Reviewed by the physics instruments team at Samtech Instruments (JAYSEE), a laboratory equipment manufacturer supplying schools and institutions across India since 2002.

A galvanometer is a sensitive electromechanical instrument used to detect and measure very small electric currents in a circuit. It works on the principle that a current-carrying coil placed in a magnetic field experiences a torque, and the resulting deflection of the coil is directly proportional to the current passing through it. This guide explains the working principle, a labelled diagram, the construction, the governing formula, types, sensitivity, and the laboratory uses of a galvanometer.

What Is a Galvanometer?

A galvanometer is a device that detects the presence and direction of a small electric current and indicates its magnitude through the deflection of a pointer or coil. Unlike an ammeter, its pointer can deflect on both sides of a central zero, so it shows not only how much current flows but also in which direction. Because of this sensitivity, galvanometers are widely used in laboratories, in Wheatstone bridge and potentiometer experiments, and as the basic building block from which ammeters and voltmeters are made.

Working Principle of a Galvanometer

The galvanometer works on the principle that a current-carrying coil placed in a uniform magnetic field experiences a torque. When current flows through the coil suspended between the poles of a magnet, the magnetic field exerts a turning force (torque) on the coil. This deflecting torque rotates the coil, while a spring or suspension fibre provides an opposing restoring torque. The coil settles at the angle where the deflecting torque equals the restoring torque. Since the restoring torque is proportional to the angle of twist, the deflection of the coil becomes directly proportional to the current — which is what makes measurement possible.

Labelled Diagram of a Moving Coil Galvanometer

A labelled diagram of a moving coil galvanometer is one of the most commonly asked figures in board exams. Mark the following parts clearly.

Construction of a Moving Coil Galvanometer

A moving coil galvanometer is built from the following main parts:

• Coil: A rectangular coil of many turns of fine, insulated copper wire wound on a light non-magnetic metallic frame (usually aluminium). This is the current-carrying, moving part.
• Permanent magnet: A strong horseshoe-shaped magnet with concave pole pieces, which produces a radial magnetic field so the field is always parallel to the plane of the coil.
• Soft iron core: A cylindrical soft iron core placed inside the coil. It concentrates the magnetic field lines and makes the field radial and stronger, increasing sensitivity.
• Suspension/spring: A fine phosphor-bronze strip suspends the coil from a torsion head and also carries current into the coil. It provides the restoring torque.
• Pointer and scale: A pointer attached to the coil moves over a graduated scale to show the deflection. In sensitive types, a small mirror and a light beam (lamp-and-scale) are used instead.
• Torsion head: Used to adjust the zero position of the pointer.

Galvanometer Formula

The deflecting torque on the coil is given by:

τ = N I A B

where N is the number of turns in the coil, I is the current, A is the area of the coil, and B is the magnetic flux density. At equilibrium, this deflecting torque is balanced by the restoring torque kθ, where k is the restoring torque per unit twist and θ is the deflection. Therefore:

N I A B = kθ, which gives I = (k / N A B) θ

Since k, N, A, and B are all constant for a given galvanometer, the current I is directly proportional to the deflection θ. This linear relationship is what allows the scale to be marked uniformly.

Sensitivity of a Galvanometer

The sensitivity of a galvanometer describes how large a deflection it gives for a given input. There are two types:

• Current sensitivity: the deflection produced per unit current (θ/I = NAB/k). It increases with more turns, larger coil area, a stronger field, or a weaker restoring torque.
• Voltage sensitivity: the deflection produced per unit voltage (θ/V). Note that increasing current sensitivity does not always increase voltage sensitivity, because adding more turns also raises the coil’s resistance.

Types of Galvanometer

Common types include the moving coil galvanometer (the most widely used, including D’Arsonval and Weston types), the ballistic galvanometer (used to measure charge from a brief current pulse), the tangent galvanometer (which uses the earth’s magnetic field), and the mirror galvanometer (which uses a light beam for very sensitive readings).

Galvanometer vs Ammeter vs Voltmeter

A galvanometer is the basic instrument from which both an ammeter and a voltmeter are built. Connecting a small resistance (a shunt) in parallel with a galvanometer converts it into an ammeter that can measure larger currents. Connecting a high resistance in series with a galvanometer converts it into a voltmeter that measures potential difference. A plain galvanometer itself detects only very small currents and deflects both ways from zero.

Uses of a Galvanometer

• Detecting the presence and direction of small currents in a circuit.
• Indicating the null (zero-deflection) point in Wheatstone bridge and potentiometer experiments.
• Acting as the core component in ammeters and voltmeters.
• Use in physics teaching laboratories for current and magnetism experiments.

Galvanometer for School and College Laboratories

Samtech Instruments manufactures laboratory galvanometers for schools, colleges, and institutions across India, supplied to dealers and through GeM tenders. To view specifications or enquire, see our galvanometer product page or contact us on WhatsApp at +91 8683 878 878.

Frequently Asked Questions

What is a galvanometer?

A galvanometer is a sensitive instrument used to detect and measure small electric currents. Its pointer deflects both ways from a central zero, showing the magnitude and direction of the current.

What is the working principle of a galvanometer?

A current-carrying coil placed in a magnetic field experiences a torque. The coil’s deflection is directly proportional to the current flowing through it.

What is the formula for a galvanometer?

The deflecting torque is τ = NIAB, and at equilibrium NIAB = kθ, so the current I is directly proportional to the deflection θ.

What is the difference between a galvanometer and an ammeter?

A galvanometer detects very small currents and deflects both ways from zero. An ammeter is a galvanometer fitted with a low-resistance shunt in parallel so it can measure larger currents.

Why is a soft iron core used in a moving coil galvanometer?

The soft iron core concentrates the magnetic field and makes it radial, which strengthens the field acting on the coil and increases the instrument’s sensitivity.

What are the types of galvanometer?

The main types are moving coil (D’Arsonval/Weston), ballistic, tangent, and mirror galvanometers.