Patent GB-1019368-A (GB)
Title: Improvements in or relating to magnetic translating devices
IPC: H04R11/00
Filed: 1964-03-23
Abstract: 1,019,368. Electromechanical transducers. TIBBETTS INDUSTRIES Inc. March 23, 1964 [Oct. 7, 1963], No. 12075/64. Heading H4J.  A magnetic translating device has means for providing a polarized magnetic field and a magnet rotatable about an axis perpendicular to the direction of the magnetic field, the rotatable magnet being magnetized perpendicular to its axis of rotation so that rotation of the magnet varies the magnetic characteristic of the field A balanced armature transducer, which may be the microphone or receiver of a miniature hearing aid, comprises a casing 1 (Fig. 2) of soft magnetic material and a folder armature 2 having one end spot welded in a recess in a plate 3, the other end 4 (Fig. 1) being free to vibrate between upper and lower magnets 5, 6 provided with polepieces 7 having tapered pole faces, the polepieces being spaced apart by shims 8. The armature 2 passes through a coil 9. A drive pin 10 has its lower end fastened to armature 2 and passes through a hole in the upper end of the armature 2 to a diaphragm (not shown). The armature 2 is damped by a pad 11. The magnets are magnetized approximately perpendicular to the plane of the free end 4 of the armature 2. Magnet 5 has its S pole adjacent plate 3 while magnet 6 has its N pole adjacent the bottom of casing 1. A cylindrical magnet 12 (Fig. 3, not shown) is transversely polarized (Fig. 3a, not shown) and fits snugly in a cylindrical hole 13 in main magnet 5. It may be of platinumcobalt wire. It may be rotated by gripping its outer end with tweezers, or the outer end of the cylinder may be provided with a screwdriver slot 14, the slot being preferably along or perpendicular to the magnetization direction of the cylinder to provide a visual indication of position. The slot 14 is preferably provided in a brass head 16 which provides a shoulder abutting the magnet 5. A groove 17 seats the free end of a U-spring 18 to friction hold the magnet 12 in position. The other end of spring 18 is welded to casing 1. This has an access aperture 19 which is covered by a disc 20 of soft magnetic material. Two cylindrical magnets 21, 22 (Fig. 5, not shown) similar to magnet 12 may be provided, one in association with magnet 5 and the other with magnet 6. Uniformity of magnetic flux may thereby be obtained from unit to unit by simultaneously adjusting both magnets 21, 22 to eliminate flux travelling along the armature. After adjustment, the magnets 21, 22 may remain in position due to the snug friction fit, but may have a drop of cement at each periphery to fix them to magnets 5, 6. The cement may be dissolvable for later readjustment. Two screwdriver tools may be interconnected by a belt drive or gearing outside the casing to ensure identical adjustment. For mechanical strength of the casing the magnets 21, 22 (and associated apertures) may be at opposite ends. The magnets may be held by an epoxy adhesive containing carbonyl iron powder. Alternatively the magnets 21, 22 may be held by a hairpin spring 27 (Fig. 7, not shown) which engage in the grooves of pulley-like discs 28, 29 on the outer ends of magnets 21, 22; Alternatively the magnets 21, 22 may be interconnected by a belt drive, rack-and-pinion arrangement or sector gear within the casing. Preferably an intermediate gear-wheel 30 (Fig. 8, not shown) has teeth meshing with teeth 31, 32 milled around magnets 21, 22 or around separate brass heads. The magnets 21, 22 may be of square or rectangular cross-section in a circular cross-section hole or vice-versa. Alternatively the main magnets 5, 6 may be omitted, and a plurality of small adjustable cylindrical magnets with brass heads 15 (Fig. 4) may be inserted in opposed polepieces 71. The cylindrical magnets and adjacent portions of the polepieces perform the flux-providing function. One or more of the cylindrical magnets may be rotated for adjustment purposes. The cylindrical magnets are held adjusted by friction springs or cement.