120 tooth worm gear reduction of course rotational drives having different particulars could be employed magnetizer 21 is moved linearly with respect to disc 12 by a rotational device such as stepper motor 22 having 40 turn per circle lead screw 23 on motor shaft 24 boss 25 is configured with a hole having internal threads to which the spiral lead screw threads are coupled the control signals are applied from microcomputer 65 to motor 22 through wires 26 again the specifics of the rotational drive are set out here as an example only other appropriate elements having different characteristics could be used in the preferred embodiment ferrite toroid core 31 fig4 which is about 30 mm in diameter in the particular embodiment being described is formed with gap 32 which is about 1 5 mm wide drive coil 33 is wound as a single layer over about 270 u00b0 of toroid 31 symmetric with respect to the gap feedback loop 34 encircles the toroid body at a location about 180 u00b0 from opposite the gap loop 34 may be outside of coil 33 or between coil 33 and the toroid core it may consist of a few or many turns as necessary and appropriate for the feedback function the purpose of the feedback loop is.