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 operates on the principle that a current-carrying coil, when placed within a magnetic field, experiences a torque. The extent of the coil's deflection is directly proportional to the current flowing through it.

Working:

• When current passes through the coil, a torque is generated due to the magnetic field.
• A phosphor-bronze wire provides a counteracting 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 strength
• $A$ = area of the coil
• $I$ = current
• $k$ = torsional constant of the wire

(i) Purpose of Radial Magnetic Field:
Ensures the coil's plane remains consistently perpendicular to the magnetic field, resulting in a constant torque throughout its rotation. This guarantees that torque is directly proportional to the current.

(ii) Purpose of Soft Iron Core:

• Concentrates and intensifies the magnetic field.
• Establishes a radial magnetic field.
• Enhances the galvanometer's sensitivity and field uniformity.