Consider the following statements about a moving coil galvanometer:
(A) Its current sensitivity is \(NAB/k\).
(B) If the number of turns of its coil is doubled, its voltage sensitivity also doubles.
(C) To use it as an ammeter, a small resistance called a shunt is joined in parallel with its coil.
(D) It gives a full-scale deflection for a current of the order of one ampere.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
In a galvanometer, the spring torque \( k\phi \) balances the magnetic torque \( NIAB \). So the deflection is \( \phi=(NAB/k)I \).
(A) Its current sensitivity is \( NAB/k \). — True. Current sensitivity is the deflection per unit current, \( \phi/I=NAB/k \).
(B) If the number of turns of its coil is doubled, its voltage sensitivity also doubles. — False. Voltage sensitivity is \( \phi/V=NAB/kR \). Doubling \( N \) also doubles the length of wire, so \( R \) doubles too. The voltage sensitivity stays the same.
(C) To use it as an ammeter, a small resistance called a shunt is joined in parallel with its coil. — True. Most of the current then passes through the shunt, and the combined resistance is small.
(D) It gives a full-scale deflection for a current of the order of one ampere. — False. A galvanometer is a very sensitive device. A current of the order of microamperes is enough for full-scale deflection.
NCERT (2026-27):
The Moving Coil Galvanometer – “We define the current sensitivity of the galvanometer as the deflection per unit current. … this current sensitivity is,”
\( \frac{\phi}{I}=\frac{NAB}{k} \)
The Moving Coil Galvanometer – “… However, the resistance of the galvanometer is also likely to double, since it is proportional to the length of the wire. … thus the voltage sensitivity, … remains unchanged.”
The Moving Coil Galvanometer – “… To overcome these difficulties, one attaches a small resistance rs, called shunt resistance, in parallel with the galvanometer coil; so that most of the current passes through the shunt.”
The Moving Coil Galvanometer – “… (i) Galvanometer is a very sensitive device, it gives a full-scale deflection for a current of the order of μA.”