832,609. Bridge measurements. WALTON, J. H. D. May 25, 1956 [Feb. 25, 1955; March 9, 1956], Nos. 5762/55 and 7473/56. Class 37. Apparatus for testing impedances comprises a bridge having two branches of which one comprises a standard impedance and the test impedance in series, and the other contains plural series resistances; detectors for responding to out-of-balance signals from the bridge network, and indicating means operating from the detector outputs to determine whether the test impedance lies between predetermined limits; a common terminal of the detectors being connected in circuit with the standard impedance and the irnpedance to be tested, and the remaining detector terminals being respectively connected either direct to junctions of the ratio arm series resistances or to junctions of series resistance pairs connecting the junction of the standard and test impedances to the appropriate junctions of the ratio arm resistances; such resistances being so proportioned as to establish plural values of test impedance at which the bridge is balanced, so that such values determine the required limits. In Fig. 1 (Provisional Specification 5762/55) a bridge network comprises standard impedance Z 1 in series with test impedance Z 2 in parallel with series-ratio resistances R 3 , R 4 , R 5 , and amplifier detectors D 1 , D 2 energized from a common connection to the junction Z 1 , Z 2 and independent connections to junctions R 3 , R 4 and R 3 , R 5 to operate polarized relays RLA, RLB, the ratio resistances being proportioned so that at the upper and lower limit values of the test impedance the respective bridges are balanced. When the test impedance value lies between its limits the bridge unbalance signals are of opposed polarities so that both relays are operative to close an indicator circuit C 2 , but when the impedance value is outside its limits the unbalance signals are of similar polarity so that the relays are inoperative. In Fig. 2 (Provisional Specification 5762/55) not shown, the bridge is A.C. energized and the detectors supply signals to a phase-sensitive ring demodulator operating a relay to close distinct indicator circuits when the unbalance signals are co-phased for a test impedance value outside its limits and antiphased for a value inside such limits. In Fig. 1 (Provisional Specification 7473/56) an A.C. energized bridge network has series standard and test impedance arms Z 1 , Z 2 in parallel with series ratio resistances R 3 , R 4 , R 5 and equal series resistances R 1 , R 2 earthed at their junction. The junction Z 1 , Z 2 is connected to junctions R 3 , R 4 , R. over equal series resistances R 6 , R 7 and R 8 , R 9 , whose respective junctions are returned to earth through amplifier detectors D 1 , D 2 whose outputs proportional to the bridge unbalance voltages at their inputs supply opposite ends of a phase-sensitive ring demodulator. As before R 3 , R 4 , R 5 are proportioned so that at the limit values of the test impedance respective bridges are balanced, and the unbalance voltages are co-phased when the impedance value is outside the limits and antiphased when it is within them, so that the polarity of the D.C. output of the demodulator operates relay RLC to correspondingly close circuit QR or QP. A meter may be connected across the demodulator output, and resistances R 4 , R 5 may be equalized and R 3 made zero to reduce the system to a simple bridge. In Fig. 2 (Provisional Specification 7473/56) not shown, the bridge is energized from a transformer secondary centre tapped to earth, and R 1 , R 2 are eliminated. Fig. 3 (Provisional Specification 7473/56) shows a bridge system energized from an earthed centre tap transformer secondary, wherein the outputs of the respective amplifier detectors D 1 , D 2 are compared in distinct demodulators M with antiphased reference signals from the transformer secondary to develop unidirectional outputs energizing polarized relays RLD, RLE ; a double-pole switch enabling either junction R 3 , R 4 or R 3 , R. to be earthed for upper or lower limit measurements, or R 3 to be shorted and earthed for simple bridge measurement. Contacts RLE1, RLD1 of the relays operate an indicator circuit so that when the test impedance is within limits, lamp L 3 is energized over contacts C, D; above limits lamp L 1 is energized over contacts B only; and below limits lamp L 2 is energized over contact A only. In Fig. 4 (Provisional Specification 7473/56) similar to Fig. 3, resistance R 3 is split into R 31 , R 32 and series resistances R 11 , R 12 are connected between junction Z 1 , Z 2 and junction R 31 , R 32 ; a further detector D 3 , demodulator M, and relay RLF being energized between junction R 11 , R 12 and earth to indicate whether the test impedance value lies above or below another limit by means of signal lamps operated by contacts RLF 1 of relay RLF. In Fig. 5 (Provisional Specification 7473/56) there are provided three separate bridge circuits having common test and standard impedance arms Z 1 , Z 2 and distanct pairs of ratio arms R 4 , R 5 ; R 41 , R 51 ; R 42 , R 52 ; all energized in parallel from the earthed centre tap secondary of a mains transformer. Unbalance signals from the junctions of series resistances R 6 , R 7 ; R 61 , R 71 ; R 62 , R 72 ; across the outputs of the respective bridges are applied through amplifier detectors to ring demodulator bridges P, P 2 , P 1 for phase comparison with a reference signal from the upper end of the transformer secondary, to operate relays RLG, RLH and from the lower end of the secondary to operate relay RLJ; the resistances R 5 R 51 , R 52 being variable to adjust limit values at which the respective relays operate in response to the unidirectional outputs of the ring demodulators. Indicator lamps L 1 , L 2 , L 3 are selectively operated by relay contacts RLJ1, RLG1 as before to indicate impedance values below, and between limits while relay RLJ indicates the relation of the impedance value to a further limit. The apparatus is applicable to initiate visual signals or automatic controls, e.g. for indication or control of strip thickness in a rolling mill in response to the impedance Z 2 of a coil having a magnetic circuit incorporating the strip. Flaws in metal may similarly be detected by inserting the test sample in the magnetic circuit of the coil, e.g. by moving the same over the internal or external surface of an aircraft spar section and setting the circuit so that the presence of a flaw causes the impedance to deviate from the test limits. The flatness of a surface may be tested by applying thereto at the corners of a square four coils wound on magnetic cores and connected diagonally in parallel as impedances Z 1 , Z 2 so that on a flat surface the impedances are equal and on a curved surface one or both of the cores on one diagonal are out of contact therewith to unbalance the impedances; deviation beyond a preset limit indicating an intolerable degree of curvature.