Question

Assertion (A): When a ferromagnetic material is inserted into the coil of a moving coil galvanometer, both the current and voltage sensitivity of the galvanometer decreases.

Reason (R): Current sensitivity and voltage sensitivity are directly proportional to the magnetic field in the coil.

In the light of above statements, choose the correct answer.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A)
• B. Both (A) and (R) are true but (R) is not the correct explanation of (A)
• C. (A) is true but (R) is false
• D. (A) is false but (R) is true

Answer 1

Answer: (C)

(A) is true but (R) is false

Answer 2

A moving‐coil galvanometer is calibrated for the uniform magnetic field provided by its permanent magnet arrangement. Inserting a ferromagnetic (soft iron) core into the coil distorts this uniform field so that the effective flux linking the coil is reduced. Hence, for a given current, the deflection (or sensitivity) decreases.

On the other hand, the statement in (R) that the current and voltage sensitivity are directly proportional to the magnetic field is generally valid in the ideal case (since deflection ∝ B); however, when a ferromagnetic core is inserted, the field becomes nonuniform and distorted, so the assumption behind (R) does not hold in this practical modification.

Thus, (A) is true but (R) does not correctly explain (A).