Wheat Stone Bridge Report

ELECTRICAL TECHNOLOGY ELEC 1033  ______________________________________________________________________________________ 

AIM The objective of the experiment was to learn and apply the basic theory of the dc resistances bridge and also to assess the advantages and disadvantages of this method of  measuring resistance.

EXPERIMENT The terminals on the module were connected as shown below:

Figure 1

The value of R S was measured for different values of ratios R 1 : R 2 when there is null deflection on the galvanometer.

 ____________________________________________________________________________________ 1 GROUPE B2

ELECTRICAL TECHNOLOGY ELEC 1033  ______________________________________________________________________________________ 

TABLE OF RESULTS

R 1 : R 2

R S / Ω

1:1

102

1:10

10

1:100

1

100:1

10092

10:1

1011

The value of R X1 can be calculated by using the formulae: R 1R X1 = R SR 2

R 1 : R 2

R S / Ω

R X1 / Ω

1:1

102

102.00

1:10

10

100.00

1:100

1

100.00

100:1

10092

100.92

10:1

1011

100.10

The exact value of R X1 was measured using an ohm meter. Its value was 101.3 Ω.

INTERCHANGING THE CIRCUIT  ____________________________________________________________________________________ 2 GROUPE B2

ELECTRICAL TECHNOLOGY ELEC 1033  ______________________________________________________________________________________ 

The circuit was re-arranged in such a way that the galvanometer is connected across A and B and the power supply across C and D. This is shown by the circuit below:

Figure 2

The experiment was repeated to measure the value of R S. The same values as the first experiment were obtained. Thus, even the circuit is interchanged the same results are obtained.

BRIDGE EQUATION  ____________________________________________________________________________________ 3 GROUPE B2

ELECTRICAL TECHNOLOGY ELEC 1033  ______________________________________________________________________________________ 

Considering Figure 1.

Suppose the current entering the network at A divides up into I1 through R 1 and I2 through R 2. If no current flows through the galvanometer, then the current through R S and R X1 must be I1 and I2 respectively. Also, since no current flows through the galvanometer, the   potential of C and D must be equal. Hence, for a balance, or null deflection of the galvanometer: Potential difference across R 1 = Potential difference across R 2 Potential difference across R S = Potential difference across R X1

Therefore, I1R 1 = I2R 2

I1R S = I2R X1 Dividing, I1R 1 RESULT: R 1

/ I1R S = I2R 2 / I2R X1

/ R S= R 2 / R X1

The bridge equation for figure 1 is given by:

R 1 / R 2 = R S / R X1

Deriving the bridge equation when the circuit is interchanged.  ____________________________________________________________________________________ 4 GROUPE B2

ELECTRICAL TECHNOLOGY ELEC 1033  ______________________________________________________________________________________ 

Considering figure 2.

When the circuit is interchanged current enters at C. Let the current through R 1 be I 1 and the current through R S be I2. Therefore, the current through R 2 and R X1 are I1 and I2 respectively. For null deflection of the galvanometer: Potential difference across R 1 = Potential difference across R S Potential difference across R 2 = Potential difference across R X1

R 1I1 = R SI2 R 2I1 = R X1I2 Dividing, R 1I1

/ R 2I1 = R SI2 / R X1I2

Therefore, the bridge equation for figure 2 is given by:

R 1 / R 2 = R S / R X1

Thus, it can be seen that the same equation is obtained when the position of the supply (E) and the balance indicator (G) are interchanged. This is why the same value of R S was measured in both experiments.

ADVANTAGES OF USING BRIDGE METHOD  ____________________________________________________________________________________ 5 GROUPE B2

ELECTRICAL TECHNOLOGY ELEC 1033  ______________________________________________________________________________________ 

Due to their outstanding sensitivity, Wheatstone bridge circuits are very advantageous for  the measurement of resistance. The Wheatstone bridge is well suited also for the measurement of small changes of a resistance and, therefore, is also suitable to measure the resistance change in a strain gauge. It is commonly known that the strain gauge transforms strain applied to it into a proportional change of resistance. It is widely used across industry even today. The Wheatstone bridge is used to measure electrical resistance with a very high   precision- precision is much higher than attainable with voltmeters, ammeters and ohmmeters. One application of the precision resistance measurement is an electronic thermometer which makes use of resistance variation with temperature. In what way does the variable ratio-arm improve the usefulness of the bridge?

The experiment can be repeated for different values of R 1 and R 2 without disconnecting the circuit. Thus, different values of R S will be obtained and the values of R X1 can be calculated. A more accurate value of R X1 will be obtained by calculating and an average of all the values.

 ____________________________________________________________________________________ 6 GROUPE B2