234 MR. Q. H. LIVENS ON THE 



Similar results apply in the magnetic case. The general expression for the forcive on 

 the magnetic media is, per unit volume, equal to 



! 



J* 



In' these expressions the parts depending on E and I 2 , representing as they do forces 

 on the elements of the media determined solely by the conditions in those elements, 

 would be neglected in a mechanical theory.* The expression for the effective forcive 

 thus reduces to 



3H\ 1 d r L 



This expression is only equivalent to MAXWELL'S expression in the statical case he 

 considers. It is, however, practically equivalent to that derived by counting the 

 forces on the constituent poles, but even here the general result rather suggests a 

 modified conception of the force on a magnetic pole, this force involving in the 

 general case a term due to the electric force. The question of the existence of forces 

 on a magnetic pole due to its motion in an electric field does not appear to have been 

 investigated on an independent basis, although it is definitely contained in the 

 relations of transformation involved in the theory of relativity, which require the form 

 for this forcive 



It will however be proved below in the next paragraph that such forces do probably 

 exist and are in fact of precisely the correct type. 



It may, of course, be objected that the last term in the equation 



dE dtt dl 



which is the origin of the discrepancy obtained for the magnetic forcive, does not in 

 reality exist, but yet the other results derived from this equation are almost certainly 



* Cf. LARMOR, ' JSther and Matter,' p. 98. 



