GB660470A - Improvements in or relating to wheatstone bridges - Google Patents

Abstract

660,470. Bridge circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Jan. 31, 1950 [Jan. 31, 1949], No. 2679/49. Class 37. A bridge comprises two fixed resistance arms 13, 14 in series between two D.C. supply points 11, 12 and two further resistance arms 17, 18 which are in series between the same points and are adjusted in accordance with two variables, the ratio between which is to be indicated by a meter 19. The resistance of the meter circuit, which may include a series resistance 20, is high compared with the maximum resistance of either adjustable resistance arm 17, 18 in order to limit the response of the meter when the resistances 19, 18 are reduced without varying their ratio. A resistance 21 less in value than the overall resistance of the bridge between the supply points 11, 12 when both adjustable arms 17, 18 are at their minimum value is included in the supply circuit. Fixed resistances 15, 16 are included in series with the variable resistances 17, 18 and are of such value that the minimum value of each adjustable arm is greater than that of the corresponding fixed resistance arm 13, 14. The bridge may be supplied from a 12-volt accumulator, suitable values for the resistances 13, 14 being 10 ohms each, the resistance 21 being 3 ohms, the resistances 15, 16 being 20 ohms each, and the resistances 17, 18 being each variable between zero and 3980 ohms. The meter 19 may be of the moving-coil type having a resistance of 80,000 ohms, which may be provided in part by the resistance 20. In order to compensate the meter 19 for any variation in supply voltage the moving system may be acted on by an electromagnet connected across the supply points 11, 12, as shown at 22, 23, so that the control torque is also dependent on the supply voltage.