A device for loading a volume of interest (VOI; 1,4; 2.4; 4.7; 9.3) with a constant magnetic field, comprising at least one magnetic field source (1.2; 2.2; 3.2a; 3.2b; 4.2; 7.2a; 7.2b; 7.2c; 7.2d; 9.1;) and a permeable yoke (1.1; 2.1; 3.1; 4.1; 5.1; 6.1; 7.1; 8.1; 9.2; 11.1) which guides the magnetic flux generated by this magnetic field source into the volume of interest (VOI; 1,4; 2.4; 4.7; 9.3), wherein the yoke (1.1; 2.1; 3.1; 4.1; 5.1; 6.1; 7.1; 8.1; 9.2; 11.1) is guided through at least one closed conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d), wherein the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b, 11.2c; 11.2d) is arranged between the magnetic field source (1.2; 2.2; 3.2a; 3.2b; 4.2; 7.2a; 7.2b; 7.2c; 7.2d; 9.1) and the volume of interest (VOI; 1,4; 2.4; 4.7; 9.3), and wherein the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d) can be transferred into the superconducting state in such a way that, in the superconducting state of the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d), a change of the flux by the yoke (1.1; 2.1; 3.1; 4.1; 5.1; 6.1; 7.1; 8.1; 9.2; 11.1) causes a current along the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d), which current counteracts this change, characterized in that switching means (2.7; 9.5) are provided for local switching of a part of the at least one conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d) from the superconducting to the normally conducting state, wherein the switching means comprise • at least one Josephson contact and/or • a heating element and/or • an electromagnetic radiation source for local heating of the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d) and/or • a local conductor cross-section narrowing with reduced critical current on the circumference of the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d), through which a magnetic flux, during the transition to the normally conducting state, is able to penetrate the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d) • at least one Josephson contact and/or • a heating element and/or • an electromagnetic radiation source for local heating of the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d) and/or • a local conductor cross-section narrowing with reduced critical current on the circumference of the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d), through which a magnetic flux, during the transition to the normally conducting state, is able to penetrate the conductor loop (1.3; 2.3; 3.5a; 3.5b; 4.3; 5.3a; 5.3b; 6.3; 7.3a; 7.3b; 7.3c; 7.3d; 8.3; 9.4; 11.2; 11.2a; 11.2b; 11.2c; 11.2d)