470 
MR. 0. HEAVISIDE ON THE FORCES, STRESSES, AND 
indeterminate problem to find the magnetic stress inside a body from the existence of 
a known, or highly probable, stress outside it. All one could do was to examine the 
consequences of assuming certain stresses, and to reject those which did not work 
well. After going into considerable detail, the only two which seemed possible were 
the second and third above (those of equations (186) and (188) above). As regards 
the seventh (Maxwell’s stress equation (198) above), the apparent simplicity produced 
by the union of intrinsic and induced magnetisation, turned out, when examined into 
its consequences, to lead to great complication and unnaturalness. This will be 
illustrated in the following example, a simple case in which we can readily and fully 
calculate all details by different methods, so as to be quite sure of the results we 
ought to obtain. 
A ivorJced-out Example to Exhibit the Forcives contained in Different Stresses. 
§ 36 . Given a fluid medium of inductivity in which is an intrinsic magnet of 
the same inductivity. Calculate the attraction between the magnet and a large solid 
mass of different inductivity /x.,. Here it is only needful to calculate the force on a 
single pole, so let the magnet be infinitely thin and long, with one pole of strength m 
at distance a from the medium which may have an infinitely extended plane 
boundary. By placing a fictitious pole of suitable strength at the optical image in 
the second medium of the real pole in the first, we may readily obtain the solution. 
Let PQ be the interface and the real pole be at A and its image at B. We have 
first to calculate the distribution of R, magnetic force, in both media due to the 
pole m, as disturbed by the change of inductivity. We have div p 1 R 1 = m in the 
first medium, and div p. 2 IL 2 = 0 in the second, therefore R has divergence only on 
the interface. Let a be the surface density of the fictitious interfacial matter to 
correspond ; its force goes symmetricallv both ways; the continuity of the normal 
induction therefore gives, at distance r from A, the condition 
