GB385982A - Improvements in and relating to the measurement of alternating currents - Google Patents

Abstract

385,982. Current and pressure meters. COSENS, C. R. G., 13, Millington Road, and CAMBRIDGE INSTRUMENT CO., Ltd., Chesterton Road, both in Cambridge. May 7, 1931, No. 13513. [Class 37.] Apparatus for measuring an alternating - current voltage comprises a thermionic valve having a source to be measured connected between the grid and cathode, and an auxiliary source of the same frequency applied between the anode and cathode, the grid being biased so that the' apparatus absorbs no power from the circuit under 'test, and the continuous-current instrument 'in the anode circuit indicating the component of voltage in phase with the auxiliary voltage. A pair of thermionic valves A<1>, A<2> are connected to impedances R<1>, R<2> to form a bridge, the voltage to be measured being applied at C, and a further voltage being applied in opposite phase relationship to the anode coils B<1>, B<2>. Grid bias is obtained by resistances R<3>, R<4>, and the continuous current instrument D is provided with a condenser and choke to exclude any alternating current. The grid voltages may be applied in opposite phase, the anode voltages being in phase, and the apparatus may be made sensitive to all frequencies except one, or to one frequency. When a single valve is employed the second valve is replaced by a variable impedance, or a continuous current voltage and variable impedance may be employed to balance out any pulsating anode current flowing in the instrument before measurement is effected. Valves with separately heated cathodes, or with multiple arrangements of electrodes may be employed.