GB701091A - Improvements in electrical circuit arrangements of electro-magnetic devices for indicating the magnitude of a deflection - Google Patents

Abstract

701,091. Measuring bridges. BLACK & DECKER MANUFACTURING CO. June 30, 1948 [April 5, 1948], No. 17548/48. Class 37 [Also in Group XXXVIII] In a bridge circuit for indicating the deflections of a balance or scale of the kind described in Specification 698,793, [Group XXXVIII], an armature is displaced by a moving member of the scale between the poles of two magnetically cored coils to differentially vary their inductances. The coils 1, 2 are connected in series with fixed resistances 7, 8 as adjacent arms of an A.C. fed bridge whose remaining arms comprise the potentiometer 11 symmetrically in series with condenser 13 and condenser 14 each of which is in series with a pair of variable resistances correspondingly ganged in opposition and in tandem, respectively for zero and calibration adjustment, and the unbalance voltage between the junction of the coils and the potentiometer slider is applied through switch 24, 25 to an indicating circuit. The latter comprises two bridge rectifiers whose D.C. outputs are connected in series through the fixed and variable resistances 31 and 32 with a galvanometer 30 connected between the junctions of the resistances and the rectifiers. The A.C. terminals of the rectifiers, respectively in series with fixed resistances 33, 34 are connected across the secondary of a transformer 20 whose primary is energized from the A.C. supply through a current reducing resistance. Initially, the switch 24, 25 is open and the resistance 32 is adjusted so that the galvanometer connects equipotential points and reads 'zero. With no load on the scale and the armature symmetrical between the coil cores, the switch is closed and the resistances 14, 15 are varied until the bridge is balanced and the galvanometer reads zero. With a load on the scale the bridge unbalance voltage due to the armature displacement adds to the A.C. voltage on one rectifier and subtracts from that on the other, so that the galvanometer is deflected. This deflection is zeroized by adjusting the potentiometer slider, and the load is indicated thereby in a calibrated dial linearized by the pre-set resistances 37, 38 shunting the potentiometer. The condensers 13, 14 equalize the phases of the two branches of the bridge to eliminate at balance any residual voltage in quadrature with the excitation voltage.