formula_4. The identity of the functional form of the stationary electric and magnetic fields has led to defining the magnitude of the magnetic dipole moment equally well as formula_5, or in the following way, imitating the moment p of an electric dipole: The magnetic dipole can be represented by a needle of a compass with fictitious magnetic charges formula_6 on the two poles and vector distance between the poles formula_7 under the influence of the magnetic field of earth formula_3. By classical mechanics the torque on this needle is formula_9 But as previously stated formula_10 so the desired formula comes up.