Patent GB-632353-A (GB)
Title: Improvements in or relating to electrical measuring apparatus
IPC: G01R1/30
Filed: 1946-12-13
Abstract: 632,353. Electric indicating systems. REASON, R. E., and KAPELLA, Ltd. Dec. 13, 1946, Nos. 36860/46 and 746/48.. [Class 40 (i)] In measuring apparatus in which a stylus &c. moved in accordance with the quantity to be measured causes the output from one impedance to increase and that from a second impedance to decrease, these outputs are rectified and connected in series in such a sense that at balance there are two terminal points at equal potentials connected to the grids of a pair of valves in push-pull and control the measuring instrument, the mid-point of the push-pull system being connected to a third point the potential of which is between that of the terminal points. As shown in Fig. 1, a stylus-operated armature B, when moved from its central position, increases the inductance of one of a pair of coils A, A1, wound on iron cores and decreases that of the other coil. The coils A, A1 in parallel with the primaries C, D of two intermediate transformers, are energized from an A.C. source E through a mains transformer and swamping resistances F, F1. The junction of the coils A, A1 and of the primaries C, D is earthed to a conductor G, to which is also connected the mid-point of .an amplifying system comprising two triodes H, H1 in push-pull. The cathodes of the valves H, H1 are connected through resistances H2, H3 to the earthing conductor G to provide negative bias. The anodes are connected together through a centre-zero measuring instrument J. Each intermediate transformer has two secondaries C1, C2, D1, D2. The outer ends of the main secondaries C1, D1 are connected to the grids of the valves H, H1. The inner ends of the auxiliary secondaries C2, D2 are connected to the earthing conductor G. The inner end of the main secondary C1 is connected to the anode of a diode K1 and the associated condenser L1 and resistance M1 are connected in parallel between the outer end of the secondary C1 and the cathode of the diode K1. The secondaries C2, D1, D2 are similarly connected to diodes K2, N1, N2, condensers L2, O1, O2, and resistances M2, P1, P2. The outputs of the diodes are crossconnected so that each main secondary is in series opposition with the opposite auxiliary secondary. When the armature B is in its central or zero position the rectified secondary voltages balance out and the instrument J gives zero indication. Movement of the armature B to one side or the other causes the grids of the valves H, H1 to be equally and oppositely biassed and the instrument J gives a corresponding deflection in the appropriate direction. The four separate diodes may be replaced by two double diodes or voltage doubling rectifier circuits may be employed. In a modification, Fig. 4, each intermediate transformer has a single secondary C3, D3, the output of which is rectified by a diode circuit S, T, U or S1, T1, U1 insulated from the earthing conductor G. The outer ends of the load resistances U, U1 are connected together through an impedance V, V1 whose mid-point is connected to the earthing conductor G. A double diode may be used in place of the two diodes S, S1 or voltage-doubling rectifier circuits may be used. The movement of the armature B may vary the magnetic circuit of one coil only, the other coil having an invariable magnetic circuit. One Provisional Specification describes also an arrangement employing a reflex amplifier including a cross-connected backing-off arrangement. An A.C. signal determined by each coil is backed-off by cross connections and applied to the grid of an amplifier. The A.C. outputs from the amplifiers are rectified and the resulting D.C. signals are backed-off by cross-connections and reapplied to the amplifier grids so that the amplifier acts both as an A.C. and as a D.C. amplifier. Specification 541,628 is referred to.