The coil (W) has a thin plate on which there is a shaped former 1 to guide the coil's winding. The widening has a rectangular cross section and comprises multi strand wires 26 coated with insulation. The assembly is completed by a top plate 11 that has 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 using mechanical holders.
