GB816242A - Improvements in or relating to electrical measuring bridges - Google Patents

Abstract

816,242. Measuring bridges. WAYNE KERR LABORATORIES Ltd., and CALVERT, R. Aug. 19, 1957 [Aug. 31, 1956], No. 26713/56. Class 37. Input transformer 10 of an A.C. energized impedance bridge has series secondary windings 14, 15 forming ratio arms whose junction is connected over a neutral line to one end of primary 17 of transformer 18 energizing detector 20; the free ends of windings 14, 17 being interconnected by unknown inductive impedance 19, while winding 15 is shunted by variable potentiometer 22 energizing a standard reactance T-network comprising series input and output resistances 24, 25 shunted to neutral over capacitance 28 and connected between potentiometer tap 23 and a variable tap 26 on further primary 27 of the detector transformer 18. By adjustment of tap 23, the detector transformer ampere turns derived from the T-network are balanceable against those from the inductive component current in the unknown 19, and it is shown that for zero detector output, where L is the inductance, E u the voltage applied to the unknown, Es the voltage applied to the T-network, C the capacitance (of small impedance compared to the resistances), R 1 and R 2 the first and second resistances, and N is the turns ratio of the detector transformer windings associated with the third and fourth bridge arms. Capacitance 28 may be adjustable in value and the inductance of the unknown 19 is measurable by reference to the setting of potentiometer 22 for balance. The resistive component of the impedance 19 is measurable by reference to the setting of a variable impedance 30 connected between a tap on detector transformer winding 17 and the free end of winding 15; the variable in-phase current component through which is balanceable against the in-phase current component through impedance 19. If the latter is of high Q, accurate measurement of its loss resistance necessitates balancing in transformer 18 the in-phase current component through 24, 25 by a potentiometer 31 shunted across the input to the T-network whose variable tap is connected through resistance 33 to a further adjustable tap on detector transformer winding 17. The loss resistance is thereafter measured by adjustment of standard resistive impedance 30 for further balance.