at PQ is I a . The area of the end of the fibre on 



the sphere is ^ , and if the surface density on that area is ^ 



<ra 

 cos 6 



la or a- = I cos 0. 



We may therefore represent the action of the magnetisation out- 

 side the sphere by a layer 



or = I cos 0, 



which will be positive on one hemisphere through which increases 



FIG. 173. 



FIG. 171. 



from to 90. and negative on the other hemisphere through \\ hich 

 increases from 90 to 180. 



We may obtain the potential of this distribution by a simple 

 device due to Poisson. 



Imagine a sphere equal fo the magnetised sphere, but consisting 

 of positive polarity of density p, uniformly distributed through iK 

 volume. Let its centre be at O. Superpose on this an equal sphere 

 consisting of uniformly distributed negative polaritv densitv p 



