of Physical Quantities to Directions in Space. 251 



But raH is the force experienced by a pole m when placed in 

 a field of strength H, and similarly for k"E. Hence 



F =mH = 



J(w}w-{(i) 



where F m and F are respectively the forces between two 

 poles m, or two charges q, at distances r apart. In other 

 words, since in expressing the force between two poles or two 

 charges we have to regard each pole or charge as an isolated 

 point source of displacement, we should regard the one pole 

 or charge as producing radially a field of given strength, and 

 then express the force experienced by the other when placed 

 in this field. 



If, now, the unit pole and the unit charge be defined re- 

 spectively by the relations 



m = r V47r/xF m , q = r \ / 4tt^F 9 , 



instead of, as usual, 



the effect is to redistribute ir in electromagnetic equations as a 

 whole. It is found, however, that all those relations into which 

 it is now made to enter depend upon and involve the consider- 

 ation of circuital or radial fluxes, and it obviously enters as a 

 plane or solid angle in connexion with circles and spheres of 

 reference. It has thus a definite physical meaning, and is 

 always definitely related to the other quantities in the relation. 

 On the other hand, in the case of the relations from which it is 

 removed, it previously entered only because of its suppression 

 elsewhere, and had no fundamental connexion with the other 

 quantities. 



The relation between the new and the old units thus defined 

 are given in the papers above referred to. For the purpose 

 of the present paper it is sufficient to notice that the relation 



becomes 





Id*/ 



p = 47rm becomes p = m : B = /jl K + 47rl becomes B = /x H + 1, 

 so that B and I are identical ; the electrostatic energy of the 



-j— ) becomes ED, thus harmonizing with BH. 

 Since, in these relations, it essentially preserves its primi- 



