OK, how about it. Put a ring of compasses around the wire with needles strongly enough magnetised that they will stay pointing N-S around the ring regardless of the Earth's field. Can you then flip them with a current? A very good visual demonstration of how core works! Philip. BTW Siemens make some torroidal cores down to 1mm thick ferrite, slightly larger in other materials. RS Components stock some - http://rswww.com/ and go in through the catalogue: "Electronics", "Transformers/Inductors/Ferrites", "Ferrites", "Torroids" (sic). There are even data sheets but I didn't manage to download them.