468 
MR. 0. HEAVISIDE OH THE FORCES, STRESSES, AND 
fP N = R.BN - N.iRB,.(193) 
( 5 ) < 
|_F .- VJB - i {i (RVjB - BViR) + j (RV 2 B - BV 2 R) + k (RV S B - BV,R)}. . (194) 
= VJB - (V B - V k )ARB. 
This we need not discuss, as it is merely a transition to the next form. 
§ 34 . To the stress (193) add h^.NB ; we then get 
fP x = H.NB - NARB,.(195) 
(6) -j F = VJB + [i.BV^j + j.BV/o + k.BV/q} 
L - i {i. (RVjB - BVjR) + j. (RV 2 B - BV 2 R) + k. (RV 3 B - BV 3 R)}, .... (196) 
- VJB + BV.h 0 - (V B - V R ) 1RB, 
where /q, h 2 , h 3 are the components of h 0 . 
Now if to this last stress (195) we add — N.^h 0 B, we shall come back to the 
third stress, (188), of the simple type. 
Perhaps the most instructive order in which to take the six stresses is (l), (2), (4), 
(5), (6), and (3); merely adding on to the force, in passing from one stress to the 
next, the new part which the alteration in the stress necessitates. 
To the above we should add Maxwell’s general stress, which is 
fP N = R.NB - N.-pt 3 ,.(197) 
(7) J F = VJB + {i.IVjR + j.IV 3 R + k.IV s R} 
L + {i-MVjR + j.MV.R + k.MV 3 Rj..(198) 
= VJB + V r [R(I+ M)], 
where M = (g — l)R=: intensity of induced magnetisation. There is a good deal 
to be said against this stress ; some of which later. 
Remarks on Maxwell’s General Stress. 
§ 35 . All the above force formulae refer to the unit volume; whenever, therefore, 
a discontinuity in the stress occurs at a surface, the corresponding expression per unit 
surface is needed; i.e., in making a special application, for it is wasted labour else. 
It might be thought that as Maxwell gives the force (198), and in his treatise 
usually gives surface expressions separately, so none is required in the case of this his 
force system (198). But this formula will give entirely erroneous results if carried 
out literally. It forms no exception to the rule that all the expressions require 
surface additions. 
Maxwell’s general stress has the apparent advantage of simplicity. It merely 
requires an alteration in the tension parallel to R, from R' 3 to RB, whilst the lateral 
pressure remains 1,R 3 , when we pass from unmagnetised to magnetised matter. The 
