Page 15 - Physics - XII

EXPERIMENT - 2

Aim
To fi nd resistance of a given wire/standard resistor using metre bridge.

Apparatus and Materials Required
A metre bridge, a resistance box, a jockey, a one-way key, a wire whose resistance to be determined, a
galvanometer, a cell, a metre scale, sand paper, and thick connecting wires.

Description of Apparatus

Metre Bridge
A metre bridge, also called a slide wire bridge, is an instrument that works on the principle of wheatstone
bridge. It is used to fi nd the unknown resistance of a conductor.
K
+ –
E Rheostat
Q
P RB

X B
E F

G

0 10 20 30 40 50 60 70 80 90 100
R D S
Jockey (J)
A C
l cm (100–l) cm
Fig. 2.1: Circuit diagram of metre bridge

A metre bridge has metallic strips for providing electrical contact. A constantan wire AC having a length of
1 m and a uniform cross-section is mounted on a wooden board with a scale. Two gaps E and F are provided
in between metallic strips for connecting the resistors. A jockey is connected from the galvanometer and
the other end of the galvanometer is connect to terminal B.

Principle/Theory B
A metre bridge works on the principle of wheatstone’s bridge. A
wheatstone’s bridge is an electrical network in which four resistors P, Q, P K Q

R, and S are connected to form the four arms of quadrilateral ABCD as I 1 2 I 1
shown in the Fig. 2.2. A G C
When the galvanometer shows zero defl ection, then no current fl ows I 2
through the galvanometer and the bridge is said to be balanced. Suppose I R S I
this condition when jockey is set at the point D called null point in the I 2
metre bridge as shown in Fig. 2.1. E D
+ –
K
1
Fig. 2.2: Wheatstone’s bridge

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