GB670027A - Improvements in or relating to electromagnetic transducers - Google Patents

Abstract

670,027. Receivers and transmitters; diaphragms. GENERAL ELECTRIC CO., Ltd., and POLGREEN, G. R. March 27, 1950 [March 25, 1949], No. 8217/49. Class 40 (iv). [Also in Group XXXV] The permanent magnet and pole-pieces of an electromagnetic transducer are constructed from ferro-magnetic powder material which is bonded by a non-magnetic, electrical insulating medium such as a synthetic resin plastic, the powder material used in the magnet having a higher remanence and coercivity than that used in the pole-pieces. According to the Specification, the consequential reduction of eddy currents in the magnetic structure permits a more compact construction and enables the transducer to be used for the direct modulation of radio frequency currents by audio-frequencies or to operate at ultrasonic frequencies. As shown in Fig. 1, a telephone receiver comprises an axially-magnetized, hollow cylindrical permanent magnet 1 and a central pole-piece 2 which are secured together by an adhesive, the pole-piece having part of its surface insulated by a layer of paper or synthetic resin and carrying a coil 3 on a stem portion 8. A circular diaphragm 4 of magnetic material is supported adjacent to the magnet and polepiece. In a modified arrangement, Fig. 2, the diaphragm is non-magnetic and carries a central armature 14 formed of magnetic powder, the axial length of the central pole-piece being reduced and an annular pole-piece 13 secured by an adhesive to the magnet. An alternative arrangement, Fig. 4, comprises a radially-magnetized ring magnet 16 fitted between concentric pole-pieces 19, 20. The diaphragm 29 may be magnetic, or alternatively may consist of a circular disc of magnetic dust material fitted within an annular disc of plastic or phosphor bronze. A further arrangement, Fig. 5, usable as a telephone receiver or transmitter includes a circular magnetic diaphragm 32 secured between two hollow cylindrical magnets 33 which are both axially magnetized in the same direction. Annular pole-pieces each carrying a coil 40, 41 on a hollow, inwardlyextending portion 38 are secured by an adhesive to the outer ends of the magnets.