response to changes in the magnetic field of the stator caused by rotation of the rotor application_number US 912398 A description in the rotation detector 10 of the first embodiment shown in fig1 and 2 the magnetic flux conducting means is a flux ring 15 and the coil 11 is wound around the flux ring the flux ring 15 is of the full shell type meaning that the ring substantially surrounds the motor casing 22 in use in practise the inner diameter of the flux ring in its relaxed state is smaller than the diameter of the motor casing so that the flux ring 15 is stretched to fit the motor casing 22 with the resilience of the ring utilised to secure the ring to the motor additional detents may be provided the shape of the ring substantially matches the shape of the motor casing to give a good contact between the casing and the ring to minimise the resistance of the magnetic flux path the motor 20 shown is a flat sided motor meaning that the casing 22 has two arcuate or part circular portions connected by two straight portions the straight portions being parallel to each other the stator has two arcuate permanent magnets 23 fitted to the circular portions of the casing 22 the.