Meissner effect, the complete expulsion of a magnetic field from a metallic conductor when the latter becomes superconducting (as the temperature and the magnetic field strength fall below the critical value Hk). The Meissner effect was first observed by the German physicists W. Meissner and R. Ochsenfeld in 1933. The relation B=H+4πJ=(1+4πμ)H between the magnetic induction B, the magnetic field strength H and the magnetisation J of the metal shows that, according to the Meissner effect (when B=0), an ideal superconductor behaves as an ideal diamagnet with an anomalously large magnetic susceptibility μ=-1/4π. In the Meissner effect the external magnetic field is screened by diamagnetic currents that arise in a thin surface layer of the superconductor. In metals that are not sufficiently pure, and in alloys, a partial “freezing-in” of the magnetic field in the bulk of the superconductor is observed, that is, the Meissner effect is incomplete. (see Superconductivity, Superconductors).