The torque on the coil remains constant irrespective of the coil's orientation during rotation due to

Q: The torque on the coil remains constant irrespective of the coil's orientation during rotation due to

**Correct option: (B) radial magnetic field.** A moving coil galvanometer uses concave (cylindrical) pole pieces together with a soft-iron core to produce a **radial magnetic field**. In such a field the plane of the coil is **always parallel to B** (equivalently, B is always along the plane), so the angle between the field and the plane stays 90° for every deflection. The deflecting torque $\tau = NIAB \sin \alpha$ becomes $\tau = NIAB$ (with $\sin 90^\circ = 1$) for all orientations — hence the torque stays constant during rotation, and the deflection $\phi \propto I$, giving a linear (uniform) scale.

Moving Charges and Magnetism

Moving coil galvanometer — radial field and constant torque

Options

(A)use of soft iron core which increases the magnetic field.

(B)radial magnetic field

(C)hair spring which provides the counter torque

(D)eddy current in the iron core which causes damping.

Official Answer

Correct option: (B) radial magnetic field.

A moving coil galvanometer uses concave (cylindrical) pole pieces together with a soft-iron core to produce a radial magnetic field. In such a field the plane of the coil is always parallel to B (equivalently, B is always along the plane), so the angle between the field and the plane stays 90° for every deflection.

The deflecting torque $\tau = NIAB \sin \alpha$ becomes $\tau = NIAB$ (with $\sin 90^\circ = 1$ ) for all orientations — hence the torque stays constant during rotation, and the deflection $\phi \propto I$ , giving a linear (uniform) scale.

radial magnetic fieldmoving coil galvanometerconcave pole piecessoft iron coretorque NIABcoil plane parallel to Blinear scalesin 90 = 1

Marking Scheme

11 mark: option (B) radial magnetic field.
2No step-by-step required (single-correct MCQ); reasoning: radial field keeps coil plane parallel to B so $\tau = NIAB$ is orientation-independent.

Hint

Curved pole pieces + soft-iron core make the field radial, so the coil's plane is always parallel to B and $\sin \alpha = 1$ .

Quick Oral Answer

A galvanometer uses a radial magnetic field from its concave pole pieces and soft-iron core, keeping the coil's plane always parallel to the field so that $\sin \alpha = 1$ and the deflecting torque $NIAB$ stays constant for every deflection, giving a linear scale.

Analysis & Explanation

Why (B) is right: The radial field is the whole point of the curved pole pieces and soft-iron core in a galvanometer. It keeps the coil's plane always parallel to B, so $\sin \alpha = 1$ always and $\tau = NIAB$ is independent of the deflection angle. This makes deflection directly proportional to current and gives a linear scale.

Why the distractors are wrong:

(A) The soft-iron core does increase the field strength (boosting sensitivity), but merely increasing B does not make the torque orientation-independent — it is the radial shape of the field, not its magnitude, that keeps τ constant.
(C) The hair spring supplies the restoring (counter) torque that balances the deflecting torque at equilibrium; it does not keep the deflecting torque constant with orientation.
(D) Eddy currents in the metallic frame provide damping (a dead-beat galvanometer) so the pointer settles quickly; they have nothing to do with keeping the torque constant.

Common Mistakes

1Choosing (A) — assuming increasing field strength (rather than its radial geometry) is what keeps the torque constant.
2Confusing the restoring torque of the hair spring (option C) with the constant deflecting torque asked about.
3Thinking eddy-current damping (option D) affects the magnitude of the deflecting torque rather than just settling the pointer.

Interesting Facts

A radial field turns a galvanometer's scale linear, which is why analogue meters have evenly spaced divisions rather than crowded ends.

The moving-coil galvanometer is based on the d'Arsonval design of the 1880s, still the heart of analogue multimeters.

The soft-iron core also concentrates magnetic field lines to make the field radial and strong, doing two jobs at once.

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Frequently Asked Questions

Why does a radial magnetic field keep the torque constant?

In a radial field, produced by concave pole pieces and a soft-iron core, the plane of the coil is always parallel to the field no matter how much it turns. The angle between the field and the plane stays 90°, so $\sin \alpha = 1$ and the deflecting torque $\tau = NIAB$ is the same for every deflection.

What advantage does the constant torque give?

Because $\tau = NIAB$ is constant, the deflection is directly proportional to the current ( $\phi \propto I$ ). This makes the galvanometer scale linear with equally spaced divisions, which is much easier to read than a non-uniform scale.

What is the role of the soft-iron core then?

The soft-iron core helps make the field radial and also concentrates the magnetic field lines, increasing the field strength and hence the sensitivity. But it is the radial shape, not the increased magnitude, that keeps the torque orientation-independent.