FLUXES OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
465 
The effect of continuing the stress into the interior of a conductor of unit induc¬ 
tivity, according to the same law, instead of stopping it on its boundary, is to 
distribute the translational force bodily, according to the formula 2VJR, instead of 
superficially, according to 2P N . In either case, of course, the conductor must be 
strained by the magnetic stress, with the consequent production of a mechanical stress. 
But the strain (and associated stress) will be different in the two cases, the applied 
forces being differently localised. The effect of the stress on a straight portion of a 
wire supporting current, due to its own field only, is to compress it laterally, and to 
lengthen it. Besides this, there will be resultant force on it arising from the different 
pressures on its opposite sides due to the proximity of the return conductor or rest of 
the circuit, tending to move it so as to increase the induction through the circuit per 
unit current, that is, the inductance of the circuit. 
§ 28. If now, we bring an elastically magnetisable body into a magnetic field, it 
modifies the field by its presence, causing more or less induction to go through it 
than passed previously in the air it replaces, according as its inductivity exceeds or is 
less than that of the air. The forcive on it, considered as a rigid body, is completely 
accounted for by the simple stress P N in the air outside it, reckoned according to the 
changed field, and supposed to terminate on the surface of the disturbing body. This 
is true whether the body be isotropic or heterotropic in its inductivity ; nor need the 
induction be a linear function of the magnetic force. It is also true when the body is 
intrinsically magnetised ; or is the seat of electric current. In short, since the 
external stress depends upon the magnetic force outside the body, when we take the 
external field as we may find it, that is, as modified by any known or unknown causes 
within the body, the corresponding stress, terminated upon its boundary, fully 
represents the forcive on the body, as a whole, due to magnetic causes. This follows 
from the equality of action and reaction ; the force on the body due to a unit pole is 
the opposite of that of the body on the pole. 
If we wish to continue the stress into the interior of the body, surrounded on all 
sides by the unmagnetised medium of unit inductivity, as we must do if we wish to 
arrive ultimately at the mutual actions of its different parts, and how they are modified 
by variations of inductivity, by intrinsic magnetisation, and by electric current in the 
body, we may, so far as the resultant force and torque on it are concerned, do it in 
any way we please, provided we do not interfere with the stress outside. For the 
internal stress, of any type, will have no resultant force or torque on the body, and 
there is merely left the real external stress. 
Practically, however, we should be guided by the known relations of magnetic 
force, induction, magnetisation, and current, and not go to work in a fanciful manner; 
furthermore, we should always choose the stress in such a way that if, in its expres¬ 
sion, we take the inductivity to be unity, and the intrinsic magnetisation zero, it 
must reduce to the simple Maxwellian stress in air (assumed to represent ether here). 
MLCCCXCII.-—A. 3 O 
