A generator (13A, . . . 13C) of a time-variable magnetic field (B1) of a magnetic resonance device (1) which has an examination space (9,9A) for accommodating at least one area to be examined of an examination subject (7), in which examination area the time-variable magnetic field (B1) is generated by means of a current through a conductor (15, 19, 31). The magnetic field lines (23) of the magnetic field (B1) close with the aid of a field flux return space (17), in which there is arranged at least one sub-assembly (27) which has magnetizable material. A reduction of a magnetic energy density in the field flux return space (17) is produced by this means. This increases e.g. the efficiency in the generation of high-frequency magnetic fields (B1) or of gradient fields with the aid of the conductor (15,19,31).