Patent GB-252228-A (GB)
Title: Improved electro-magnetic sound recording apparatus and record produced thereby
IPC: H04R9/12
Filed: 1925-02-09
Abstract: 252,228. Sykes, A. F. Feb. 9, 1925. Magnetic instruments.-Relates to electromagnetic sound-recording apparatus of the kind comprising a coil energized by an acoustically-controlled current and movable in a magnetic air gap. The invention is mainly characterized in that electromagnetic damping is effected by winding the coil, which carries the recording member, on a closed-circuit conducting layer, such as a metal bobbin, or by energizing the coil by a circuit of equal or less impedance. A further feature of the invention consists in improved means for mounting the coil. In the construction shown in Figs. I, II, and IV, a vibratory coil 9 is wound on a metal bobbin 34, preferably of aluminium or magnesium, which is secured to a lever 31. The lever, which carries a stylus 32, is resiliently mounted on a floating weight 22 by means of a pair of parallel springy rods such as fine steel needles 33. The lever 31 lies in a slot in the weight 22, the coil and bobbin being disposed in an annular air gap between a central magnet pole 6 and a surrounding pole 4. A distance-piece 7 of brass or the like is mounted on the pole 6, between the magnet winding 1 and the pole 4, and within a recess in the member 7 is located a coil 8 connected in series with the coil 9, the coil 8 being so wound and arranged that it exactly neutralizes any induction effects that might arise from the other coil. The weight 22 is mounted by pivot pins (not shown) engaging a lug 25 on the magnetic casing 2, the pivotal axis lying between the upper and lower planes of the pole-piece 4. A pivoted catch 27, 28 is so arranged that when turned into the position shown it a a pin 29 on the end of the weight, so raising the weight and the stylus-lever. The whole instrument is carried by a support 14, with which rotatably engages a plate 12, on which a lug 10 projecting from the casing 2 is hinged at 13. Adjustments about the two perpendicular axes of rotation provided at 13 and 14 are effected by means of two nuts 17, 20. In order to damp relative vibration between the parts 22, 31, the space, between the end of the weight and the part of the lever to which the springs 33 are attached is preferably packed with petroleum jelly or grease. A modified mounting of the stylus lever is described. in which the cross-piece of the lever and the resilient rods are enclosed by the weight, the intervening space being filled with a damping medium. The leads to the coil 9 are also preferably thickly coated with grease or the like. Instead of winding the vibrating coil on a metal bobbin, it may be wound upon another winding of wire short-circircuited upon itself, which layer is located on a non-metallic bobbin; or the coil may be wound upon a non-metallic bobbin and enclosed within a metallic sheath or a short-circuited winding. Instead of a vibrating coil 9 arranged as shown, a torsionally-acting coil may be employed. Water or oil cooling of the magnetic field coils may be provided. The invention may be applied to apparatus for producing a photographic sound record, for example as shown diagrammatically in Figs. X and XII. In this apparatus, an elongated vibrating coil 46. wound on an aluminium former 47, is resiliently pivoted between blocks 50 depending from a bridge piece 51 mounted above the poles of a magnet 45. The former 47 has a central aperture 53, to the front of which is attached a curved piece 54. This passes over a slit 55 in a sheet 56 beneath which moves a photographic film. A lamp 58 is arranged as shown, and light passes through the aperture 53 and the slot 55, the effective width of which is varied bv the vibrating member 54. A similar apparatus to that just described may be adapted for mechanically cutting wax records. A transmitter such as is described in Specification 220.420 may be used for operating the sound-recording apparatus, and tone-control circuits suitable for correcting the mechanical characteristic of the recorder may be employed. Specification 160,223 is referred to.