and 68 windings 78 further identified by the legends a b and c for respectively the inductors 46 66 and 68 and an encapsulation 80 of a rigid insulation material such as polyurethane further identified by the legends a b and c respectively in the inductors 46 66 and 68 the inductors 46 66 and 68 are each fabricated in the form of a toroid so that their respective magnetic fields are essentially unaffected by the presence of the rod 28 which passes through each of the inductors 46 66 and 68 the inductor 66 is secured within the front member 24 by a cap 82 which is threadedly secured to the front member 24 the cap 82 having an aperture 84 for passage of the wires 36 the inductor 46 is secured to the support element 44 by a ring 86 threadedly secured to the support element 44 the inductor 68 is secured to the rear member 26 by means of a cap 88 which is threadedly secured to the rear member 26 and has an aperture 90 for passage of the wires 36 the core 76 in each of the inductors is fabricated of a heavy magnetic material such as iron and the windings 78 are fabricated from a heavy electrically conducting metal such as copper the mass of the core 76c plus that of the winding 78c approximates the total mass of the material typically iron which was removed from the rear member 26 to provide the void wherein is nested the inductor 68 as shown in the figure the volume of the inductor 68 is approximately one quarter.