GB527039A - Improvements in and relating to electric measuring instruments - Google Patents

Abstract

527,039. Current, voltage; and resistance measurements. FRAENKEL, H., and AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO., Ltd., March 29, 1939, No. 9797. [Class 37] The moving coil assembly of a quotient meter, for example an ohmmeter, comprises a control coil formed in two series-connected oppositely wound parts extending from one polar gap to the axis of rotation together with a deflector coil disposed about the two parts of the control coil in the same plane, the magnetic system being so arranged that movement of the moving coil system brings the individual parts of the control coil into fields of different strengths. The permanent magnet 10 has pole pieces 11 with polar faces concentric with the axis of rotation of the moving system. Across the top of the pole pieces 11 is arranged a non-magnetic bridge 13 carrying pins 14 on which is mounted the field concentrator 15. The outer surface of this is cylindrical about its own axis, which is displaced relatively to the axis of rotation of the movement so that the air gaps between the pole pieces and the concentrator are symmetrical about a vertical centre line but increase in width from the top to the bottom. The concentrator 15 has a cylindrical bore 17 with a radial slot 18. The moving system comprises a control coil formed of two parts 20, each having one side within the bore 17 and the other side within a polar gap. These coil parts are oppositely wound, connected in series, and arranged on opposite sides of the core 15. They may be identical, or may have different numbers of turns. Over the parts of the control coil is wound a single deflector coil 21 extending across the whole core 15. In the absence of current in the deflector coil the coil assembly moves to a position in which the torques set up by the two parts of the control coil balance one another, so that a true zero position is obtained. , Zero adjustment may be effected by varying the displacement of the core 15, e.g. by inserting small washers between the bridge 13 and the pole pieces 11. In a modification, the field concentrator core is coaxial with the axis of rotation, one of the pole faces is concentric with the core, and the other pole face is eccentric thereto, so that one airgap is of constant length while the other progressively increases. According to the Provisional Specification only a single small pressure coil coplanar with the larger current coil is employed.