Patent GB-813076-A (GB)
Abstract: 813,076. Measuring bridges. GENERAL RADIO CO. Feb. 18, 1957 7 [Oct. 19, 1956], No. 5478/57. Class 37. In a bridge adapted to measure a complex impedance through sliding-null balance adjustments of a pair of variable impedances in a corresponding pair of arms of the bridge, mechanical linkage means are provided for varying the first variable impedance whilst simultaneously varying the second impedance so as to keep constant a product of said impedances or reciprocals thereof and means for varying the second impedance independently of the first. In Figs. 1 and 2 the unknown complex impedance comprises a real resistance R x  and an imaginary series component, e.g. an inductance L x  of impedance X x  = j#L x , where j = #- 1 and # = angular frequency of the A.C. voltage e in  energizing the bridge. In Fig. 2 R b  is a known resistance, C p a known capacitance, Rp a variable resistor in shunt therewith having a scale calibrated in terms of Q, i.e. the ratio of reactance to resistance, R n  is a variable resistor whose scale mav be calibrated in inductance values. The general balance condition for A.C. bridges of this type is the left-hand side representing the real and imaginary components of the unknown impedance, the right-hand side a function of the three bridge arms not containing the unknown, M and R s  being real variables and X s  an imaginary constant quantity. Usually for balance successive adjustments are made to M and Rs to reduce the unbalance as much as possible, but variation in MX s  resulting from varying M also varies MR s  which term also varies from varying R s . Neither variations in M or R s  will move the point M(R s  + X s ) directly toward the desired R x  + X x  position but will slowly converge to it (Fig. 7, not shown); a false balance may be arrived at since a measurable variation in either R s  or M may produce a greater unbalance, and this is avoided by adjusting the terms MR s  and MX s  independently and a more rapid balance is achieved, the sliding null condition being eliminated. In the Maxwell bridge of Figs. 1 and 2 the general equation (1) becomes The apparatus of Fig. 1 comprises two cylindrical logarithmically wound potentiometers having sliders 5 and 6 tapping variable resistances R n <SP>1</SP> and R p <SP>1</SP> between terminals 7 and 9 and 8 and 10 which, with the fixed resistors R n <SP>11</SP> and R p <SP>11</SP> make up R n  and R p . Sliders 5 and 6 are adjusted by knobs 13 and 14 via shafts 11 and 12 mounted in a chassis 15, scales CRL and Q are mounted below the knobs 13 and 14. Variations in R n <SP>1</SP> cause variations in R p <SP>1</SP> through friction plates F 1  on shaft 11 and chassis 15, gears 16, 18 and 17, and friction plates F 2  on gear 17 and shaft 12 so that R n /R p  is constant. By causing the friction of plates F 2  to be less than that of F 1 , the slider 6 may be moved independently of slider 5, so varying R p <SP>1</SP> independently of R n <SP>1</SP>. Hence the terms R b R n  (1/R p ) and R b R n j#C p  in equation (2) may be varied independently. The gear 17 is of larger diameter than gear 16, gear 18 being an idler gear. In an embodiment in Fig. 3 (not shown) a differential gear mechanism is used instead of the differential friction means. Other forms of A.C. bridges may employ the device in determination of (a) unknown impedance resistance and reactive components connected in shunt (Fig. 4, not shown); (b) unknown impedance resistive and capacitive components in series (Fig. 5, not shown); (c) inductance and resistance in series by comparison with known inductance and resistance (Fig. 6, not shown).