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Measurement of unknown resistance using Carey-Foster’s Bridge
06-05-2011, 02:29 PM
Post: #1
Measurement of unknown resistance using Carey-Foster’s Bridge
1 Aim of experiment
We are going to determine the value of an unknown resistance using a meterbridge
and then make furthur accurate measurements using Carey Foster
bridge to take into account the specific resistivity () of the bridge wire.
Finally measuring the radius and length of the unknown resistance we can
determine the specific resistivity of the material given.
2 Apparatus required
a)An unknown resistance
b)A known resistance(variable)
c)A galvanometer
d)A very high resistance of the order of 15000
to protect the galvanometer.
e)A dry cell
f)A plug key
g)A meter bridge with jockey
h)A sorting plate
i)A copper strip
3 Theory of Experiment
Brief discussion and relevant formulae:

The Meter Bridge is a device for measurement of resistance
using the principle of Wheatstone Network.AB is a uniform wire 100 cm(1m)
long, stretched in two segments of 50 cms.AA0,BB0,CC0,DE are thick metal
strips which have zero resiatance A0D and BE are gaps in which resistances
to be compared are connected. The dry cell is connected through a plug
key between A0 and B0. A pivoted galvanometer is connected between the
central terminal (F) and sliding contact J through a high resiatance. With
a standard resiatance in R the position of jockey is found for which the
current in the galvanometer is zero. Then,
= ResistanceofAJ
= l
100 − l
Using the bridge one can calculate the resistance of the wire from which the
specific resistance  of the material of wire can be obtained.
The Carey-Foster Bridge is a modified form of meter bridge. The shorting
plate DE is removed so that we get four gaps AD,DC,CE,EB in which
resistances can be connected. Two equal resistances P and Q can be connected
in gaps DC and CE suppose an unknown resistance and a known
resistance are connected in gaps AD and EB. Let the balane point be founf
at J(AJ = l)

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