Abstract: A device (1) for generating a variable composite magnetic field comprises top and bottom concentric ring members (3, 4) which define a main central axis (5) and a central area (4) within which the composite magnetic filed is formed. Rod-like permanent magnets are located thin respective annuli (15) of the respective ring members (2, 3), and are equi-spaced circumferentially along a pitched circle (21). The permanent magnets (18) are rotable about corresponding secondary axes (19) which extend parallel to each other and parallel to the main central axis (5). The permanent magnets (18) are magnetised with their respective direction of magnetisation extending transversely of their secondary axes (19) and are independently rotatable by corresponding stepper motors (22) in respective of annuli (16) of the ring members (2, 3) for orienting the permanent magnets (18) about their respective secondary axes (19) for in turn altering the composite magnetic field generated in the central area (4).