GB691862A - Improvements in or relating to electrical measuring instruments - Google Patents

Abstract

691,862. Bridge circuits; current and voltage measurements. BUTLER, Ltd., H., and BUTLER, H. April 27, 1951 [April 27, 1950], No. 10361/50. Class 37. An instrument is connected in a network employing non-linear resistors in one or more arms so as to use a major portion of the available deflection to measure a selected portion of the current or voltage range, the resistance of one or more of the linear arms of the network being variable by control means on the instrument so as to vary the limits of the selected range. A Wheatstone bridge has two opposite arms formed by lamps L1, L2, the other pair of opposite arms being formed by linear resistances R1, R2. For testing the voltage of an accumulator the lamps L1, L2 and resistances R1, R2 are so chosen that the bridge is balanced when the voltage applied across terminals E is equal to that of the accumulator in the tow condition of charge. If the voltage is higher than this the current through the lamps L1, L2 and consequently their resistance, rises. The bridge becomes unbalanced and current flows through the instrument G. A switch S in the position shown connects the instrument G in series with a resistance r3 across the diagonal of the bridge. In the alternative position of the switch S the resistance r2, forming part of the resistance arm R2, is short-circuited so that the resistance r1 forms this arm. At the same time the sensitivity of the instrument G is altered by substituting a resistance r4 for the resistance r3. The change in the resistance of the arm R2 changes the lower limit of the selected range. Shunt resistances may be employed instead of the series resistances r3, r4. Instead of employing a bridge circuit the instrument may be shunted by a non-linear resistance in series wth a variable linear resistance.