Torque on a Current Loop — Part 2: The moving coil galvanometer, ammeter and voltmeter: visual lesson, ISC Class 12 Physics

Lesson

The Moving Coil Galvanometer

Part 2 — the torque on a coil put to work: a sensitive current detector, and the ammeters and voltmeters built from it.

In this lesson you will

Describe the construction and working of a moving coil galvanometer and the purpose of its radial field
Derive I = kθ and use current and voltage sensitivity
Convert a galvanometer into an ammeter with a shunt and into a voltmeter with a series resistance

Diagram

Construction

Main parts of a moving coil galvanometer:

A rectangular coil of many turns of fine insulated copper wire on a light frame, free to turn about a vertical axis.
A strong magnet with concave (cylindrical) pole pieces.
A soft-iron cylinder fixed inside the coil.
A phosphor-bronze suspension strip (or hairsprings) giving the restoring torque, with a mirror or pointer to read the deflection.

Diagram

Why a radial field?

The concave pole pieces and the soft-iron core make the field in the narrow gap radial. Whatever the deflection, the plane of the coil contains $\vec B$ , so $\theta = 90^\circ$ and the deflecting torque is always $\tau = NIAB$

The soft-iron core also concentrates the field lines, making $B$ larger and the galvanometer more sensitive.

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