Abstract
An earlier study of the magnetic properties of fine dispersed powders of acicular grains of magnetite and of γ-ferric oxide, derived by reduction and re-oxidation of dehydrated pigment oxides, has been revised, taking account of the probable cavities in the powder particles caused by the loss of oxygen during the chemical and structural transformations. Excellent quantitative agreement is now found between theoretical and measured values of coercivity, shape anisotropy alone seeming to be involved. Moreover, the qualitative agreement between theoretical values of intrinsic induction and remanence and those derived from actual powder dispersions is much better than with the earlier treatment.