The coil (W) has a thin plate on which there is a shaped former (1) which guides the coil's windings. These have a rectangular cross section and are formed from multi strand wires (26) coated with insulation. The assembly is completed by a top plate (11) with a locating centre aperture that fits on the former and has a slot (30) that runs to the edge and is provided for the end (24) of the wire. A number of the coils may be stacked together.
