480 
ON THE FORCES, STRESSES, AND FLUXES OF ENERGY. 
But if the curve in the rod be of the type of the first figure, and the straight 
line ac be the air curve to correspond, it is the area abc that now represents the 
outward force per unit area when the magnetic force has the value ad. If the 
straight line can cross the curve ab, we see that by sufficiently increasing H we can 
make the external air pressure preponderate, so that the rod, after initially expand¬ 
ing, would end by contracting. 
If the rod be a ring of large diameter compared with its thickness, the forcive 
would be approximately the same, viz., an outward surface force equal to the 
difference of the lateral pressures in the rod and air. The result would then be 
elongation, with final retraction when the external pressure came to exceed the 
internal. 
Bidwell found a phenomenon of this kind in iron, but it does not seem possible 
that the above supposititious case is capable of explaining it, though of course the 
true explanation may be in some respects of a similar nature. But the circumstances 
are not the same as those supposed. The assumption of a definite connexion between 
H and B, and elastic storage of the energy T, is very inadequate to represent the facts 
of magnetisation of iron, save within a small range. 
Magneticians usually plot the curve connecting H — h 0 and B, not between H and B, 
or which would be the same, between H — h 0 and B — B 0 , where B 0 is the intrinsic 
magnetisation. Now when an iron ring is subjected to a given gaussage (or magneto 
motive force), going through a sequence of values, there is. no definite curve connecting 
H — h 0 and B, on account of the intrinsic magnetisation. But, with proper allowance 
for h 0 , it might be that the resulting curve connecting H and B in a given specimen, 
would be approximately definite, at any rate, far more so than that connecting H — h 0 
and B. Granting perfect definiteness, however, there is still insufficient information 
to make a theory. lire energy put into iron is not wholly stored ; that is, in 
increasing the coil current we increase B 0 as well as B, and in doing so dissipate 
energy ; and although we know, by Ewing’s experiments, the amount of waste in 
cyclical changes, it is not so clear what the rate of waste is at a given moment. 
There is also the further peculiarity that the energy of the intrinsic magnetisation at 
a given moment, though apparently locked up, and really locked up temporarily, 
however loosely it may be secured, is not wholly irrecoverable, but comes into play 
again when H is reversed. Now it may be that the energy of the intrinsic magnetisa¬ 
tion plays, in relation to the stress, an entirely different part from that of the elastic 
magnetisation. It is easy to make up formulae to express special phenomena, but 
very difficult to make a comprehensive theory. 
But in any case, apart from the obscurities connected with iron, it is desirable to be 
apologetic in making any application of Maxwell’s stresses or similar ones to 
practice when the actual strains produced are in question, bearing in mind the 
difficulty of interpreting and harmonising with Maxweil’s theory the results of 
Kerr, Quincke, and others. 
