Patent GB-334131-A (GB)
Title: Improvements in and relating to alternating current potentiometers
IPC: G01R1/28
Filed: 1929-11-13
Abstract: 334,131. Tinsley, H., and Gall, D. C. Nov. 13, 1929. Potentiometers. - In an alternating-current potentiometer, two potentiometers A, B, Fig. 4, having subdivided resistances a, b and slide-wire resistances a<1>, b<1> are connected in series, the potentiometer A providing the in-phase component of the desired electromotive force and the potentiometer B providing the quadrature component. Reversing switches C, D are provided so that either or both components may be reversed. An electromotive force of any magnitude and phase angle may thus be read off, a detector e being included in the circuit for balancing an unknown electromotive force against the adjusted potentiometers. The in-phase potentiometer A has a current meter f, Fig. 5, in circuit and is standardized by direct current against a standard cell. The instrument is then transferred to alternating current, and reads correctly provided the resistances a, a<1> are non-inductive. The quadrature potentiometer B is standardized in the same manner, but in addition a mutual inductance g is inserted in one or both circuits in order to test the quardrature relationship between the potentiometers A, B, the secondary electromotive force of the mutual inductance being in quadrature with the primary current. The phase of the current in the quadrature potentiometer may be controlled by feeding the in-phase potentiometer A directly from the secondary winding of a transformer h, Fig. 6, and the quadrature potentiometer B from the secondary winding of a second transformer j, the primary winding of which is in series with a variable condenser k and resistance l. Another method is shown in Fig. 7, in which the quadrature potentiometer is fed from a transformer j, the primary winding of which is connected to the source l through an iron-cored mutual inductance o and variable resistances m, n. In a modification, Fig. 8, the phase of the current in the quadranture transformer B is varied bv means of a self-inductance p, and its magnitude is adjusted by means of a variable mutual inductance M. Variable resistance m<1> n<1> enable the instrument to be used over a wide range of frequencies. The power from the source may be amplified by one or more thermionic valves before being fed to the potentiometers as shown, for example, in Fig. 9, in which a variable coupling condenser r and a variable grid leak q are connected as shown. The potentiometers A, B are supplied from the secondary windings of transformers A<1>, B', the primary windings of which are in the plate circuits of two thermionic valves. The alternating current source l<1> is applied between the filament and grid of the first valve, quadrature between the transformers A', B<1> being effected by varying the resistance q and condenser r. In a modification, Fig. 11 quadrature between the voltages applied to the grids of the amplifying valves is obtained by adjusting tappings on resistances r<1>, r<2> in series with a condenser C and an inductance L respectively in two parallel circuits connected to the alternating current source l. The values of the resistances r<1>, r<2> are chosen so that their product is equal to the ratio of the inductance L to the capacity C, the currents in the two parallel circuits then being in quadrature. The two potentiometers may alternatively be fed from a polyphase supply through a phase-shifting transformer; the in-phase potentiometer may be fed through a resistance, and the quadrature potentiometer through an isolating transformer.