“…Dzyaloshinskii (1959) reasoned, from the effect of symmetry operators on the free energy of the crystal, that the magnetoelectric effect should be present in materials which have combined space and time inversions as symmetry elements of their magnetic point groups. He noted that chromium oxide was such a material and predicted that the magnetoelectric effect should be present, a prediction that was confirmed experimentally by Astrov (1960) and since then the effect has been observed in a number of materials (Al'shin and Astrov (1963), Rado (1964Rado ( , 1969, Ascher et al (1966), Mercier et al (1967aMercier et al ( , 1967bMercier et al ( , 1968aMercier et al ( , 1968b). …”