Rotary electromagnetic indicator

Claim 1

In a rotary electromagnetic indicator having a stator with a ferromagnetic core with at least one salient pole and means for selectively magnetizing the stator core to selectively establish substantially reverse magnetic fields through the stator pole and a rotor indicator having a permanent magnet with substantially diametrically spaced salient magnetic poles of opposite polarity and a diametrically extending magnetic axis and mounted to be rotated in general alignment with the stator pole with an air gap therebetween by the magnetic interaction of the stator and rotor magnet and to be rotated approximately 180* by substantially reversing the magnetic field established by the stator through said one salient pole, the improvement wherein the rotor comprises a separate elongated generally radially extending ferromagnetic element of low magnetic retentivity associated with each of the salient poles of the permanent magnet and mounted in close association therewith, each ferromagnetic element lying in a common transverse plane with the associated salient pole of the permanent magnet and being angularly spaced from the associated salient pole of the permanent magnet and extending from adjacent an edge of the associated salient pole of the permanent magnet generally radially outwardly beyond the associated salient pole of the permanent magnet toward the stator pole and generally between the permanent magnet and stator poles and completely to one side of said magnetic axis when the poles are in general alignment, each ferromagnetic element having a circumferential cross section substantially less than and related to the circumferential cross sections of both the stator pole and associated permanent magnet pole such that when the associated permanent magnet pole and statOr pole are in general alignment, the ferromagnetic element provides an angularly offset low reluctance path for a portion only of the magnetic field extending between the associated pole of the permanent magnet and the stator pole when the stator pole and the associated rotor pole have opposite magnetic polarity and with the remainder of the magnetic field therebetween passing across the air gap between the associated salient pole of the permanent magnet and the stator pole and the ferromagnetic element is magnetically coupled at least in part to the stator pole when the magnetic field through the stator pole is reversed to rotate the rotor substantially 180*.