Demagnetizing fields in uniformly magnetized rectangular prismatic crystals

In their 1954 paper Rhodes and Rowlands calculated demagnetizing energies for uniformly magnetized rectangular prismatic crystals and derived effective demagnetizing factors Nb with no reference to the internal fields. It is shown that the demagnetizing field at the centre of a cube is precisely NbIs and that the fields at the centres of rectangular blocks with square cross section lie near to the values of NbIs and NdIs, where Nd is the demagnetizing factor of the inscribed ellipsoid.