GB1203745A - Permanent magnet assembly - Google Patents

Abstract

1,203,745. Moving coil instruments. R. A. PARNELL. 15 Jan., 1969 [12 Feb., 1968], No. 2301/69. Heading H4J. A permanent magnet assembly 10, suitable for a moving coil loud-speaker or microphone, comprises a magnetic core body 19 having a polepiece 18, preferably cylindrical, projecting forwardly. Around the core body 19 are permanent magnet wafers 20 having flat inner sides 20a of like polarity abutting against the core body 19 and outer sides 20b of the opposite polarity. A front member 17 of magnetic material, preferably rectangular, has a hole 15 into which the polepiece 18 extends. It cooperates with the polepiece 18 to define an air gap 14. An outer return case of magnetic material contacts the outer sides 20b of the wafers 20 and the front member 17 to define an air gap 14. The case preferably comprises four magnetically conducting plates 21. The core body 19 has flat sides abutting the flat inner sides 20a of the wafers 20. The core body 19 may be rectangular, triangular or of other polygonal section. Integrally joined plates 21 may cover more than one wafer 20. The core body 19 and polepiece 18 may be formed in one piece or adhered together. The other parts may be secured together adhesively or mechanically. A clearance gap, between 1/2 and 2/3 of the thick. ness of the wafers 20 is provided between the wafers 20 and the front member 17. The wafers 20 are rectangular and identical and wider than the contiguous sides of the core body 19 by 1/3 to 2/3 of the thickness of the wafers 20. Each wafer 20 has one side edge flush with an adjacent side of the body 19 and another side edge partially overlapping an adjacent wafer 20.