  A permanent magnet assembly in which the flux density can be altered by a
mechanical operation is often significantly smaller than comparable
electromagnets and also requires no electrical power to operate. In this paper
five permanent magnet designs in which the magnetic flux density can be altered
are analyzed using numerical simulations, and compared based on the generated
magnetic flux density in a sample volume and the amount of magnet material
used. The designs are the concentric Halbach cylinder, the two half Halbach
cylinders, the two linear Halbach arrays and the four and six rod mangle. The
concentric Halbach cylinder design is found to be the best performing design,
i.e. the design that provides the most magnetic flux density using the least
amount of magnet material. A concentric Halbach cylinder has been constructed
and the magnetic flux density, the homogeneity and the direction of the
magnetic field are measured and compared with numerical simulation and a good
agreement is found.
