OPERATING INSTRUCTIONS FOR
MEASURE LOW RESISTANCE BY
CROMPTON POTENTIOMETER
OBJECT:
To measure low resistance by Crompton Potentiometer.
APPARATUS:
Crompton potentiometer New Tech Type NTI – 501, two battery eliminators, two
rheostats, two one way keys, one two way key, One Galvanometer, one known
resistance of comparatively higher value and given small resistance.
THEORY:
Let I current is passing through known resistance R of comparatively higher value
R and small unknown resistance r. The balancing length corresponding to potential
drop on R is l1.
.·. IR = σ l1 …………….. (1)
B1
+
-
Rh1
K1
C
A
B
Jockey
G
Two way
K
Key
a
r
R
I
b
B2
+
-
K2
Rh2
Fig. (1)
Where σ is potential gradient.
If balancing length corresponding to potential drop on (R + r) resistance is l2.
I (R + r) = σ l2 ……………………….. (2)
Then
(2) / (1)
R + r
R
or
=
r
1 +
r
=
R
=
l1
l2
l1
R
or
l2
l2 - l1
l1
or
r =
l2 - l1
l1
R ............ (3)
Putting the values of l1, l2 and R in this formula the unknown low resistance r is
calculated.
PROCEDURE:
(1) Complete the connections as shown in Fig. (1).
(2) Close the keys K1 and K2 put plug in ‘a’ gap of two way key K.
(3) Note down the balancing length l1 for potential drop on resistance R.
(4) Now take out plug from ‘a’ gap and put it in ‘b’ gap of two way key.
Adjustments of Rheostats Rh1 and Rh2 should not be disturbed. Note down
the balancing length l2 for potential drop on resistance (R + r).
(5) Calculate the value of unknown small resistance r using formula:-
r =
l2 - l1
l1
R
(6) Different sets of observation are taken by changing currents in primary and
secondary circuits.
(7) Mean value of small resistance r is calculated.
OBSERVATIONS:
Sr.
No.
Balancing length
l1 for P.D. on R
Cm
Balancing
length l2 for
P.D. on R Cm
r =
l2 - l1
l1
Ohm
1.
2.
3.
4.
5.
CALCULATIONS:
R
Putting values of l1, l2 and R in the formula the value of unknown small resistance r
is calculated in each set of observations. Then mean value of r is determined.
RESULT:
The value of given small resistance as determined using Crompton potentiometer is
= ……………. Ohm.
PRECAUTIONS:
1. The E.M.F. of the cell used in primary circuit should be kept more than
E.M.F. of the cell in secondary circuit.
2. All the positive terminals should be connected to the same point (A) of the
potentiometer.
3. Jokey should not be moved along the potentiometer wire.
4. Electrical connections should be tight.
C R O M P T O N P O T E N T IO M E T E R
N T I
+
GALVANOMETER
A
C
PRIMARY
CIRCUIT
-
+
5
B
7
8
9
10
1
0
6
4
3
2
1
SECONDARY
D
CIRCUIT - I
FOR STANDARDISATION
+
E
SECONDARY
CIRCUIT - II
FOR UNKNOWN P.D.
11
F
2
49
-
50
N T I - 501
Fig. (2) Panel Diagram
*************
G
C R O M P TO N P O T EN TIO M ET ER
N TI
+
GALVANOMETER
A
B1 K1 .
PRIMARY
CIRCUIT
Rh1
-
+
C
+
B
5
6 7
4
3
2
SECONDARY
D
CIRCUIT - I
FOR STANDARDISATION
8
+
E
+
SECONDARY
CIRCUIT - II
FOR UNKNOWN P.D.
1
F
b
-
11
1
49
2
Jockey
50
N T I - 5 01
Fig. (3) Connections for Crompton Potentiometer
**************
B2
K3
r
Rh2
K
0
.
a
9
10
-
R