Patent GB-385603-A (GB)
Title: Improvements in apparatus for the conversion of electrical oscillations into mechanical oscillations
IPC: H04R11/00
Filed: 1931-05-20
Abstract: 385,603. Current and pressure meters. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, and OLIVER, D. A., of General Electric Co., Wembley, Middlesex. May 20, 1931, No. 14923. [Class 37.] Relates to apparatus such as a loud-speaker or a vibration galvanometer, for the interconversion of electrical and mechanical oscillations, of the type comprising an armature which is normally held balanced in a magnetic circuit by opposing magnetic forces, and is vibrated as a result of relative variations in these forces In such apparatus, the armature has a resonant frequency dependent on the geometrical relations of the parts of the magnetic circuit, and according to the invention, means are provided for the mechanical adjustment of the said parts, whereby the resonant frequency of the apparatus may be varied at will. The invention is shown as applied to a loud-speakei of the " inductor type." Two armature masses 11, 12, Figs. 2 and 3, connected by bars 13, are arranged in air gaps between pairs of opposed polepieces 8, 9, each upper or lower pair being provided on poles 5, 7 connected by C-shaped magnetic yokes. The bars 13 are held by flexible supports 14 which permit the masses to oscillate across the air gaps when varying currents pass through coils 16, 17, each surrounding a polepiece as shown. A rod 15 connects the mass 12 to a diaphragm (not shown). The polepieces 9 are integral with the poles 5, 7, respectively, but the polepieces 8 are slidably mounted in contact with the poles. They are adapted to be adjusted in the direction of movement of the armature by means of a spring-controlled threaded spindle 18 passing through a brass member 19 secured to the poles 5, 7. The adjustment is made by rotating past a pointer 25 a dial 20 which is screwed on to the spindle 18. By this means, the spacing apart of the magnetic air gaps may be varied, and the resonant frequency of the loud-speaker thereby adjusted. In the modification shown in Figs. 5 and 6, there are only three polepieces, the flux from the polepiece 7 dividing and passing through the pole pieces 8, 9. A single armature 11 is adapted to oscillate in the air gap. A liner 26 comprising two iron portions 27 and two brass portions 28 is adapted to slide between the pole-pieces 8, 9 perpendicularly to the plane of Fig. 5, so that in effect the spacing apart. of the polepieces 8, 9, is varied by varying the extent of the iron parts 27 in contact with the polepieces. The apparatus shown in Figs. 2 and 3 may be modified for use as a. vibration galvanometer by adapting the rod 15. to vibrate a mirror &c. to detect an alternating current flowing in the coils 16, 17. The Provisional Specification describes and illustrates another construction of loud-speaker, in which each pole corresponding to the parts 5, 7 of Fig. 2 is split, and the two portions are adapted to be adjusted towards or away from each other. It also states that the distance between the armature masses may be varied by sliding them along a connecting bar.