1961-02-08

Abstract of GB860755 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.

2

1961-02-08

G01R17/10 ; G01R17/00

G01R17/10B

GB19560032527 19561025

GB19560032527 19561025

1	Improvements in or relating to apparatus for measuring changes in electrical resistance
	Inventor: PARRAMORE THOMAS STUART	Applicant: NAT RES DEV
	EC:G01R17/10B	IPC: G01R17/10;G01R17/00
	Publication info: GB860755 A - 1961-02-08

**WARNING** start of CLMS field may overlap end of DESC **.

comparative method, does not rely upon absolute measurements of voltage and current and, moreover, by employing a bridge method of measurement it is independentof varied tions in applied voltage. With thesymrnetry of the circuit and the employment of closed bridges, a high degree of temperature cosensation is achieved whatever the physical distancebetween the bridges and whatever the relative temperatures of the transducer and the measuring instrument might be. In consequence drift in resistance change indications due to temperature variations is reduced to a minimum.

WHATWE CLAIM IS:-

1. Apparatus for measuring electrical resistance changes which apparatus includes a primary electrical resistance bridge arranged to incorporate the circuit whose resistance change is to be measured; a measuring electrical resistance bridge whose resistance arms are adjustable by means of twin ganged slidewires connected, one at each of two opposite apices, in series with the adjacent resistance arms and means for connecting thebridg s, eachthrough one pair of opposite apices, in parallelwith a voltage source and, each through its other pair of opposite apices, in parallelwith a null balance indicator; whereby a measured degree of out of balance can be applied to the measuring bridge, through the slide wires, to nullify any current whichflows through the indicator due to resistance changes in the primary bridge circuit.

2. Apparatus for measuring changes in electrical resistance substantially as herein describedvvith reference to the accompanyingdrawings.

PROVISIONAL SPECIFICATION Improverents iz ior aetnng to Apparatus and Method for

Measurraig Charies inEllectrical Resis.anXce

We, NATIONALRESEARCH DEVELOPMENTConPoRAaoN, a British Corporation established by Statute, of 1,Tilney Street, London,W.1, do hereby declare this invention to bedescribed in the followingstatement : -

This invention relates to methods of measuring changes in electrical resistance and to apparatus for measuring such changes to a high degree of accuracy.

An object of the invention is to provide a method of measuring changes in resistance of an electrical circuit which isunaffected by variations or fluctuations of the supply voltage applied to the circuit. It is a further object of the invention to provide an apparatus for malting such measurements.

According to the invention there is provided an apparatus for measuring changes in the electrical resistance of a circuit, including a primary balanced electrical resistance bridge having two of its opposite apices connections firstly in parallel with two opposite apices connections of an electrical resistance bridge with corresponding resisance values which can be adjusted (herein referred to as a measuring bridge) and secondly in parallel with a voltage source, the other two apices connections of the bridges being in parallel with a single galvanometer. Further, the invention provides a method of employing this apparatus to effect ameasurement of change of resistance occurring in the circuit such as results in an out of balance of the primary electrical resistance bridgewhich comprises effecting a controlled degree of out of balance of the measuring bridge until a null deflection of the galvanometercommon to both bridges is obtained, the degree of out of balance imparted to the measuring bridge being employed asmeasure of resistance change in the circuit.

A preferredmethod of effecting a controlled degree of out of balance to the measuring bridge is toprovide slidewire devices in serieswith the resistance arms at twoopposite apices, any out ofbalance current due to change in resistance of the other bridgebeing applied through sliders engaging the slidewire devices. Then, when an out of balance current flows, the sliders are moved so as to vary theeffective resistance values of the arms of themeasuring bridge until a null deflection of the galvanometer isobtained. The movement of the sliders is proportional to the out of balance current and to the resistance change. Hence, a measure of resistance change may be obtained.

The primary electrical resistance bridge may form part of a transducer and the invention may accordingly be considered as an arrangement for measuring changes in resistance in a transducer by means of a double D.C.bridge circuit having two closed bridges whereof one bridge is included in the transducer and a second bridge incorporates four standard resistance arms and means such as twin ganged slide wire devices foreffecting a controlled degree of out of balance of these resistance arms.

Thearrangement is such that the balance of the second bridge may be varied in proportion to any resistance change in the

transducer to give a null deflection on a galvanometer connected in parallel with both closed bridges.

In practice it is convenient to provide the resistance change measuring bridge circuit as a unitary assembly comprising the measuring bridge, slide wire devices and certain other associated equipment, within a casing with external terminals arranged for ready connection to a galvanometer, the primary bridge circuit, in which a resistance change is to be measured and a source of potential.

One form of such unitary assembly, described here by way of example, is provided by an outerboxAike casing providing on one wall thereof a control panel which presents, on its outer face, suitable controls as will be described below. Within the casing are mounted the measuring bridge resistances, the arms of which are each of substantially equal value and each of a similar value tothcse resistances in the externally locatedprimary bridge in which the resistance change is to occur. Two opposite apices of the measuring bridge are connected bytwo conductors across externally mounted terminals for connection to a source of potential, one of these conductors being connected in series with a potentiometer at oneaPeX and the other with a millammeter for convenience in checking battery current. The potentiometer is controlled at the control panel for zero adjustment of the galvanometer and allows for initial slight out of balance in the primary bridge circuit or transducer. In addition, a parallel connection is made between the twoaforementioned terminals and further terminals for connection across a pair of apices of the primary bridge.

In series with the resistances of the measuring bridge at the pair of apices thereof which are not adapted for connection to the source of potential, shunted slide wires are connected. These slide wires are wound helically on a hollow former fixed within the casing with its axis normal to the control panel. An inner portion of the hollow former is formed to provide an internally threaded hole extending coaxially of the former and this is engaged by a complimentarily threaded axially moveable spindle which extends within the hollow former. At its end adjacent the control panel, the axially moveable spindle is formed with an axial, internal splineway or recess into which is fitted an external spline on an operating spindle. The latter is coupled through a suitable insulated coupling to a control knob mounted on the outer face of the control panel such that rotation of the control knob effects axial displacement and rotation of the spindle relative to the fixed former by virtue of its screw threaded engagement therewith. Due to the splined coupling between the axially moveable spindle and the operating spindle the control knob merely rotates as this axial displacement takes place. A main dial is mounted adjacent the inside face of the control panel on the operating spindle so as to give an indication of the degree of rotation performed by the control knob. A subsidiary dial divided into ten divisions is geared to the operating spindle to indicate complete revolutions of the main dial. Both these dials are visible to the operator through a window in the control panel.

The axially moveable spindle terminates within the casing a short distance beyond the end of the hollow former and carries at, or near, such end an arm which has a part extending radially of the former and a cranked part, cranked at90 , extending parallel to, but spaced above, the surface presented by the helically wound slide wires on the former. A shoulder is formed on the spindle a short distance from the end thereof which carries the aforementioned arm and between this shoulder and the arm is mounted a slip ring assembly. This assembly comprises two mutually insulated slip rings which are mounted on a short sleeve of insulating material such as that material marketed under the Registered Trade Mark "Tufnol". This sleeve extends round the spindle and is attached to a steady plate through which plate the spindle extends.

The steady plate provides a mounting for a number of spaced pins which project therefrom parallel with the spindle axis and slidably engage suitably positioned holes in the frame of the former. The steady plate and slip ring assembly are thus restrained from rotation. The outer peripheries of the slip rings are engaged by wiper arms which are secured to the radially extending part of the arm. Electrical connection is made between each wiper arm and each of the two slider contacts which are fixed at appropriately spaced positions along the cranked part of the arm.

The slider contacts engage the surface presented by the helically wound slide wires.

Thus, as the control knob is rotated, an axial and rotational motion is imparted to the spindle thereby to impart through the cranked arm a similar motion to the slider contacts. The contacts are connected through the slip ring assembly to a pair of external terminals for connection to one pair of opposite apices of the external bridge. Also, electrical connection is made between the slip rings and externally mounted terminals for connection to a galvanometer.

In operation, the appropriate connections are made between the external terminals on the casing and the external primary bridge circuit in which a resistance change is to be measured. In addition, a source of potential is connected to the unit. Initially, both the external bridge circuit and the measuring bridge are in balance and no deflection of the galvanometer occurs. When a change in resistance in the external bridge circuit occurs a current due to the consequent out of balance of external bridge flows through the galvanometer. The resultant deflection is observed and at the control panel the operator rotates the control knob to move the two sliders over the helical slide wires so as to unbalance the measuring bridge to such an extent as wilI counteract the out of balance current in thegalvanometer, that is, until a null deflection is obtained. The amount of rotation that is required to be given to the control knob is indicated and is thus proportional to the change in resistance in the external bridge circuit.

In modification the control knob incorporates a slow motion drive by means of a knob concentric with the control knob. The concentric knob is geared down 5:1 to facilitate fine adjustment. A vernier scale may be added to the dial.

The invention is applicable to any circuit wherein a resistance change is to be measured provided that circuit is connectable as one armof, or actually forms, an electrical resistance bridge. For example, in measuring the response from a transducer adapted to serve as a load cell having electrical resistance strain gauges where change in resistance of one or more of these gauges connected into a primary bridge circuit may be used to give an indication of load on a member to which the strain gauge or load cell is applied. If the strain gauge bridge is balanced whilst the member is unloaded the change in resistance due to an applied load can be measured and employed to give a measure of that load with a high degree of accuracy.

- Such an arrangement enabling as it does change in resistance to be measured by a comparative method, does not rely upon absolute measurements of voltage and current and, moreover, by employing a bridge method of measurement it is independent of variations in applied voltage. With the symmetry of the circuit and its employment of closed bridges, a high degree of temperature copensation is achieved whatever the physical distance between them and whatever the relative temperatures of the transducer and the measuring instrument might be. In consequence drift in resistance change indications due to temperature variations is reduced to a minimum.

COMPLETE SPECIFICATION Inaprovemeni:s fln orreiatrng toApparatus foriAeasurng

Changes inEltectrical Resistance

We, NATIONAL RESEARCH DEVELOPMENTCOP\PORATION, a British Corporation established by Statute, of 1, Tilney Street, London,

W.1, do hereby declare the invention, forwhich we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the followingstatement:

This invention relates to apparatus for measuring changes in electrical resistance to a high degree of accuracy.

An object of the invention is to provide an apparatus for measuring changes in resistance of an electrical circuit, measurements obtained by which apparatus areunaffected by variations orfluctuations in the supply voltage applied to the circuit.

In accordance with the invention there is provided an apparatus for measuring electrical resistance changes which apparatus includes a primary electrical resistance bridge arranged to incorporate the circuit whose resistance change is to be measured; a measuring electrical resistance bridge whose resistance arms are adjustable by means of twin ganged slide wires connected, one at each of two opposite apices, in series with the adjacent resistance arms and means for connecting the bridges, each through one pair of opposite apices, in parallel with a voltage source and, eachthrough its other pair of opposite apices, in parallel with a null balance indicator; whereby a measured degree of out of balance can be applied to the measuring bridge, through the slide wires, to nullify any current which flows through the indicator due to resistancechanges in the primary bridge circuit.

Preferably both primary and measuring bridges are individually balanced and in order to obtain maximum sensitivity all eight arms in the two bridges should have similar resistance values. With this arrangement the galvanometer will initially show a null deflection and when a current flows through the galvanometer due to resistance changes in the primary bridge the resistance armsd the measuring bridge can be adjusted until a null deflection of the galvanometer is obtained.

A preferred method of producing a cotrilled degree of out of balance in the measuring bridge is to provide ganged slide wire devices in series with its resistance arms at two opposite apices, any current due to resistance changes in the primary bridge being applied through sliders engaging the slide wires. When a current flows in the galvanometer the sliders may be moved so as to vary the effective resistances of the arms of the measuring bridge until the galvanometer deflection is reduced to zero.

The movement of the sliders is a function of the current and of the resistance change in the primary bridge and hence maybe used as a measure or that resistance change.

The primary electrical resistance bridge may incorporate or form part of a transducer and the invention may accordingly be considered as anarrangement for measuring changes in resistance in a transducer by means of a double D.C. bridge circuit having two closed bridges whereof one bridge is included in the transducer and a second bridge incorporates four standard resistance arms and means such as twin ganged slide wire devices for effecting a controlled degree0i out of balance of these resistance arms.

The arrangement is such that the balance of the second bridge may be varied in pro-portion to any resistance change in the transducer to give a null deflection on a galvanometer connected in parallel with both closed bridges.

An apparatus in accordance with the invention will now be more particularly described, by way of example only with reference to the accompanying drawings in which:

Figure 1 is a circuit diagram illustrating the essential features of the apparatus,

Figure 2 is a more detailed circuit diagram of a measuring bridge, and

Figure 3 is a view of a convenient unitary assembly showing the construction of the slide wire device.

The circuit as shown in Figure 1 comprises a primary electrical resistance bridge 1, a measuring bridge 2, a galvanometer G connected in parallel with both bridges through apices 3, 4, 7 and 8, and a voltage source V also connected in parallel with both bridges through apices 5, 6, 9 and 10.

The arms of the primary bridge 1 are formed by resistancesRi, R, R, andR4 which include the parts of the circuit in which resistance changes are to be measured.

The arms of the measuring bridge include resistancesR,, Rss, R7 andR8. Preferably each resistance bridge is substantially balanced and the eight resistances forning their arms are all of substantially equal value. A potentiometer 11 is connected in series with resistancesR, andRi across the apex 9 of the measuring bridge 2 in order to adjust the zero reading of the galvanometer when any slight initial out of balance is present in the circuit. Slide wires 12, 13 are connected across apices 7, 8 of the measuring bridge 2 in series with the adjacent resistance arms, connections to the galvanometer being made through sliders 14, 15. When changes occur in the resistances of the arms of the primary bridge 1 which produce an out of balance current through the galvanometer, the sliders 14, 15 can be adjusted until a null deflection of the galvanometer is obtained and the amount of adjustment required serves as a measure of the resistance changes in the primary bridge.

In the more detailed circuit of the measuring bridge asshove in Figure 2 resistancesR,, R,, R7) Ri constitute the major portion of the resistancesof its arms. Across one apex of the bridge in series with resistancesR5, R6 are connected a slide wire 12 shunted by a low resistanceS" and a resistanceRi whose value is equal to that of the shunted slide wire. This arrangement locates the zero point at one end of the slide wire for purposes in which the direction of the out of balance which will occur in the primary bridge isknown and enables the full length of the slide wire to be used for measurement. A second slide wire 13 shunted by resistanceS2 is similarly connected in series with resistanceR10 across the opposite apex of the bridge. Sliders 14, 15 respectively in contact with slide wires 12, 13 are connected through slipring devices 16, 17 and pigtails 18, 19 to the terminals of a galvanometer G and to two terminals of a four pin socket 20 into which can be plugged the leads from the primary bridge. A potentiometer 11 is connected across one of the remaining apices of the bridge in series with resistances R,,Ri and its slider and the fourth apex of the bridge are connected to the terminals of a voltage source V and to the other terminals of the socket 20.A milliameter mA is introduced into the battery circuit to check the current.

In practice it is convenient to provide the resistance change measuring bridge circuit as a unitary assembly comprising the measuring bridge, slide wire devices and certain other associated equipment, within a casing with external terminals arranged for ready connection to a galvanometer, to the primary bridge circuit in which a resistance change is to be measured and to a source of potential.

One form of such unitary assembly, described here by way of example, is provided by an outerbox-like casing providing on one wall thereof a control panel which prsent, on its outer face, suitable controls as will be described below. Within the casing are mounted the measuring bridge resistances, the arms of which are each of substantially equal value and each of a similar value to those resistances in the externally located primary bridge in which the resistance change is to occur. Two opposite apices of the measuring bridge are connected by two conductors across externally mounted terminals for connections to a source of potential, one of these conductors being connected in series with a potentiometer at one apex and the other with a milliameter for convenience in checking battery current. The potentiometer is controlled at the control panel for zero adjustment of the galvanometer and allows for initial slight out of balance in the primary bridge circuit or transducer. In addition, a parallel connection is made between the twoaforementioned terminals and further terminals for connection across a pair of opposite apices of the primary bridge.

In series with the resistances of the measuring bridge at the pair of apices thereof which are not adapted for connetion to the source of potential, shunted slide wires are connected.- As shown in Figure 3, these slide wires 12, 13, which are shunted by resistancesSi, i, are wound helically an a hollow former 21 fixed within thecasing with its axis normal to the control panel 22. An inner portion of the hollow former 21 is formed to provide an internally threaded holeextending coaxially of the former and this is engaged by a complimentarily threaded axially moveable spindle 23 which extends within the hollow former 21. At its end adjacent the control panel 22, the axially moveable spindle 23 is formed with an axial, internal spineway or recess into which is fitted an external spline on an operating spindle. The latter is coupled through a suitable insulated coupling to a control knob mounted on the outer face of the control panel 22 so that rotation of the control knob effects axial displacement and rotation of the spindle 23 relative to the fixed former 21 by virtue of its screw threaded engagement therewith. Due to the splined coupling between the axially moveable spindle and the operating spindle the control knob merely rotates as this axial displacementtakes place. A main dial is mounted adjacent the inside face of the control panel on the operating spindle so as to give an indicationd the degree of rotation performed by the control knob. A subsidiary dial divided into ten divisions is geared to the operating spindle to indicate complete revolutions of the main dial. Both these dials are visible to the operator through a window in thecontrol - panel.

The axially moveable spindle 23 terminates within the casing a short distance beyond the end of the hollow former 21 and carries at, or near, such end an arm 24 which has a part 25 extending radially of the former and a cranked part 26, cranked at90 , extending parallel to, but spaced above, the surface presented by the helically wound slide wires 12, 13 on the former 21. A shoulder is formed on the spindle 23 a short distance from the end thereof which carries the aforementioned arm 24 and between this shoulder and the arm is mounted a slip-ring assembly. This assembly comprises two mutually insulated slip-rings 27, 28 which are mounted on a short sleeve of insulating material. This sleeve extends round the spindle 23 and is attached to a steady plate 29 through which plate the spindle 23 extends. The steady plate 29 provides a mounting for a number of spaced pins 30 which project therefrom parallel with the spindle axis and slidably engage suitably positioned holes 31, in a disc 31a, serving as part of the former. The steady plate and slip-ring assembly are thus restrained from rotation. The outer peripheries of the slip-rings 27, 28 are engaged by wiper arms 32, 33 which are secured to the radially extending part 25 of the arm 24 and are insulated from the arm 24 and from each other. The wiper arms 32, 33 are respectively connected to two slider contacts 14, 15 which are fixed at appropriately spaced positions along the cranked part 26 of the arm 24 and which are similarly insulated from the arm 24 and from each other. The slider contacts 14, 15 engage the surfaces presented by the helically wound slide wires 12, 13.

Thus, as the control knob, is rotated, an axial and rotational motion is imparted to the spindle 23, thereby to impart through the cranked arm 26 a similar motion to the slider contacts 14, 15. The contacts are connected through the slip-ring assembly to a pair of external terminals for connection to one pair of opposite apices of the external bridge. Also, electrical connection is made between the slip-rings and externally mounted terminals for connection to a galvanometer.

In operation, the appropriate connections are made between the external terminals on the casing and the external primary bridge circuit in which a resistance change is to be measured. In addition, a source of potential is connected to the unit. Initially, both the external bridge circuit and the measuring bridge are in balance and no deflection of the galvanometer occurs. When a change in resistance in the external bridge circuit occurs a current due to the consequent out of balance of external bridge flows through the galvanometer. The resultant deflection is observed and at the control panel the control knob is rotated to move the two sliders over the helical slide wires so as to unbalance the measuring bridge to such an extent as will counteract the out of balance of the primary bridge, that is, until a null deflection of the galvanometer is obtained. The amount of rotation that is required to be given to the control knob is indicated and is a function of the change in resistance in the external bridge circuit.

In a modification the controlknob incorporates a slow motion drive by means of a knob concentric with the control knob.

The concentric knob is geared down 5:1 to facilitateflue adjustment. A vernier scale may be added to the dial.

The invention is applicable to any circuit wherein a resistance change is to be measured provided that circuit is connectable as the whole or part of an electrical resistance bridge. For example, it may be used to measure the response from a transducer adapted to serve as a load cell havingelectti- cal resistance strain gauges connected into a primary bridge circuit, where change in resistance of one or more of these gauges is used to give an indication of load on a member to which the strain gauge or load cell is applied. If the strain gauge bridge is balanced whilst the member is unloaded the indicator dial of the measuring bridge can be calibrated to give a measure of that load with a high degree of accuracy.

Such an arrangement, enabling as it does, change in resistance to be measured by a comparative method, does not rely upon absolute measurements of voltage and current and, moreover, by employing a bridge method of measurement it is independentof varied tions in applied voltage. With thesymrnetry of the circuit and the employment of closed bridges, a high degree of temperature cosensation is achieved whatever the physical distancebetween the bridges and whatever the relative temperatures of the transducer and the measuring instrument might be. In consequence drift in resistance change indications due to temperature variations is reduced to a minimum.

WHATWE CLAIM IS:-

1. Apparatus for measuring electrical resistance changes which apparatus includes a primary electrical resistance bridge arranged to incorporate the circuit whose resistance change is to be measured; a measuring electrical resistance bridge whose resistance arms are adjustable by means of twin ganged slidewires connected, one at each of two opposite apices, in series with the adjacent resistance arms and means for connecting thebridg s, eachthrough one pair of opposite apices, in parallelwith a voltage source and, each through its other pair of opposite apices, in parallelwith a null balance indicator; whereby a measured degree of out of balance can be applied to the measuring bridge, through the slide wires, to nullify any current whichflows through the indicator due to resistance changes in the primary bridge circuit.

2. Apparatus for measuring changes in electrical resistance substantially as herein describedvvith reference to the accompanyingdrawings.

PROVISIONAL SPECIFICATION Improverents iz ior aetnng to Apparatus and Method for

Measurraig Charies inEllectrical Resis.anXce

We, NATIONALRESEARCH DEVELOPMENTConPoRAaoN, a British Corporation established by Statute, of 1,Tilney Street, London,W.1, do hereby declare this invention to bedescribed in the followingstatement : -

This invention relates to methods of measuring changes in electrical resistance and to apparatus for measuring such changes to a high degree of accuracy.

An object of the invention is to provide a method of measuring changes in resistance of an electrical circuit which isunaffected by variations or fluctuations of the supply voltage applied to the circuit. It is a further object of the invention to provide an apparatus for malting such measurements.

According to the invention there is provided an apparatus for measuring changes in the electrical resistance of a circuit, including a primary balanced electrical resistance bridge having two of its opposite apices connections firstly in parallel with two opposite apices connections of an electrical resistance bridge with corresponding resisance values which can be adjusted (herein referred to as a measuring bridge) and secondly in parallel with a voltage source, the other two apices connections of the bridges being in parallel with a single galvanometer. Further, the invention provides a method of employing this apparatus to effect ameasurement of change of resistance occurring in the circuit such as results in an out of balance of the primary electrical resistance bridgewhich comprises effecting a controlled degree of out of balance of the measuring bridge until a null deflection of the galvanometercommon to both bridges is obtained, the degree of out of balance imparted to the measuring bridge being employed asmeasure of resistance change in the circuit.

A preferredmethod of effecting a controlled degree of out of balance to the measuring bridge is toprovide slidewire devices in serieswith the resistance arms at twoopposite apices, any out ofbalance current due to change in resistance of the other bridgebeing applied through sliders engaging the slidewire devices. Then, when an out of balance current flows, the sliders are moved so as to vary theeffective resistance values of the arms of themeasuring bridge until a null deflection of the galvanometer isobtained. The movement of the sliders is proportional to the out of balance current and to the resistance change. Hence, a measure of resistance change may be obtained.

The primary electrical resistance bridge may form part of a transducer and the invention may accordingly be considered as an arrangement for measuring changes in resistance in a transducer by means of a double D.C.bridge circuit having two closed bridges whereof one bridge is included in the transducer and a second bridge incorporates four standard resistance arms and means such as twin ganged slide wire devices foreffecting a controlled degree of out of balance of these resistance arms.

Thearrangement is such that the balance of the second bridge may be varied in proportion to any resistance change in the

**WARNING** end of DESC field may overlap start of CLMS **.