Theory: In
the arrangement as shown in Fig. 01 if X
and R be the unknown and known
resistances respectively and 
be the
distance of the null point measured from the left end A. lf the meter bridge,
then by the principle of the whetstone’s network. We get,
be the
distance of the null point measured from the left end A. lf the meter bridge,
then by the principle of the whetstone’s network. We get,
= 
Where x and y are end-errors.
When the resistances X and R are interchanged, we get
= 
Or, X = 
. . . .
. . . . . . . . . . .(2) Fig:
01
. . . .
. . . . . . . . . . .(2) Fig:
01
The mean of equations (1) and (2), after end-corrections, give the value of the unknown
resistance.
If now L is the length of the experimental wire in centimeters then
X = 
Or, ρ = 
. . . . . . . . . . . . . . . . . . .
. . . (3)
. . . . . . . . . . . . . . . . . . .
. . . (3)
Where ρ is the
specific resistance of the material of the wire and r is the radius of the
cross-section of the wire.
Thus ρ may be determined from equation
(3) after measuring X, r and L.
Apparatus : Meter bridge,
Leclanche’s cell (E), zero-centre
galvanometer (G), rheostat (Rh), commutator (K), resistance box (R) , the
specimen wire (X), connecting wires, screw-gauge etc.
Procedure:
1) Collect all the instruments and make the connection as
shown in figure above
2) Take out some suitable resistance ‘R’ form the resistance box (R. B.).
3) Touch the jockey at point A; see that there is a deflection in the galvanometer on one side, and then touch the jockey on the point C of the wire, the deflection in the galvanometer should be on the other side. If it is so, your connections are correct.’
2) Take out some suitable resistance ‘R’ form the resistance box (R. B.).
3) Touch the jockey at point A; see that there is a deflection in the galvanometer on one side, and then touch the jockey on the point C of the wire, the deflection in the galvanometer should be on the other side. If it is so, your connections are correct.’
4) Now
find the position of null point where deflection in galvanometer becomes zero.
Note length AB (l) BC will be (100 – l).
5) Repeat the above procedure for different values of ‘R’. Take at least 6 readings
6) Note the point where the galvanometer shows 0 deflections, this is called the balance point.
7) Measure the length of the given wire using ordinary scale and the radius of the wire using screw gauge (Take five readings)
Note length AB (l) BC will be (100 – l).
5) Repeat the above procedure for different values of ‘R’. Take at least 6 readings
6) Note the point where the galvanometer shows 0 deflections, this is called the balance point.
7) Measure the length of the given wire using ordinary scale and the radius of the wire using screw gauge (Take five readings)
Data Collection:
(A). Length of
the wire (L):
(і)
L1 = cm,
(іі). L2
= cm, (ііі). L3 = cm
Mean L = cm
Calculation of the
least count:
Pitch (P) = 
No. of divisions in the circular
scale, n = 100
Least count (L.C) = 
= mm
= cm
(B).Reading for Radius of the wire r:
No.
of obs.
|
Linear Scale Reading ( x )
|
Circular Scale Divisions
( C.D
)
|
Least
Count
(L.C)
|
Diameter
d = x + C.D ×
L.C
|
Mean
Diameter (d)
|
Mean Radius
r = d/2
|
cm
|
cm
|
cm
|
cm
|
cm
|
||
01
|
||||||
02
|
||||||
03
|
(C). Readings
for the balance point:
Known resistance
R
|
Positions
Balance point
|
Mean
100-
 |
Resistance
X
=
 |
Mean Unknown
Resistance
X
|
|||||
Unknown
Resistance
X
|
Known
Resistance
R
|
Direct
Length
 |
Reverse
Length
|
Mean
Length (
) |
|||||
X=
 |
|||||||||
Ω
|
cm
|
cm
|
cm
|
cm
|
Ω
|
Ω
|
|||
Left
|
Right
|
||||||||
Left
|
Right
|
||||||||
Left
|
Right
|
||||||||
Right
|
Left
|
||||||||
Right
|
Left
|
||||||||
Right
|
Left
|
||||||||
Calculation: From equation (3), we get
ρ = 
Ω-cm
Ω-cm
= Ω-cm
= Ω-cm
Result: The specific resistance of a wire is ……………….. Ω-cm
Error analysis: 
X 100%
X 100%
Discussions:
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