2dix-The Student Choice
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Measurement of the low resistance by using Kelvin Double bridge method.

 

Objective: Measurement of the low resistance by using Kelvin Double bridge method.

Apparatus:

Regulated dc supply, Standard resistance coil, Kelvin’s double bridge kit Digital multi-meter Patch codes.

Theory:

Kelvin’s bridge is a modification of whetstone’s bridge and always used in measurement of low resistance. It uses two sets of ratio arms and the four terminal resistances for the low resistance consider the ckt. As shown in fig. The first set of ratio P and Q. The second set of ratio arms are p and q is used to connected to galvanometer to a pt d at an Approx. potential between points m and n to eliminate the effects of connecting lead of resistance r between the known std. resistance ‘s’ and unknown resistance R.

The ratio P/Q is made equal to p/q under balanced condition there is no current flowing through galvanometer which means voltage drop between a and b, Eab equal to the voltage drop between a and c, Eamd.

Now                            Ead=P/P+Q;

Eab=I[R+S+[(p+q)r/p+q+r]]                                        -------------- (1)

Eamd= I[R+ p/p+q[ (p+q)r/p+q+r]]                               ---------------- (2)

For zero deflection->              Eac=Ead

[ P/P+Q]I[R+S+{(p+q)r/p+q+r}]=I[R+pr/p+q+r]                                                      ---- (3)

Now, if                                                P/Q=p/q

Then equation (3) becomes

                                       R=P/Q=S – (4)

For the Equation (3) is useful however as it shows the error that is introduced in case the ratios are not exactly equal. It indicates that it is desirable to keep r as small as possible in order to minimize the error in case there is a difference between the ratio P/Q and p/q.

R=P/QS Double Bridge

Circuit Diagram:-

Fig.1.1 Circuit Diagram of Kelvin double bridge method

Procedure: -

1) The circuit configuration on the panel is studied.

2) Supply is switched on and increased up to 5v.

3) The unknown resistance is connected as shown.

4) The value of P, Q was selected such that

 P/Q=p/q

5) S was adjusted for proper balance and balance value of s was balanced.

6) The value of known resistance was calculated.

Observation Table:- 

P(ratio arm resistance)

Q(ratio arm resistor)

Standard resistor

S

R measured value

R actual

 

 

 

 

 

 

 

 

 

 

Result- The observed value of unknown resistance.

Precautions:-

1) Check all the connections before turning ON the power supply.

2) Do not exceed the value of 5v.

3) Note the readings accurately.

Questions

  1. Write the applications of Kelvin double bridge method?
  2. What is principal of Wheatstone bridge?

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