Patent GB-378145-A (GB)
Title: Improvements in or relating to permanent magnets
IPC: H04R9/02
Filed: 1931-06-22
Abstract: 378,145. Loud-speakers. DARWINS, Ltd. and CATHERALL, A. C., Fitzwilliam Works, Templeborough, Sheffield. June 22, 1931, No. 17974. [Class 40 (iv).] Relates to a permanent magnet, more particularly for a moving coil loudspeaker, telephone relay or the like, of the type which comprises a base supporting a central polepiece of high permeability material and an outer series or a pair of arms of magnet steel with an outer polepiece connecting the free ends of the arms and secured thereto and consists in forming a portion of the magnet which merely conducts the magnetic flux, such as the base and/or outer polepiece, as distinguished from the parts which are used to maintain the magnetic flux in the system, from a number of laminations. Fig. 1 shows a construction of magnet in which the base comprises a number of laminations 11 of high permeability metal clamped by screws 12 between a cover-plate 13 of like material and a non-magnetic carrier 14. The central polepiece 15 is arranged as a tight fit in apertures in the laminations 11 and is supported by the plate 13. The outer polepiece also comprises a series of laminations 16 of high permeability metal having mutually registering central apertures which form an annular gap 21 with the central pole 15. The laminations 16 are clamped by screws 17 between a magnetic cover-plate 18 and a non- magnetic carrier 19 which is preferably cast on to the central pole 15 so as to engage a groove 20 therein. The base 11 and outer polepiece 16 are connected by four straight bar permanent magnets 22, each of which is connected by a screw 23 to a flange 24 on the carrier 14 and by a screw 25 to a flange 26 on the carrier 19. The laminations of both the base and the outer polepiece are notched to receive the ends of the magnets 22 which are covered by the plates 13, 18. Fig. 3 shows a modification in which the laminations 11 are clamped by the screw 12 between the central pole 15 and the cover-plate 13, the magnets 22 being of channel formation. One of the inwardly turned ends of each magnet fits into notches in the laminations 11 and is secured to the plate 13 by a screw 23 whilst the other inwardly turned end of each magnet fits into notches in the laminations 16 and is secured to the outer polepiece by a screw 17 which clamps the laminations 16 between the plate 18 and carrier 19. According to a further modification, Fig. 5, L-shaped magnets 22 are employed having the outer laminated polepiece 16 secured thereto as described with reference to Fig. 1 whilst the bent portions form part of a composite base and are clamped between the coverplate 13 and the central pole 15 by a screw 12. These bent portions are also secured directly to the plate 13 by screws 23, the bent ends of two of the magnets abutting against the edges of the other two which are somewhat longer. If desired, however, the base ends of the magnets may abut against a suitable block on the plate 13 or against a faced portion of the central pole 15. According to a further modification, the magnets 22 and the base are integrally produced in star-like form, in which case the plate 13 may be omitted, the central pole being connected directly to the base portion.