GB383348A - Improvements of the carbon microphone - Google Patents

Abstract

383,348. Microphones. SIEMENS BROS. & CO., Ltd., Caxton House, Tothill Street, and WIGAN, E. R., 18, Cambridge Road, Lee, both in London. Sept. 12, 1931, No. 25633. [Class 40 (iv).] In a two-electrode granular microphone having a modulating element or moving electrode coupled to a diaphragm by a driving support, the modulating element is so dimensioned and/or shaped as to vibrate substantially as a whole and in phase with that end of the support to which it is fixed and is totally immersed in the granules in such manner that its motion in either direction is opposed by granules. Figs. 1 (III), (V), and (VII) show various forms of the moving and fixed electrodes me and fe, g being the granules, the chamber for which is closed at the front end by a relatively rigid closing member cm. Thus, as shown in Fig. 1 (III) both electrodes are of disc form, the moving electrode me having a conducting surface on one or both faces, or the fixed electrode fe may be cylindrical as shown in Fig. 1 (V), the moving electrode being of disc form as shown or conical. Alternatively, the fixed electrode may be of conical form as shown in Fig. 1 (VII), the moving electrode being either a hollow or solid cone. The constructions shown in Figs. 1 (V) and (VII) are such as to cause a radial, or partially radial flow of current as shown by heavy lines in a direction which is more or less coincident with the lines of gravitational force and in that part of the chamber in which the pressure on the granules induced by that force is greatest. Fig. 2 shows a preferred embodiment of the invention, the closing member cm being replaced by a silk cup cu so that the inertia of the mass. of granules is relied on to produce a proper resistance modulation. In this construction, the moving electrode me is connected to the diaphragm d by a spider support s, screw sc and resilient member rm. Granules are introduced into the chamber through the filling hole fo and a cavity may be provided so that granules therein help to maintain the pressure on the moving electrode when the microphone is in an inverted position. An annular chamber occupied by granules may also be formed between the inner surface of the barrel ba and the fixed electrode. According to a modification two fixed electrodes are employed and a totally immersed modulating element not forming part of the electric circuit is provided for modulation of the resistance of the granules. Specification 308,630, [Class 40 (iv), Telephones &c.], is referred to.