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Expt. No. 4 Measurement of Resistance Using Kelvin'S Double Bridge Aim

The document describes an experiment to measure low resistances using a Kelvin double bridge. The Kelvin double bridge incorporates a second set of ratio arms to eliminate errors from lead resistance. It provides a more accurate measurement than a standard Wheatstone bridge. The procedure involves connecting an unknown resistance to the bridge, adjusting the ratio arms and standard resistance dial to find the null point, and calculating the unknown resistance value using the standard formula.

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2K views3 pages

Expt. No. 4 Measurement of Resistance Using Kelvin'S Double Bridge Aim

The document describes an experiment to measure low resistances using a Kelvin double bridge. The Kelvin double bridge incorporates a second set of ratio arms to eliminate errors from lead resistance. It provides a more accurate measurement than a standard Wheatstone bridge. The procedure involves connecting an unknown resistance to the bridge, adjusting the ratio arms and standard resistance dial to find the null point, and calculating the unknown resistance value using the standard formula.

Uploaded by

mamoon
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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EXPT. NO.

4 MEASUREMENT OF RESISTANCE USING KELVIN’S DOUBLE


BRIDGE

AIM:
To measure the given low resistance using Kelvin’s double bridge method.

OBJECTIVE:
To study the working of bridge under balanced and unbalanced condition and to study
the sensitivity of bridge.
APPARATUS REQUIRED:

NAME OF THE
SL.NO APPRATUS
1 Kelvin Double bridge kit
2 Unknown resistance
3 Multimeter
4 Connecting Wires

CIRCUIT DIAGRAM:
THEORY:
The kelvin double bridge is a modified version of Wheatstone bridge and provides greatly
increased accuracy in measurement of low resistance values

The kelvin double bridge incorporates the idea of a second set of ratio arms - hence the
name double bridge- and the use of four terminal resistors for the low resistance arms.
Fig.1. shows the schematic diagram of kelvin bridge. The first ratio arms is P and Q. The
second set of ratio arms p and q is used to connect the galvanometer to a point d at the
appropriate potential between points m and n to eliminate the effect of connecting lead
resistance r between the unknown resistance R and the standard resistance S.

The ratio p/q is made equal to P/Q. Under balance conditions there is no current through
the galvanometer which means that the voltage drop between a and b, Eab is equal to voltage
drops Eamd between a and c.

for zero galvanometer deflection, Eab=Eamd

now if

Eq (2) is the usual working equation for the kelvin bridge. It indicates that the resistance
of connecting lead, r, has no effect on the measurement, provided that the two sets of ratio
arms have equal ratios.

FORMULA USED:

Rx = (P/Q) S ohms
Where
P, Q  first set of ratio arms.
p, q  Second set of ratio arms.
S  Standard resistance,
Rx  unknown resistance.

PROCEDURE:

1. The resistance to be measured is connected such that the leads from +C and + P are
connected to one end and those from –C and –P are connected to the other end in the
kit.
2. The P/Q ratio (multiplier) is initially kept at position ‘1’ and the deflection of the
galvanometer is observed by pressing the galvanometer key.
3. The ‘S’ arm (main dial) is adjusted and two positions are identified for which the
deflection of the galvanometer is on either side of the null point. [If not some other P/Q
ratio is to be tried].
4. The lowest of the two position indicates the coarse value of the unknown resistance
and the null point is obtained by adjusting the Vernier scale, with the galvanometer
sensitivity knob at the maximum position.
5. The value of unknown resistance is read. [‘S’ Value]
6. Steps 3, 4, 5 are repeated for some other P/Q ratio for the unknown resistance. The
mean value is taken.
7. The above procedure is repeated with another sample.

TABULAR COLUMN:

SAMPLE P/Q RATIO S VALUE UNKNOWN


S.NO (Multiplier) COARSE() FINE ( ) RESISTANCE
Rx ()

.
CALCULATION:

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