GB351723A - Improvements in or relating to permanent magnets for electrodynamic loud-speakers - Google Patents

Abstract

351,723. Loud-speakers. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN, 13A, Emmasingel, Eindhoven, Holland. June 11, 1930, No. 17943. Convention date, June 22, 1929. [Class 40 (iv).] Relates to a permanent magnet for an electrodynamic loud-speaker, the walls of which are formed with two coaxially arranged apertures adapted to be engaged by a soft iron mandrel supported by the wall of the one aperture and forming with the other an annular gap, and consists in forming the magnet as a box-shaped body, cast in one piece, which, apart from the above-mentioned apertures, is entirely closed. The body may be cylindrical or have the shape of a polyhedron and is preferably of cobalt steel containing 9 to 15 per cent of cobalt. As shown in Fig. 1, the magnet comprises a box-shaped body 10, the walls of which are formed with axial apertures 11, 12. A core 13 is secured by a conical bolt 17 in the aperture 12, the upper end of the core forming with the wall of the aperture 11 an annular gap 14 in which is located a coil 15 attached to the diaphragm 16. The body 10 is formed so that for the permanent flux the section is substantially constant at any point except in the neighbourhood of the air gap, where the section attains a minimum. For this purpose, the thickness of the wall 30 containing the aperture 12 is shaped so that its thickness increases towards the aperture. Fig. 3 shows a device for magnetizing the body 10 comprising a heavy U-shaped magnet 20 provided with apertures 21, 22 through which are slid pins 23, 24 that engage in the apertures 11, 12 of the body. When the coil 25 is energized by a direct current, the magnetic flux traverses the upright walls 26 of the body in the same direction so that the latter is magnetized, whereupon the pin 24 is pushed through the apertures 11, 12 and the body removed from the device. The pin 24 is then removed by sliding the core 13 into the aperture 12 and the bolt 17 is tightened to secure the core in position.