MR, G. F. C. SEARLE ON PROBLEMS IN ELECTRIC CONVECTION. 
695 
Thus, in this case also, although there is no true magnetic potential, still the 
mechanical force on a moving pole has a potential. 
Mechanical Force on a Moving Electric Current , 
12 . If c denote the current, the force on it per unit length is simply 
YcB or fVcR . ... (75). 
Values of E and H in terms of F and It. 
13. The electric and magnetic forces E and H can now be at once expressed in 
terms of the mechanical forces F and It experienced by a moving unit electric charge 
and by a moving unit magnetic pole respectively. For, since F = — V'P and E — — V£l, 
equations (26) to (31) become 
(77). 
Meaning of curl F = 0 and curl K, = 0. 
14. We now perceive the true meaning of the two equations (16) and (17), viz.,— 
curl (E — /xVHu) = 0, curl (H + KVEu) = 0, 
or, as we may now write them, 
curl F = 0, curl E = 0. 
They simply express the fact that the work done in taking a unit quantity of either 
electricity or magnetism round any closed path is zero, the path itself moving forward 
with the velocity u which is common to the whole system. This implies that F and 
