GB821227A - Improvements in or relating to substitution bridge circuits - Google Patents

Abstract

821,227. Measuring bridges. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 3, 1956 [Oct. 4, 1955], No. 28293/55. Class 37. A substitution bridge circuit comprises in closed series a signal source, a signal detector and a first impedance network, a first winding of a transformer in parallel with the signal source, and a second winding of the transformer in series with a second impedance network across the signal detector; the transformer connections being such that the current in the detector from the second winding and network is in antiphase to that in the detector from the signal source and the first network; which includes a bridge balancing variable impedance and switching means disconnecting or connecting an unknown impedance to be measured, whereby the difference of impedance necessary for a detector null reading with the unknown impedance respectively in and out of circuit gives a measure of the magnitude of a component of the unknown impedance. Figs. 4, 5 show a substitution bridge circuit for measuring a capacitance C1 by adjusting a variable standard capacitance V to two balancing values with C1 disconnected and connected in parallel therewith. Windings T1W, T2W of 1: 1 transformer T are connected in series with their junction earthed and winding T 1 W is energized from an oscillator while the parallel capacitances V, C1 and a fixed capacitance CA are respectively connected between the live ends of T1W and T2W to a detector which is returned to earth. At balance the detector current through V, C1 is equal and in antiphase to that through CA, and stray capacitances from C1 to earth are shunted across the oscillator and the detector, thus eliminating error. In a modification (Fig. 6) for measuring a capacitance having shunt conductance, resistances R1, R3 are shunted across T1W and resistance R2 connected from the junction to the live side of the detector; R3 being adjustable for conductance balance while resistances R4, R5 are connected in series and in shunt with capacitance CA to assist in balancing for imperfect capacitances having an appreciable power factor. Resistances R1, R3 may be replaced by equivalent fixed and variable capacitances (Fig. 7, not shown). For increased accuracy at high frequencies, stray capacitances not appearing across the oscillator or the detector are reduced by symmetrically arranging the wiring from the oscillator, detector, transformer and fixed capacitance to the terminals at which the capacitances to be compared are connectible (Fig. 8, not shown). In Fig. 9 stray earth capacitances are represented by CS, CT of which CS is variable. An additional detector D2 is provided in a subsidiary bridge comprising stray capacitance CS, and capacitance CB, so that with switch S2 open and S1 closed to insert unknown impedance Cl in the main bridge, variable capacitance C4 in shunt with CS is adjusted until combined impedance of C4 and CS is balanced against that of CB for null of detector D2, which is then short-circuited by S2 and the main bridge rebalanced for null of detector D1 by adjustment of V. S1 is opened and the subsidiary bridge rebalanced after which S2 is closed and the main bridge unbalanced until balance is attained in both conditions so that the stray capacitance effect is eliminated. The device may be utilized for measuring permittivity and power factor of dielectrics inserted within the electrodes of a standard capacitance whose capacitance is determined, or whose plates are movable to achieve the same capacitance, with and without dielectric.