This is the problem of § 670 in Clerk Maxwell's Electricity and Magnetism. The author proposes to proceed by another method and to obtain the result in a different form. Let O be the centre of the spherical surface on which the shell lies and Z the point where the magnetic potential Vm is to be found. Also let ∅ be the strength of the shell (magnetic moment per unit area), α its internal, and α + δα its external radius. To represent the magnetic distribution let a layer of negative magnetic matter of density σ cover the inside face, and a corresponding positive layer the outside face. Finally, let Z be without the matter of the shell and on the positive side.