Question

With the help of a labelled diagram, explain the principle of working of a moving coil galvanometer. Write the purpose of using (i) radial magnetic field, and (ii) soft iron core, in it.

Hint:

A radial magnetic field and a soft iron core ensure that the torque on the coil is always proportional to current and independent of the coil's position, which enhances accuracy and sensitivity.

Answer 1

Principle:
A moving coil galvanometer works on the principle that a current-carrying coil placed in a magnetic field experiences a torque. The deflection of the coil is proportional to the current passing through it.

Working:

• When current flows through the coil, it experiences a torque due to the magnetic field.
• A phosphor-bronze wire provides a restoring torque.
• At equilibrium, the deflection is proportional to the current:

\[ \tau = nBIA \quad \text{and} \quad \tau_{\text{restoring}} = k\theta \Rightarrow \theta = \frac{nBIA}{k} \]

where:

• $n$ = number of turns
• $B$ = magnetic field
• $A$ = area of the coil
• $I$ = current
• $k$ = torsional constant of the wire

(i) Purpose of Radial Magnetic Field:
To ensure that the plane of the coil always remains perpendicular to the magnetic field, providing a constant torque throughout the rotation. This ensures that torque is directly proportional to current. 

(ii) Purpose of Soft Iron Core:

• Concentrates and strengthens the magnetic field
• Makes the field radial
• Increases sensitivity and uniformity of the galvanometer