As regards the classical problem of the Wiedemann effect in polycrystalline ferromagnetics, any satisfactory theory has not yet been proposed, except Fromy\u27s theory for the case of thin-walled circular tube specimens. We derive, in a simple way, a general expression for the Wiedemann effect or the torsion angle per unit length, θ_γ, of a cylindrical layer of the radius γ in cylindrical rod of ferromagnetic substance, fixed at its one end and magnetized by a longitudinal magnetic field, H_l, parallel to, and by circular field, H_, arround, its rod axis, and show that, when the elastic energy is negligible as compared with the magnetic field energy as in normal ferromagnetics, the general expression is reduced to θ_γ= (2/γ){λ_l(H_γ) - λ_t(H_γ)}(H_lH_/H_γ^2) where H_γ= (H_l^2+H_^2)^ and λ_l(H_γ) and λ_t(H_γ) are the longitudinal and transverse magnetostrictions accompanied with H_γ. This expression may be written, for the surface of the rod of the radius a, as θ_α = (2/α){λ_l(H_α) - λ_t(H_α)}( H_lH_/H_α^2), which holds also for a thin-walled tube and is the expression derived already by Fromy. This expression is further reduced to θ_α = (3/α)・{λ_l(H_α)・( H_lH_/H_α^2) when the volume magnetostriction may be negligible as in normal ferromagnetics. It is shown that the above expressions can explain, qualitatively completely and also to a considerable extent quantitatively, all of the available experimental facts concerning the Wiedemann effect
