Page 139 - Physics - XII

Sources of Error

1. There may be eddy current in the soft iron coil.
2. The resistance of the circuit may increase slightly due to heating eff ect of current.

PROJECT - 8

Aim
To study the Earth’s magnetic fi eld using a compass needle-bar magnet by plotting magnetic fi eld lines and

tangent galvanometer.

Apparatus and Materials Required
A tangent galvanometer, a battery, a rheostat, spirit level, a reversing key, an ordinary key, an ammeter,
connecting wires, sand paper, and a metre scale

Principle/Theory

Compass Circular coil
box
S N

Base
terminals
0 500
2 50
Levelling
Screws
Fig. 8.1: A tangent galvanometer

The working of a tangent galvanometer is based on the tangent law. It works on the principle that when the

coil of the tangent galvanometer is kept in magnetic meridian and current passes through any of the coil,
then the magnetic needle at the centre gets defl ected and comes to an equilibrium position under the action
of two perpendicular fi elds: one due to horizontal component of Earth (B ) and the other due to the fi eld (B)
H
set up by the coil due to current.
In equilibrium, B = B tanθ
H
The above relation is known as tangent law.
NI
Magnetic fi eld at the centre of coil, B 0
2 R
where, N = number of turns
R = radius of tangent galvanometer or coil
I = current to be measured
θ = angle made by needle from the direction of B in equilibrium
H
µ NI RB tan
Hence, 0 = B tanθ or I 2 H ⇒ I = K tanθ
2R H N
0

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