Match List-I with List-II for a moving coil galvanometer. (In each case, the quantities not named stay the same.)
| List-I (change) | List-II (effect) |
|---|---|
| (A) Number of turns of the coil doubled; effect on current sensitivity | (I) Becomes half |
| (B) Torsional constant of the spring doubled; effect on current sensitivity | (II) Remains the same |
| (C) Area of the coil doubled and its resistance also doubled; effect on voltage sensitivity | (III) Becomes double |
| (D) Magnetic field doubled and resistance of the coil halved; effect on voltage sensitivity | (IV) Becomes four times |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A)-(III), (B)-(I), (C)-(II), (D)-(IV)
The current sensitivity of a galvanometer is \( \phi/I=NAB/k \). The voltage sensitivity is \( \phi/V=NAB/kR \), where \( R \) is the resistance of the coil.
(A) Number of turns of the coil doubled; effect on current sensitivity — This matches (III) Becomes double, because \( \phi/I \) is proportional to \( N \).
(B) Torsional constant of the spring doubled; effect on current sensitivity — This matches (I) Becomes half, because \( \phi/I \) varies as \( 1/k \).
(C) Area of the coil doubled and its resistance also doubled; effect on voltage sensitivity — This matches (II) Remains the same. \( A \) is in the numerator and \( R \) in the denominator of \( NAB/kR \), and both become double.
(D) Magnetic field doubled and resistance of the coil halved; effect on voltage sensitivity — This matches (IV) Becomes four times. Doubling \( B \) gives a factor 2. Halving \( R \) gives another factor 2, because \( R \) is in the denominator.
NCERT (2026-27):
The Moving Coil Galvanometer – “… We define the current sensitivity of the galvanometer as the deflection per unit current. …”
\( \frac{\phi}{I}=\frac{NAB}{k} \)\( \qquad (4.27) \)
The Moving Coil Galvanometer – “… We define the voltage sensitivity as the deflection per unit voltage. …”
\( \frac{\phi}{V}=\left(\frac{NAB}{k}\right)\frac{1}{R} \)\( \qquad (4.28) \)