Read the following passage and answer the questions.
A school laboratory has a moving coil galvanometer. Its rectangular coil has 100 turns, and each turn has an area of 4.0 cm2. The coil can turn between the curved pole pieces of a magnet, with a soft iron cylinder inside it. The magnetic field at the coil is 0.25 T. A spring gives a restoring torque of \(2.0\times10^{-5}\ \mathrm{N\,m}\) for each radian of twist. The coil has a resistance of 50 Ω. The pointer reaches the end of the scale for a current of 2 mA. A student first uses the galvanometer as a current detector in a Wheatstone bridge. Later she plans to change it into an ammeter and into a voltmeter, so that she can measure larger currents and voltages in other circuits.
When the galvanometer is used as a detector in the Wheatstone bridge, the zero of its scale is in the middle. This is because
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → the current through the galvanometer can flow in either direction
In the bridge, the galvanometer only shows whether a current flows in its arm. Before balance, this current may flow one way or the other.
Option (1) readings near the middle of the scale are the most accurate — Incorrect. A detector is not used to read a value. It only has to show a current or no current.
Option (2) the current sensitivity is largest when the pointer is in the middle — Incorrect. The current sensitivity \( NAB/k \) does not depend on the position of the pointer. It is the same over the whole scale.
Option (3) the field is radial only near the middle position of the coil — Incorrect. The soft iron cylinder and the curved pole pieces keep the field radial at every position of the coil.
Option (4) the current through the galvanometer can flow in either direction — Correct. The pointer must be able to turn to the right for one direction of current and to the left for the other. At balance it rests in the middle.
NCERT (2026-27):
The Moving Coil Galvanometer – “… In this usage the neutral position of the pointer (when no current is flowing through the galvanometer) is in the middle of the scale and not at the left end … Depending on the direction of the current, the pointer’s deflection is either to the right or the left.”