Patent GB-860755-A (GB)
Title: Improvements in or relating to apparatus for measuring changes in electrical resistance
Inventor: PARRAMORE THOMAS STUART
IPC: G01R17/10
Filed: 1956-10-25
Abstract: 860,755. Measuring bridges; variable resistors. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 25, 1957 [Oct. 25, 1956], No. 32527/56. Class 37: Apparatus measuring resistance wherein a primary bridge containing the unknown resistance is eriergized from the same source as a measuring bridge whose resistance arms are adjustable by twin ganged slidewires interconnecting the resistance arms at opposite apices; the unbalance outputs of the bridges being parallel connected to a common null balance indicator so that a measurable out of balance of the measuring bridge due to adjustment of the slide wires annuls the out of balance current in the indicator due to the primary bridge. A resistance bridge 1 having equal arms R1, R2, R3, R4 is energized at opposite points 5, 6 from a D.C. source and a galvanometer is connected across opposite points 3; 4. A measurement bridge 2 comprises equal resistances R5, R6, R7, R8 similar in value to those of bridge 1, separated by potentiometers 11, 12, 13 between resistances R5, R8, R5, R6, and R7, R8 respectively ; whose sliders constitute junction points 9, 7 and 8. Opposite points 7, 8 are connected across galvanometer G while opposite points 9, 10 are connected to the D.C. source, so that unbalance of bridge 1 due to variation of one or more resistance arms is reducible to null galvanometer deflection by adjustment of sliders 14, 15 of potentiometers 12, 13 to produce a cancelling unbalance of bridge 2; bridge 1 may incorporate one or more strain gauge elements and the sliders may be ganged and their adjustment calibrated to give a measure of the applied load.  The measurement bridge may comprise (Fig. 2) resistances 5, 6 separated by slidewire 12 shunted by low resistance S1 and series resistance R9 equal to the shunted slidewire resistance, while resistances R7, R8 are similarly separated by slidewire 13 shunted by S2 and in series with RIO; energization being supplied by D.C. source V in series with a current meter between the junction of R6 and R7 and the slider of a potentiometer 11 between R5 and R8. A galvanometer G is switchable between the ganged sliders of 12, 13 through sliprings 16, 17 and pigtails 18, 19 while a 4-pin socket 20 enables a resistance bridge 1 to be connected to the energization and to galvanometer G as above. This bridge may be constructed as a unitary assembly having a front control panel 22 (Fig. 3) carrying the shunt resistances S1, S2, the potentiometer 11 (not shown) and slidewires 12, 13 wound helically on a hollow former 21 axially normal to the panel and having a central hole into which is' threaded an axially movable spindle 23 splined internally to receive an operating spindle of a control knob on the outside of panel 22. Rotation thereof by main and geared vernier dials rotates crank member 24 having arm 26 carrying insulated sliders 14, 15 over the respective slidewires in a helical path due to the axial motion of the threaded spindle; the contacts 14, 15 being connected over insulated sliprings 27, 28 to external terminals for connection to a galvanometer and to opposite points of the external bridge. Rotation of the control knob enables any deflection of the galvanometer to be nullified, and the knob may be calibrated in terms of change in resistance of the external bridge circuit. The apparatus embodies compensation for variations of temperature and applied voltage due to the symmetry of the circuit and employment of closed bridges.