Magnetic field generating assembly

An assembly according to claim 20, wherein the first and second magnets define a first pair of magnets, the third and fourth magnets define a second pair of magnets and the fifth and sixth magnets define a third pair of magnets and one pair of the first pair, second pair and third pair of magnets is magnetized in opposition to the other pairs of magnets.

An assembly which generates a magnetic field in a working volume spaced from the assembly, the assembly comprising:

• first and second magnets, each having a north and a south pole and a respective magnet axis which joins the north and south poles, the first and second magnets being positioned so that the respective magnet axes are along a main axis of the assembly and a pole of the first magnet faces a like pole of the second magnet;
• and
• third and fourth magnets, each having a north and a south pole and a respective magnet axis which joins the north and south poles, the third and fourth magnets being positioned so that the respective magnet axes are substantially parallel with the main axis, wherein the position of at least one magnet of the group comprising the first, second, third and fourth magnets is adjustable in the direction of the main axis and the relative positions and magnetic field strengths of the first, second, third and fourth magnets are chosen to control the radial dimension and the characteristics of the magnetic field generated in the working volume.