Question

In a moving coil galvanometer, the pole pieces of the permanent magnet are systematically sculpted into a cylindrical shape to create a radial magnetic field. What is the primary operational purpose of applying this radial field configuration?

• A. \( \text{To minimize the net magnetic flux passing through the coil.} \)
• B. \( \text{To ensure the magnetic deflecting torque remains constant and independent of the coil's rotation angle.} \)
• C. \( \text{To increase the overall internal electrical resistance of the galvanometer suspension.} \)
• D. \( \text{To completely eliminate electromagnetic damping effects.} \)

Hint:

A radial magnetic field guarantees that \( \theta \) is always \( 90^\circ \) (\(\sin\theta = 1\)). This ensures a perfectly linear relation between current and pointer deflection (\( I \propto \alpha \)), which is why the galvanometer scale is evenly spaced.

Answer 1

The Correct Option is B

Concept: The deflecting magnetic torque (\( \tau \)) acting on a current-carrying loop of \( N \) turns suspended inside a magnetic field \( B \) is given by the formula: \[ \tau = NIAB\sin\theta \] where \( A \) is the face area of the coil loop, \( I \) is the current, and \( \theta \) is the angle between the normal to the plane of the coil and the magnetic field lines.

Step 1: Analyze how a radial field configuration affects geometry. When a permanent magnet has curved, concave cylindrical pole faces combined with a central soft iron core, the magnetic field lines are forced to radiate out like spokes on a wheel. As the coil rotates around its central axis, the plane of the coil stays parallel to the magnetic field lines at every position.

Step 2: Evaluate the torque equation under radial field conditions. Because the field lines run parallel to the plane of the coil, the normal vector to the coil face is always perpendicular (\( 90^\circ \)) to the magnetic field lines. \[ \theta = 90^\circ \implies \sin(90^\circ) = 1 \] Substituting this value back into our torque formula yields: \[ \tau = NIAB \] This configuration keeps the deflecting torque strictly proportional to the current \( I \), removing any dependence on the angle of rotation. This allows the galvanometer to use a clean, linear measurement scale.