This paper deals with the calculation of the dependence rho = rho(mu(r
)), which is a Clausius-Mossotti type of relation for magnetic fluids,
and determines the partial derivative of the magnetization with respe
ct to density: partial-derivative M/partial-derivative rho, a term tha
t comes into consideration whenever one calculates the forces exerted
on magnetic fluids introduced in electric and magnetic fields. The pap
er also takes into account the influence of the electric field on thes
e terms, because in special cases the permeabilities of the magnetic f
luid depend on both E and H.