234 
-MR. G. 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 
pj 1 -p-] I/rr 1 3(h-i + Eo) 
= E+a, 
d.v 
dt' 
In these expressions the parts depending on Eo and P, 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 
(TV)H-i([l„„]|| 
>il p- 
It’ 
This expression is only equivalent to Maxwell’s expression in the statical case he 
considers. It is, however, practically equivalent to that derived l)y 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 foi'cive 
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 
dt 
dE , dl • . 1 ry n 
If [^'“1 
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 
* Of. Larmor, ‘ Hither and Matter,’ p. 98. 
