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


Aim: - Measurement of the low resistance by using Kelvin Double bridge method.
Apparatus: - Regulated dc supply-1no
Standard resistance coil-1no
Kelvin’s double bridge kit.
Digital multimeter-1no,
Patch codes.
Circuit Diagram: -
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 equa… (3) becomes
R=P/Q=S -
(4)
Equation (4) is the usual working equation. For the Kelvin’s Double Bridge .It indicates
the resistance of connecting lead r. It has no effect on measurement provided that the two
sets of ratio arms have equal ratios. 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 diff. between
the ratio P/Q and p/q.
R=P/QS
Observation Table: -
P (ratio arm
resistor)
Q (ratio arm
resistor)
Standard resistor
S
R measured
value
R actual
Procedure: -
1) The circuit configuration on the panel is studied.
2) Supply is switched on and increased upto 5v.
3) The unknown resistance is connected as shown .
4) The value of P,Q was selected such that
a. 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.
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.
Result- The observed value of unknown resistance is
Viva Questions:1)Why this method is called as double bridge method?
2)Can this method be beneficial for measurement of low value of
resistance or not?

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