GB377461A - Improvements in permanent magnets - Google Patents

Abstract

377,461. Loud-speakers. HURD, W. L., Linden Avenue, Woodseats, Sheffield. Aug. 20, 1931, No. 23498. [Class 40 (iv).] A permanent magnet for moving coil loud-speakers comprises a body member of muff like form having two flat and parallel faces at its opposite sides around openings in the body and a plug on a back plate on one of the flat faces projecting through the body to the opposite side where it comes centrally within a hole in a front plate secured to the other flat face. The magnet shown comprises a muff-like part a having flat parallol faces b, c at opposite sides apertured at d, e. A back plate f is held on the face b and carries a plug g forming one polepiece and projecting through the muff and the aperture e into a hole in a front plate h secured to the face c and forming the other polepiece. In producing the magnet, the metal around the aperture e is slotted and undercut and the plate h is formed with countersunk holes i. A sleeve upon a jig k is passed through the hole in the plate to fill the annular gap around the plug g which is placed in position without the back plate f thereon. The whole is then positioned on the face c and a non-magnetic metal m, such as white metal, is poured through holes n in the face b or a hole formed in the body of the muff so as to fill in the hole e around the plug g and the holes i in the plate h. After the metal has solidified, the plate f is forced on to the plug g and the jig k and sleeve j removed. If desired the body of the muff may be cut away at different points. According to the Provisional Specification, a pin is used instead of the central polepiece g during the casting operation, this pin being removed as the metal solidifies. The front end of the polepiece g may also have a series of washer-like rings thereon to produce a laminated pole.