Alejandro Portillo-Homework 4.5 Construction and Uses of the Wheatstone Bridge- Met

5/26/2018 Alejandro Portillo-Homework 4.5 Construction and Uses of the Wheatstone Bridge- Met

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Construction and uses of the Wheatstone bridge

Alejandro Portillo S.

Universidad Politecnica de Victoria

Ciudad Victoria, Mxico


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Abstract

The circuit known as a Wheatstone bridge is most commonly used to

determine the value of an unknown resistance to an electrical current. Firstly

described by British mathematician and scientist Samuel Hunter Christie in 1833,

the circuit came to bear the name of Sir Charles Wheatstone, the English

physicist who popularized it in the 1840s.

In a typical Wheatstone bridge, four resistors are positioned in a circuit

designed in such a way that the current from a battery splits, flows through the

sequence of resistors, then recombines into a single conductor. Three of these

resistors have known values, one of which is variable, or adjustable. The value of

the fourth resistor is not known. By studying and manipulating the paths the

current can take through the Wheatstone bridge grid, that fourth, unknown

resistance can be identified and with this other magnitudes can be found.

Keywords: Wheatstone, resistance, circuit


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Introduction

The Wheatstone bridge was named in honor of the English physicist Sir

Charles Wheatstone (1802-1875), who was associated with Michael faraday and

was professor at the kings college of London. The bridge circuit is widely used to

make accurate measurements of resistance. This characteristic allows using the

bridge circuit in science and industry, as a method to transform temperature,

deformation, distortion, sound, light and other physical effects into an electric

signal, for its accurate measurement.


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Development

The Wheatstone bridge circuit

Even though the potentiometer circuit has several benefits, the drawback of

having a large common-mode voltage on the top of a small signal voltage makes

its use impractical for static strain measurements, except in rare cases when used

with semiconductor gages. Even then, separating from is troublesome.

By far the most common electrical circuit used with strain gages, and forming the

basis of strain- gage instrumentation, is the Wheatstone bridge.

Samuel hunter Christie invented what was called the diamond method in

1833 and used the technique to compare resistances of wires with different

thicknesses. The method went mostly unrecognized until 1843 when Charles

Wheatstone proposed it in a paper to the royal society, for measuring resistance

in electrical circuits.

Measurements of voltages and resistances

The Wheatstone bridge is an arrangement of four resistances used for

measuring one of them in terms of the other three. Fig. 5.78 represents

Wheatstonesbridge circuit where P, Q, R and S are connected to form a mesh. A

battery of emf E is connected between the junctions A and C through a key KB

called the battery key. A galvanometer of resistance G is connected between theresistances B and D through a key KG called galvanometer key. It is always the

battery key which is closed first and then the galvanometer key.

The currents through the various branches are indicated in fig. 5.78. In order

to reduce the number of unknowns at the outset, Kirchhoffs first rule is used at

every junction.

When current through the galvanometer is zero, the potential difference

between B and D is zero.


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The resistances P, Q, R and S are so adjusted that the galvanometer gives

zero deflection. This would be possible if B and D are at the same potential and

therefore, no current would flow through the galvanometer i.e., Ig=0. In this

situation the Wheatstone bridge is said to be balanced.

Note: the Wheatstonesbridge method is unsuitable for the measurement of very

low and very high resistances.

Resistance of balanced Wheatstone bridge between A and C

(P + Q) (R + S)

P + Q + R +S


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Application of the Wheatstone bridge for temperature measurement

The Wheatstone bridge has been widely used for a long time with manualadjustment for the zero, or null, galvanometer reading. Now-a-days, it is

increasingly used with electronic and mechanicals arrangements to make it self-

balancing. The measurements are recorded on a paper chart. In this form, the

Wheatstone bridge is the basis for many measurement and control devices used

in industry. Any quantity such as temperature, humidity, strain, displacement,

liquid level in a tank etc. which can be made to produce a change in the value of

a resistance can be measured with a Wheatstone bridge.

Experimental arrangement for measurement of temperature and temperature

coefficient of resistance is shown on fig. 5.79.

We know that the resistance of a wire varies with temperature. A coil of fine

copper wire is put into a water bath. A Wheatstone bridge is used to measure its

resistance at various moderate temperatures. By plotting a graph between

temperature and resistance, we can also find temperature coefficient of

resistance.


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Transforming pressure waves into electrical signals: the electrical

strain gauge

Pressure measurements systems today generally use electrical strain gauges

based on the principle of the Wheatstone bridge. In its simple form, the strain

gauge is variable resistance transducer whose operation depends on the fact that

when an electrical wire is stretched, its resistance to the flow of current increases.

As long as the strain remains well below the elastic limit of the wire, there is a

wide range within which the increase in resistance is accurately proportional to

the increase in length.

Figure 7.7 illustrates how the Wheatstone bridge uses the principle in

converting a pressure signal to an electrical signal. In this schematicrepresentation of a pressure transducer, pressure is transmitted through port P

and acts on diaphragm D, which is vented to atmospheric pressure on its opposite

side.


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Applications of a Wheatstone bridge

The Wheatstone bridge is basically a D.C. bridge and used to measure the

resistances in the range 1 to low mega ohm.

It is used to measure the D.C. resistance of various types of wires for the

purpose of quality control of wire.

It is used to measure the resistance of motor winding, relay coils etc.

It is used by the telephone companies to locate the cable faults. The faults

may be of the type line to the line short or line to ground short


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Bibliographic references

Narinder Kumar 2004 ComprehensivePhysics XII

Laxmi Publications

Pages 398-399

Donald S. Baim 26/10/2005 GrossmansCardiac Catheterization, Angiography,and Intervention Volume 1

Lippincott Williams & WilkinsPages 138-139-140

U.A.Bakshi,A.V.Bakshi , 01/01/2009 Instrumentation Engineering

Technical Publications

Pages 2-17, 2-18

William N. Sharpe, Jr. 04/12/2008 SpringerHandbook of Experimental SolidMechanics

SpringerPages 287
http://www.google.com.mx/search?hl=es&tbo=p&tbm=bks&q=inauthor:%22U.A.Bakshi%22http://www.google.com.mx/search?hl=es&tbo=p&tbm=bks&q=inauthor:%22A.V.Bakshi%22http://www.google.com.mx/search?hl=es&tbo=p&tbm=bks&q=inauthor:%22A.V.Bakshi%22http://www.google.com.mx/search?hl=es&tbo=p&tbm=bks&q=inauthor:%22U.A.Bakshi%22


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Note about the author

Alejandro Portillo Soto

Mechatronics student at the Universidad Politecnica de Victoria (UPA)

Email: [email protected]

Alejandro Portillo-Homework 4.5 Construction and Uses of the Wheatstone Bridge- Met

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