Patent GB-558646-A (GB)
Title: Electric measuring, indicating, recording or controlling apparatus
IPC: G01R27/02
Filed: 1942-05-01
Abstract: 558,646. Impedance measurements. REDIFFUSION, Ltd., and ADORJAN, P. May 1, 1942, No. 19868/43. Divided out of 558,614. [Class 37] An apparatus for observing the impedance of a load comprises means for obtaining two direct potentials from an alternating or pulsating E.M.F. being applied to the load, these direct potentials being a function of the voltage of the applied E.M.F. and the current being passed through the load respectively, frequency discriminating means whereby the values of these two direct potentials are made primarily dependent upon alternating or pulsating E.M.F.s of a predetermined frequency, and means sensitive to the relative values of these potentials for performing an indicating, recording, or controlling operation. In the apparatus shown in Fig. 1 a transformer 4 supplies a load circuit represented by a resistance 5, inductance 6, and condenser 7, which circuit may be, for example, a public telephone line or a broadcast rediffusion network. Two pairs of terminals 8, 9 and 10, 11 are normally connected as shown in dotted lines but for measuring the impedance of the load they are connected to the measuring apparatus by terminal leads 12, 13, 14, 15. A potential transformer 16 and a current transformer 17 are connected as shown and their secondary windings are tuned by capacitors 16c, 17c to the frequency at which it is desired to make the observation, e.g. 500 c.p.s. Rectifiers 18, 19 are connected in series with the secondary windings of these transformers and a resistance 20 having a sliding contact 21 is connected between sides of similar polarity. A centre-zero micro-ammeter 22 is connected across the resistance 20. The slider 21 is adjusted until the instrument 22 gives a zero indication, the position of the slider then being dependent on the impedance of the load circuit at the frequency of the alternating E.M.F. A scale calibrated in terms of impedance may be provided. The frequency discrimination may alternatively be effected by filter networks. Fig. 2 shows a modification in which the change in load on the transformers 16, 17 by movement of the slider 21 is avoided. A rectifier 23 and resistor 24 are connected to the sliding contact 21 of a potentiometer resistance 25, which is connected in series with a fixed resistor 26 across the secondary of the transformer 16. Across the secondary of the transformer 17 is connected a tapped resistor 27 with a tap-selecting switch 28. This is connected to a rectifier 29 and resistor 30. A centre-zero micro-ammeter 31 is connected between corresponding ends of the resistors 24, 30. Balance is effected by adjusting the slider 21 of the potentiometer resistance 25. The tapping switch 28 enables the range to be changed. In the system shown in Fig. 3 the output voltages of the transformers 16, 17 are amplified by valves 32, 33 and rectified by a double diode valve 34. The measurement of the impedance may be effected at any one of a number of component frequencies of a complex wave by means of tuned circuits 43, 44, 45 to which the amplified voltage of the valve 32 is applied by a selector switch 42 and corresponding tuned circuits 52, 53, 54 to which the amplified voltage of the valve 33 is applied by a switch 51 ganged with the switch 42. Aperiodic circuits 46, 55 may be substituted for the tuned circuits for measuring the average impedance. An automatic gain control system is provided by a dry plate rectifier 57 which receives the full voltage across the secondary of the transformer 16 and produces a direct bias potential across a resistor 58, so as to bias the valves 32, 33. The output potential between the terminals 35, 36 may be applied to alternative apparatus A, B, C, D. The relay A controls a servomotor 62 which automatically moves the slider on the potentiometer resistance 25 to establish balance, and may also actuate recording or controlling means. The instrument B comprises an amplifier 65 supplying a cathode-ray tube 66. The instrument C is a graphic recorder. The relay D controls a switch 68 which switches in or out an auxiliary amplifier 69 for a main power amplifier 70 according to the prevailing conditions of the circuit. Specifications 558,614 and 558,645 are referred to.