GB696837A - Apparatus and method for accurate measuring of d.c. voltage and resistance values bycomparison - Google Patents

Abstract

696,837. Voltage and resistance measurements; bridge circuits. PYE & CO., Ltd., W. G. Jan. 16, 1952 [Jan. 23, 1951], No. 1743/51. Class 37. For comparing two D.C. voltages e1, e2, Fig. 3, which are known to be approximately equal, each voltage is applied in turn to an A.C. detector circuit D through a changeover switch S operated at constant changeover frequency, so that any difference between the voltages is applied to the detector d as an A.C. voltage. When the two D.C. voltages e1, e2 are equal the detector gives zero response. The switch S has a short open-circuit period so that the voltages e1, e2 are never connected to the same points at the same time. The switch S may be a motor-driven commutator, cam-driven contacts, or vibratory interrupted contacts, the frequency of operation preferably being over 5 cycles per second. The detector D may comprise a valve amplifier and a rectifier which may be polarized by a frequency equal to that of the changeover switch, so as to be phase sensitive. Part of its D.C. output may be applied to the input circuit to provide negative feed-back and thereby provide a linear relationship between the output and input and give an accurate indication of the difference between the voltages e1, e2. The voltages e1, e2 may be applied to the detector through a condenser to eliminate errors due to the internal resistance of the sources of voltages e1, e2. Two resistances, for example low resistances having current and potential terminals, may be compared by connecting them in series and comparing the voltages across their potential terminals by the method described. A bridge circuit as shown in Fig. 5 may be employed, where the unknown resistor R1 and standard resistor R2 form two arms of the bridge and are both four-terminal resistors. The other two arms are formed by a fixed standard resistor R3 and a variable standard resistor R4, both of which are twoterminal resistors of comparatively high value. The voltages e1, e2 across the resistances R1, R2 are compared and R4 is adjusted until balance is obtained.