Patent GB-738526-A (GB)
Title: Improvements in or relating to maxwell bridges
IPC: G01R17/00
Filed: 1953-05-20
Abstract: 738,526. Impedance measuring bridges. WAYNE KERR LABORATORIES, Ltd., and CALVERT, R. May 20, 1954 [May 20, 1953], No. 14229/53. Class 37. A Maxwell A.C. bridge for low impedance measurement comprises included in its first arm a known resistance, in its second arm an adjustable complex standard impedance, in its third arm a further known resistance, and in its fourth arm the impedance to be measured, with an A.C. source connected between the junction of the second and third arms and that of the first and fourth arms, and an unbalance transformer having a first winding connected across the ends of the second and third arms remote from the A.C. source, a second winding connected to an unbalance detector, and a third winding in series with the first arm known resistance, which is not directly connected to the second arm but is coupled into the bridge circuit in series with the third transformer winding over leads connected to the immediate ends of the impedance to be measured. In Fig. 2 an unknown impedance 30 comprising inductance 31 in series with resistance 32 is connected between one side of the secondary of an A.C. energization transformer 39 and one end of a known fixed resistance 33, while the other side of the secondary is connected to the junction of resistance 33 and an adjustable standard impedance comprising a variable resistance 36 shunted by a variable capacitance 37. Primary winding 34 of output transformer 35 energizing the unbalance detector is connected between the free end of the standard impedance and the junction of resistance 33 and unknown impedance 30, while an additional output transformer winding 42 is connected in series with a known fixed resistance 43 over leads 44, 45 connectible to the immediate ends of the unknown impedance 30 so that the measured impedance is that between the connection points 46, 47 of the leads and the impedance. The resistance component 32 is inversely proportional to the variable resistance 36, while the inductive reactance component 31 is directly proportional to the variable capacitance 37. In a modification for measuring low values of the resistance component (Fig. 3, not shown), the variable resistance of the standard adjustable impedance is replaced by a fixed resistance connected to a variable tap on a potentiometer (which may be a standard decade potentiometer) replacing the fixed resistance of the third arm; whereby adjustment of the potentiometer tap is equivalent to variation of the effective resistance component of the standard adjustable impedance between infinity and the value of the fixed resistance connected to the tap. In a further modification (Fig. 4) for cancellation of residual and parasitic impedances introduced by the bridge arms and standards, resistance 33 (Fig. 2), is replaced by parallel potentiometers 52, 77, whose taps are connected to resistances 50, 75 alternatively connectible by switches 65, 66 in parallel with variable capacitances 37, 78 alternatively connectible by switches 67, 68 to constitute the standard adjustable impedance, and a further winding 60 in antiphase to windings 34, 42 of transformer 35 is connected by the switches between the junction of potentiometers 52, 77 with the unknown impedance 30 and the free ends of the resistances and capacitances of the standard impedance not currently connected in the measuring bridge. As shown, the tapping of potentiometer 52 and the capacitance 37 are variable for measurement of the unknown impedance while the tapping of potentiometer 77 and the capacitance 78 are initially variable to balance out the effect of residual and parasitic impedances. Switches 65, 66 and 67, 68 enable the resistances and capacitances to be respectively transposed and enable impedances to be measured in all four quadrants of phase angle. The transformer windings may be variably tapped to extend the range of measurement.