GB316246A - Improvements in or relating to electro-magnetic instruments of the moving coil type, particularly microphone transmitters and loud speaking telephones - Google Patents

Abstract

316,246. Clarke, H. M. April 26, 1928. Loud-speakers; magnetic instruments.-In a loud-speaker or microphone of the moving-coil type, an additional winding coaxial with and of axial length at least equal to that of the moving coil is provided to produce a magnetomotive force in opposition to that in the latter, and having a distribution and position substantially the same as that produced by the current flowing in the moving coil. The additional coil may be of enamelled copper wire and in two parts a, b on either side of the moving coil e, the coil a being wound on the central pole c of a pot-magnet and insulated therefrom by a paper ring, the turns being cemented together by celluloid cement, while the coil b is wound on a former, removed therefrom and attached to the outer pole-piece d. Coils a and b each have half the number of turns of the moving coil, have an axial length at least equal to that of the latter, and are connected in series with each other and the moving coil. in such a way that the resulting magnetic effect is a minimum. Alternatively, the additional coil may be in the form of two highly conductive metal cylinders, one within and one around the moving coil, and of axial length equal to or greater than the latter. The additional windings may in other modifications be non-rigidly suspended in such a position relative to the moving coil as to produce the required effect. The additional windings may be electrically connected in series or parallel with the moving coil or alternatively to one winding of a transformer (e.g. a secondary winding) and the moving coil mav be connected to another winding (e.g. another secondary winding) of the same transformer, or the two coils may be connected to different tappings on one winding. In each case resistances, capacities, or other means by which an adjustment of the magnetomotive force in the additional coil and its phase can be made, may be shunted across the additional winding. Specifications 250,931 and 252,228 are referred to.