The Moving Coil Galvanometer

The assertion given is that the deflection in a galvanometer is directly proportional to the current passing through it. This is indeed true. A galvanometer works on the principle that a current-carrying coil placed in a magnetic field experiences a torque. The deflection θ in the galvanometer is given by the equation θ = kI, where I is the current and k is a constant of proportionality, indicating that the deflection is directly proportional to the current.

The reason provided is that the coil of a galvanometer is suspended in a uniform radial magnetic field. This is also true. In a galvanometer, the coil is placed between the poles of a magnet designed to create a uniform radial magnetic field, ensuring that the torque is proportional to the current for all positions of the coil.

However, the reason mentioned is not the correct explanation for the assertion regarding the proportionality of deflection to current. The arrangement of the uniform radial magnetic field allows for consistent torque at different coil positions, but it is primarily the electromagnetic torque resulting from the interaction of the magnetic field and current that causes the deflection proportionality.

Therefore, the correct option is: Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

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:

where:

• = number of turns
• = magnetic field
• = area of the coil
• = current
• = 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