FLUXES OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
469 
force to which it gives rise is also apparently simple, being merely the sum of two 
forces, one the electromagnetic, VJB, the other a force on magnetised matter whose i 
component is (I + M)V 1 R, both per unit volume, the latter being accompanied (in 
case of eolotropy) by a torque. Now I is the intrinsic and M the induced magneti¬ 
sation, so the force is made irrespective of the proportion in which the magnetisation 
exists as intrinsic or induced. In fact, Maxwell’s “ magnetisation ” is the sum of 
the two without reservation or distinction. But to unite them is against the whole 
behaviour of induced and intrinsic magnetisation in the electromagnetic scheme of 
Maxwell, as I interpret it. Intrinsic magnetisation (using Sir W. Thomson’s term) 
should be regarded as impressed (I = ph,„ where h 0 is the equivalent impressed 
magnetic force); on the other hand, “ induced ” magnetisation depends on the force 
of the field {M = (p — 1) Rj. Intrinsic magnetisation keeps up a field of force. 
Induced magnetisation is kept up by the field. In the circuital law I and M 
therefore behave differently. There may be absolutely no difference whatever 
between the magnetisation of a molecule of iron in the two cases of being in a 
permanent or a temporary magnet. That, however, is not in question. We have no 
concern with molecules in a theory which ignores molecules, and whose element of 
volume must be large enough to contain so many molecules as to swamp the charac¬ 
teristics of individuals. It is the resultant magnetisation of the whole assembly that 
is in question, and there is a great difference between its nature according as it 
disappears on removal of an external cause, or is intrinsic. The complete amalgama¬ 
tion of the two in Maxwell’s formula must certainly, I think, be regarded as a false 
step. 
We may also argue thus against the probability of the formula. If we have a 
system of electric current in an unmagnetisable (p = 1) medium, and then change p 
everywhere in the same ratio, we do not change the magnetic force at all, the 
induction is made p times as great, and the magnetic energy p times as great, and is 
similarly distributed. The mechanical forces are, therefore, p times as great, and are 
similarly distributed. That is, the translational force in the p = 1 medium, or YJR, 
becomes YJpR in the second case in which the inductivity is p, without other 
change. But there is no force brought in on magnetised matter per se. 
Similarly, if in the p = 1 medium we have intrinsic magnetisation I, and then 
alter p in any ratio everywhere alike, keeping I unchanged, it is now the induction 
that remains unaltered, the magnetic force becoming p -1 times, and the energy p _1 
times the former values, without alteration in distribution (referring to permanent 
states, of course). Again, therefore, we see that there is no translational force 
brought in on magnetised matter merely because it is magnetised. 
Whatever formula, therefore, we should select for the stress function, it would 
certainly not be Maxwell’s, for cumulative reasons. When, some six years ago, I 
had occasion to examine the subject of the stresses, I was unable to arrive at any 
very definite results, except outside of magnets or conductors. It was a perfectly 
