GB199678A - Improvements in telephone receivers and similar sound producing devices - Google Patents

Abstract

199,678. British Thomson - Houston Co., Ltd., and You.ng, A. P. May 20, 1922. Magnetic i n s t r uments.-A portion or the whole of the magnetic circuit of a telephone receiver is embedded in an insulating material such as plastic rubber or synthetic resin which is moulded to form the body of the receiver. In one form laminated pole pieces 1, Fig. 2, are moulded in insulating material so as to form a casing 2 of that material, a recess 7 being moulded on one side in which h a straight bar magnet 8, preferably of cobalt steel, is afterwards inserted and held against the pole pieces by a wedge 9. The coils 11 are then placed on the pole pieces and their terminals secured to terminal pieces 19 held by screws 18 in a recess moulded for the purpose. A slot may be provided for holding a second permanent magnet on the opposite side of the pole pieces to the magnet 8. In a modification, Fig. 5, the bar magnet 8 is in the same plane as the pole pieces 1 and its ends are shaped as shown to engage with the ends of the pole pieces. In further modifications, a straight bar magnet may be secured by screws to the pole pieces, or the magnet may be of horseshoe form with reduced ends to form pole pieces. The pole pieces in each case may consist of a cut away rectangular portion or straight or slightly tapering cylindrical pole pieces may be used. A flat portion 27 of the pole pieces forms a seat for the coils. Projecting portions of the pole pieces may be provided approaching each other to form a shunt path for the magnetic flux. In all modifications, the casing is provided with a screw thread 13 on which screws a cap to clamp the diaphragm. Where the pole pieces are laminated the laminations may be held together by rivets or by leaving holes into which the insulating material flows. According to one Provisional Specification, cylindrical coils with a conicallyshaped aperture may fit on conical cores, and according to another Provisional Specification, the moulded casing may be of non-magnetic metal such as brass or aluminium. Another Provisional Specification states that the magnetic circuit of a receiver in which voice currents pass through a coil movable in a magnetic field is formed from a central cylindrical core having a number of outwardly directed limbs against which are located the like poles of short bar magnets. The annular space between the magnets and the central core may be filled with an annular block of say aluminium or brass or synthetic resin may be cast in the annular space and around the outside of the bar magnets.