FLUXES OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
479 
connecting the force and the induction be a straight line. Thus, if the curve be 
of the type shown in the first figure, the shaded area will represent the stored 
energy and the tension, and the remainder of the rectangle will represent the lateral 
pressure. They are equal when H is small; later on the pressure preponderates, and 
more and more so the bigger H becomes 
But if the curve be of the type shown in the second figure, then, after initial 
equality the tension preponderates; though, later on, when H is very big the 
pressure preponderates. 
To obtain an idea of the effect, take the concrete example of an infinitely long rod, 
uniformly axially inductized by a steady current in an overlapping solenoid, and 
consider the forcive on the rod. Here both H and B are axial or longitudinal; and 
so, by equation (223), the translational force would be a normal force on the surface of 
the rod, acting outwards, of amount 
(HB - T) - ttK 0 B 0 
per unit area ; this being the excess of the lateral pressure in the rod over -|H 0 B 0 , the 
lateral pressure just outside it. 
In case of proportionality of force to flux, the first pressure is ^HB, and if there is 
no intrinsic magnetisation H and H 0 are equal. The outward force is therefore 
positive for paramagnetic, and negative for diamagnetic substances, and the result 
would be lateral expansion or contraction, since the infinite length would prevent 
elongation. 
rotation, that E and D should be parallel, and likewise H and B. At any rate, if such terms be allowed, 
some modification may be required in the subsequent reckoning of the mechanical force. In other respects, 
it is merely implied by (2176) that E and D are definitely connected, likewise H and B, so that there is 
no waste of energy other than that expressed by Q. 
