A magnetic field generator (50) having at least one magnet (51) extending along a longitudinal axis (101), wherein the magnetic material of the at least one magnet is arranged such that the at least one magnet produces a magnetic field with a magnetic flux density that changes substantially continuously in magnitude in the axial direction substantially along the length of the at least one magnet in the axial direction such as to enable the axial position of the magnetic field generator to be determined by a position sensing assembly comprising the magnetic field generator (50) and at least one magnetic sensor (105).