A transducer includes a core of low reluctance magnetic material formed with a narrow gap. There are first and second coils of conductive material wound on the core adjacent to and on opposite sides of the gap. A permanent magnet in and substantially filling the gap is in noncontacting relationship with...http://www.google.fr/patents/US5216723?utm_source=gb-gplus-shareBrevet US5216723 - Permanent magnet transducing
