[en] The authors show that measurements of the first frequency moment of the response function observed in magnetic neutron scattering from electrons will feature details of the electron correlations. The first frequency moment, or f-sum rule as it is often called, for magnetic scattering is therefore distinctly different from the corresponding sum rule for particle density interactions. The behaviour of the magnetic f-sum rule as a function of wave vector is illustrated, together with the mean-square energy transfer, in an approximate calculation for an electron plasma. (author)