GB293847A - Improvements in or relating to electromagnetic devices for recording sound and reproducing sound from records - Google Patents

Abstract

293,847. Electrical Research Products, Inc., (Assignees of Jones, W. C.). July 14, 1927, [Convention date]. Addition to 290,264. Magnetic instruments; diaphragms, - Relates to electromagnetic devices for reproducing sound from a record of the kind described in the parent Specification in which the mechanical vibrations are damped by means of a viscous material acted upon by the movement of two relatively movable surfaces in a casing containing the viscous material. According to the invention, the magnet system comprises a permament magnet apon one leg of which an electromagnet is carried and upon the other leg of which is located an extension serving as a return path for the magnetic flux, the magnet system being contained in the casing containing the viscous damping material. In the construction shown, the magnet system comprises a semicircular permanent magnet 5 to one pole of which is secured an angular extension 6 carrying at its free end a U-shaped extension constituting the cores 7 of an electromagnet which ane surrounded by the windings 8. Secured to the other pole of the magnet 5 is a second angular extension 12 provided at its free end with an annular shaped portion 13 which surrounds the upper ends of the windings 8 in juxtaposition to a diaphragm 14 and provides a return path for the magnetic flux. The magnet system is enclosed in an annular casing 17 comprising a large annular chamber 18 for housing the permanent magnet and a smaller annular chamber 20 provided with a seating surface for the diaphragm 14. The casing 17 is provided with a thickened portion 25, Fig. 4, to which the magnet structure is secured by bolts 27 which also serve to secure the extensions 6, 12 to the pole-pieces. A terminal block 30 is secured to the upper face of the casing by bolts 32 and nuts 33 which also serve to clamp soldering terminals 34 in position, the block 30 being insulated from the casing by an insulating block 36 and washers 37. The casing is filled with viscous material to such a level as to fill entirely the smaller chamber 20 and has its larger end closed by a removable screw-threaded plate 52. The diaphragm 14 carries an armature 15 provided with transversely extending arms 40 and a stylus carrying projection 41.