GB587581A - Improvements in instruments for measuring alternating potentials or alternating currents - Google Patents

Abstract

587,581. Thermionic ammeters and volt meters. MARCONI INSTRUMENTS, Ltd., and BROCKELSBY, C. F. June 5,1944, No. 10800. [Class 37] The invention relates to alternating current or voltage measuring arrangements wherein the quantity to be measured is applied to the control grids of a pair of thermionic valves, the anodes of which are fed with a reference alternating potential of the same frequency, so that the deflection of a D.C. measuring instrument depends on both the magnitude of the quantity being measured and its phase relationship to the reference potential. According to the invention the mutual conductances and the A.C. resistances of the valves are adjustable substantially independently of one another so that the characteristics of the two valves may be made identical. As shown, Fig. 1, the potential E1 to be measured is applied through a transformer T to the grids of triodes V, V1 in anti-phase and the reference potential E2 is fed to both anodes in phase. The mutual conductances of V, V1 are adjusted by variable resistances R1, R1<1>, respectively, and their A.C. resistances by anode resistor R2, R2<1>, in series with further resistors R3, R3<1>, if desired. The value El. cos # is measured by a D.C. instrument G which by initial adjustment of the valve characteristics is arranged to read zero.when E1 is zero, over a range of values of E2, # being the phase angle between E1, E2. For balancing an A.C. bridge network B, Fig. 2, the valves V, V1 are included in one envelope, R1, R1<1> and R2, R2<1> are formed by potentiometers and R3,- R3<1> are omitted. The source M supplies the bridge B and reference voltage E2 in phase therewith by separate transformer secondaries T1, the bridge output E1 being fed to the valve grids through transformer T. The transformer T may be omitted if the grid of one valve is connected to point A, Fig. 1, by way of source E1 and the. grid of the other valve connected directly to A. Also the grids may, alternatively, be energized in phase and the anodes in anti-phase. The meter G may, alternatively, be connected across the anode ends of R2, R2<1>. Specification 385,982 is referred to.