Abstract: A magnetic field generating assembly comprises at least two counter-running, nested coils (31-33) positioned between and spaced from opposed pole pieces (42, 43). The pole pieces (42, 43) are mounted to a wall of iron (38-41) surrounding the coils, the walls providing a substantially closed magnetic flux path. The arrangement is such that forces on the coils (31-33) due to magnetic flux in the wall are substantially balanced. The gap between the coils (31-33) and at least one of the pole pieces (42, 43) is sufficient to accommodate a human body. The opposed pole pieces (42, 43) are non-planar so that when working currents flow in the coils (31-33) a magnetic field is generated in working regions (48, 49) situated in the gap between the coils and the pole pieces having homogeneities suitable for performing a nuclear magnetic resonance experiment.