GB789567A - Improvements in or relating to methods for measuring electric potentials - Google Patents

Abstract

789,567. Voltage measurement. GUSTAFSSON, G. V. A., and OLSSON, C. O. Oct. 20, 1955 [Oct. 22, 1954], No. 30064/55. Class 37. [Also in Group XXXVIII] In apparatus for measuring instantaneous values of electric voltages, the unknown is switchably applied, for a period of time which is variable as to its duration and instant of commencement, to an indicating instrument (e.g. a cathode-ray oscilloscope) which remains shortcircuited until immediately before the commencement of the time period and is again short-circuited at its termination, and during the period the instrument indicates a comparison between a direct voltage of opposite polarity and variable magnitude and the unknown; the latter being determined with reference to the change of the direct voltage necessary for equality of comparison. In Fig. 4 a resistance strain gauge R1 is connected in a normally balanced Wheatstone bridge 1 energized from a direct voltage source at diagonal A, B to derive a variable voltage signal at diagonal C, D representing stress variations of a specimen on which is superimposed a variable direct voltage signal from potentiometer 12. The combined signal is applied over decoupling resistances to amplifier 3 and the vertical deflection plates of a cathoderay oscillograph 4, whose horizontal plates are energized from timebase 11 synchronized by impulses from trigger circuit 7 tripped by the amplified bridge unbalance signal on each passage of the latter through its arithmetical mean value. Variable delay circuits 8, 9 impulsed by the trigger circuit respectively operate and release relay 10 closing a shorting contact 5 across the oscillograph vertical plates for a time interval of variable incidence and duration adjustable for display of a narrow pulse (Fig. 1) at any point of the cycle of variation of the unbalance signal. The voltage of potentiometer 12 adjusted for coincidence of the baseline and the apex of the pulse (Fig. 2) determines the instantaneous stress signal value at the selected point, and switch 5 may alternatively be operated from the cycles of a fatigue testing machine or in dependence on extensions of a test specimen. The apparatus is also applicable to resistance thermometry. The bridge may comprise inductive or capacitative variable elements with A.C. energization and phase rectification of the unbalance signal.