Why is a radial magnetic field used in a Moving Coil Galvanometer?

A radial magnetic field is crucial in an MCG to ensure that the plane of the coil is always parallel to the magnetic field lines, or equivalently, the normal to the coil's plane is always perpendicular to the magnetic field direction. This means the angle $\theta$ between the magnetic field $\vec{B}$ and the area vector $\vec{A}$ (normal to the coil) is always $90^circ$. Consequently, the torque experienced by the coil, given by $\tau = NIAB \sin\theta$, simplifies to $\tau = NIAB$ (since $\sin 90^circ = 1$). This makes the torque directly proportional to the current $I$, resulting in a linear scale for the galvanometer, which is essential for accurate measurements.

What is the role of the soft iron core in an MCG?

The soft iron core placed concentrically within the coil serves two vital functions. Firstly, it significantly increases the strength of the magnetic field passing through the coil. Soft iron is a ferromagnetic material, which means it can concentrate magnetic field lines, effectively increasing the magnetic flux density ($B$) and thereby enhancing the sensitivity of the galvanometer. Secondly, its cylindrical shape, in conjunction with the concave pole pieces of the permanent magnets, helps in creating the desired radial magnetic field, ensuring that the torque is consistently maximum and linear with current.

How can the sensitivity of a Moving Coil Galvanometer be increased?

The sensitivity of an MCG, specifically current sensitivity ($I_s = \phi/I = NAB/k$), can be increased by several means: (1) Increasing the number of turns ($N$) in the coil, (2) Increasing the area ($A$) of the coil, (3) Using stronger permanent magnets to increase the magnetic field strength ($B$), and (4) Decreasing the torsional constant ($k$) of the suspension wire. A smaller $k$ means the suspension wire twists more easily for a given torque. Phosphor bronze is often used for suspension due to its low torsional constant and high elasticity.

Why is phosphor bronze preferred for the suspension wire in an MCG?

Phosphor bronze is an alloy specifically chosen for the suspension wire in an MCG due to its excellent mechanical and electrical properties. It possesses a very low torsional constant ($k$), meaning it requires very little torque to produce a significant twist, thus contributing to high galvanometer sensitivity. Additionally, it exhibits high elasticity, ensuring that it returns to its original untwisted position accurately after the current is removed, preventing 'fatigue' or permanent deformation. It also has good electrical conductivity, allowing current to flow to and from the coil.

What is electromagnetic damping in an MCG and why is it important?

Electromagnetic damping refers to the process where induced eddy currents within the metallic (usually aluminum) frame of the coil quickly bring the oscillating coil to rest. When the coil, along with its metallic frame, moves through the magnetic field, eddy currents are induced in the frame. According to Lenz's Law, these eddy currents produce their own magnetic field that opposes the motion causing them. This opposing force acts as a braking mechanism, preventing the coil from oscillating excessively and allowing the pointer to settle rapidly at the correct reading, making the measurement quick and stable.

Moving Coil Galvanometer

Physics

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

A Moving Coil Galvanometer (MCG) is a highly sensitive electromagnetic device used for detecting and measuring small electric currents. Its operation is fundamentally based on the principle that a current-carrying coil, when placed in a uniform magnetic field, experiences a torque. This torque causes the coil to rotate, and the extent of this rotation is directly proportional to the magnitude of t…

Quick Summary

The Moving Coil Galvanometer (MCG) is a precision instrument designed to detect and measure small electric currents. Its fundamental principle relies on the torque experienced by a current-carrying coil when placed in a magnetic field.

This torque, given by $\tau = NIAB \sin\theta$ , causes the coil to rotate. Key components include a coil wound on a non-magnetic frame, strong permanent magnets, and a soft iron core. The soft iron core concentrates the magnetic field and, along with concave pole pieces, ensures a radial magnetic field.

This radial field ensures that $\sin\theta = 1$ , making the torque directly proportional to the current ( $I$ ). A phosphor bronze suspension wire provides a restoring torque ( $k\phi$ ), leading to an equilibrium where $NIAB = k\phi$ , and thus deflection $\phi \propto I$ .

Sensitivity, defined as deflection per unit current or voltage, depends on $N, A, B,$ and $k$ . MCGs can be converted into ammeters by connecting a low shunt resistance in parallel, or into voltmeters by connecting a high series resistance.

Electromagnetic damping, caused by eddy currents in the coil's metallic frame, ensures quick and stable readings.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Current Sensitivity Calculation

Current sensitivity ( $I_s$ ) quantifies how much the galvanometer deflects for a given current. It's defined…

Ammeter Conversion (Shunt Resistance)

To convert a galvanometer into an ammeter capable of measuring a larger current range, a small resistance,…

Voltmeter Conversion (Series Resistance)

To convert a galvanometer into a voltmeter to measure a larger voltage range, a high resistance, known as a…

Principle: — Torque on current loop in B-field:

\tau = NIAB \sin\theta

Radial Field: — Ensures

\theta=90^\circ

, so

\tau = NIAB

Equilibrium: —

NIAB = k\phi \implies \phi = \frac{NAB}{k}I

Current Sensitivity ($I_s$): —

I_s = \frac{\phi}{I} = \frac{NAB}{k}

Voltage Sensitivity ($V_s$): —

V_s = \frac{\phi}{V} = \frac{NAB}{kR_g}

Ammeter Conversion (Shunt): —

R_{sh} = \frac{I_g R_g}{I - I_g}

(parallel connection).

Voltmeter Conversion (Series): —

R_{series} = \frac{V}{I_g} - R_g

(series connection).

Soft Iron Core: — Increases

B

, makes field radial.

Phosphor Bronze: — Low

k

, high elasticity for suspension.

Damping: — Electromagnetic damping by eddy currents in metallic frame.

N.A.B.K. is SENSITIVE!

N — Number of turns (Increase N, increase sensitivity)

A — Area of coil (Increase A, increase sensitivity)

B — Magnetic field strength (Increase B, increase sensitivity)

K — Torsional constant (Decrease K, increase sensitivity)

Shunt for Ammeter (Parallel, Low R) Series for Voltmeter (Series, High R)