GB202418A - Improvements in and relating to telephone receivers and similar sound producing devices - Google Patents

Abstract

202,418. British Thomson-Houston Co., Ltd., and Young, A. P. May 20, 1922. Magnets; electromagnets; homopolar machines. -In electrical apparatus, including mercury motors, of the type employing a permanent magnetic circuit, the circuit comprises a central core having a flange, against which is located a number of short bar magnets arranged parallel to the central core. Fig. 3 shows the invention applied to a loud speaker telephone. The bar magnets are arranged against the flange 2 of the core 1 and are secured to a block 6 of non-magnetic material e.g. aluminium. The block 6 also supports an annular pole-piece 4, which co-operates with the core 1 to form an air gap 5. Instead of the annular pole-piece, a number of pole-pieces may be provided. In a modification, instead of employing the block 6, the space is filled with synthetic resin, such as the material known under the registered Trade Mark " Bakelite," which retains the magnetic members in position. Also synthetic resin may be moulded around the outside of the structure. As applied to a mercury motor, which can be used as in integrating meter, an amalgamated copper cup 60 is rotatably mounted in a mercury chamber 5. Eddy-current braking is effected by the provision of grooves 66 in the annular pole-piece 4; or by a copper disc mounted on the shaft 61 and rotating in the air gaps of permanent magnets. Specifications 15937/91, 24212/95, and 14124/02, [all in Class 40, Electric telegraphs &c.], are referred to. The first Provisional Specification states that the core has a separate tapered pole-piece, and the second Provisional Specification that projecting limbs may be used instead of the flange, and that a single annular magnet may be employed instead of the bar magnets.