see fig1 the pushing device may have a fork shaped portion that may be positioned to straddle the connector behind the flanges 21 thereof after the winding operation the pushing device moves in the direction p parallel to the axis a of the mandrel so as to position the coil 2 precisely on the magnetic core 3 if the coil is made of wire with a thermo adhesive layer the coil may be heated or maintained hot to enable deformation thereof during insertion on the core 3 the tore shaped core 3 is made of a conventional magnetically permeable material such as iron nickel and may have any appropriate shape cylindrical wire flattened wire metal sheet or an assembly of such elements typically in order to have good magnetic properties the material is annealed after deformation to an open tore shape since large plastic deformation degrades magnetic properties it is however not possible to anneal the tore once the coil is mounted thereon in the present invention plastic deformation of magnetic material is reduced to a minimum in order to reduce the influence of such deformation on the magnetic properties of the core to this end the magnetic material has for example the shape of a.