Page 28 - Physics - XII

Moving Charges
2 Moving Charges
and Magnetism
and Magnetism

EXPERIMENT - 4

Aim
To determine resistance of a galvanometer by half-defl ection method and to fi nd its fi gure of merit.

Apparatus and Materials Required

A weston type galvanometer, resistance box of the range 0 to 10,000 Ω, resistance box of the range 0 to
200 Ω, a battery, two one-way keys, connecting wires, and sand paper

E K R
+ – 1 R.B. Battery Key
+ –
I I K
Ig G Ig Ig 1 Ig

R
I-Ig I-Ig R.B.
S Resistance box G
K 2
Fig. 4.1: Circuit diagram for fi nding Fig. 4.2: Determination of fi gure
of merit of galvanometer

the resistance of galvanometer

Principle/Theory
Galvanometer
It is a sensitive device (instrument) used for detecting feeble electric current in an electric circuit. It is made
up of a rectangular coil that has many turns, and it is usually made of thinly insulated or fi ne copper wire

that is wounded on a metallic frame. The coil is free to rotate about a fi xed axis.
The working principle of a galvanometer is that “A current-carrying coil, when placed in a uniform magnetic
fi eld, experiences a torque”.
When a coil carrying current I is placed in a radial magnetic fi eld, it experiences a defl ection θ. Then,
I ∝ θ
or I = Kθ
where, K is a constant of proportionality and is called as fi gure of merit of the galvanometer.
Figure of Merit: It is defi ned as the current required for defl ecting the pointer by one division. It is denoted
I
by K and K = .
θ
The resistance of the galvanometer G by half-defl ection method is given by
RS
G =
RS

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