Galvanometer – Construction, Working, and Applications of Galvanometer

Definition

The galvanometer is an electrical device that is used to detect the passage of current through a circuit. It is regarded as the historical name of moving coil, used to detect electric current.

History
• Hans Christian Ørsted

In 1820, Danish physicist and chemist Hans Christian Ørsted first discovered the instrument to observe and calculate current to produce magnetic fields. Its concept was taken from the compass needle which shows deflection near the current.

• Johann Schweiger

Johann Schweiger who was the German mathematician and physicist assembled the first galvanometer and named it multiplier.

• André-Marie Ampère

André-Marie Ampère, a French physicist gave the mathematical expression for Hans Christian observation and called it “Luigi Galvani”.

Construction of Galvanometer
Moving Coil

The moving coil is the current-carrying part of the galvanometer. It is rectangular or round and has a number of turns of copper wire. The coil is freely moved about its upright axis of symmetry between the poles of a permanent magnet.

The iron core offers the reduced reluctance flux path and thus supplies the strong electromagnetic field for the coil to move in.

Suspension

The coil is held by a flat ribbon which carries the current to the coil. The other current-carrying coil is the reduced suspension whose torque result is negligible.

The upper suspension coil is composed of gold or copper wire which is made in the form of a bow. The strength of the wire is not really strong, and therefore the galvanometers handle it meticulously without any jerks.

Mirror

The suspension carries a little mirror that casts the beam. The beam of light is positioned on the scale on which the deflection is gauged.

The torsion head is made use of for managing the placement of the coil as well as for adjusting the zero setting.

Working of Galvanometer

When current passed through the coil, a couple; two equal and opposite forces acting on two different points on a body, act on the coil, and the coil tends to rotate. This is known as deflecting couple.

Deflecting Torque

If the number of turns of the coil is N and A is the area then the torque due to deflecting couple is given by

τ = NIBA cos α

where I, is the current and B is the density of flux.

Since the field is made radial by making pole of magnet concave, therefore the plane of the coil is always parallel to the field, so α = 0ᵒ

τ = NIBA cos 0ᵒ

τ = NIBA cos (1)

τ = NIBA

Restoring Torque

As the coil rotates under the action of deflecting torque, the suspension wire is twisted which gives rise to the torsional couple which tends to untwist the suspension wire and restore the original position. This is called restoring couple.

The torque produced by restoring couple is proportional to the angle of deflection θ as long as the suspension wire obeys Hook’s Law,

Restoring torque θ

Restoring torque = C θ

Here C is the constant of proportionality called the torsional constant. It depends on the nature of the suspension wire.

Equilibrium State

When the deflecting couple balances the restoring couple then the coil will come to rest. So, in equilibrium

Since,

C / NAB = constant

Current passing through the coil is directly proportional to deflection θ.

Applications
• Used to measure current
• Determination of voltage between two points in the circuit
• Detects null point of the circuit
• Modern uses include detection of errors in telecommunication cables
• Open-loop galvanometers are used in space systems, imaging, bar code scanners, etc.

MCQs:

• What is a galvanometer used for?
• A) To measure voltage
• B) To detect the passage of current
• C) To generate magnetic fields
• D) To regulate resistance
• Answer: B) To detect the passage of current
• Who first discovered the instrument that later became known as a galvanometer?
• A) André-Marie Ampère
• B) Hans Christian Ørsted
• C) Johann Schweiger
• D) Luigi Galvani
• Answer: B) Hans Christian Ørsted
• Which part of a galvanometer is responsible for detecting current?
• A) Mirror
• B) Suspension
• C) Moving Coil
• What does the suspension in a galvanometer do?
• A) Provides a reduced reluctance flux path
• B) Holds the coil and carries current to it
• C) Carries a mirror for deflection measurement
• D) Controls the position of the coil
• Answer: B) Holds the coil and carries current to it
• Which physicist gave the mathematical expression for the observations made by Hans Christian Ørsted and named it after Luigi Galvani?
• A) André-Marie Ampère
• B) Johann Schweiger
• C) Hans Christian Ørsted
• D) Luigi Galvani
• What type of torque tends to rotate the coil in a galvanometer when current passes through it?
• A) Deflecting Torque
• B) Restoring Torque
• C) Equilibrium Torque
• D) Torsional Torque
• In a galvanometer, what does the restoring torque do?
• A) It rotates the coil under the action of deflecting torque
• B) It balances the deflecting torque to bring the coil to rest
• C) It increases the deflection angle of the coil
• D) It provides resistance to the passage of current
• Answer: B) It balances the deflecting torque to bring the coil to rest
• What condition is required for a galvanometer to be in equilibrium?
• A) Deflecting torque exceeds restoring torque
• B) Restoring torque equals deflecting torque
• C) Deflecting torque is zero
• D) Restoring torque is zero
• Answer: B) Restoring torque equals deflecting torque
• What is the application of a galvanometer in detecting errors in telecommunication cables?
• A) Measuring voltage
• B) Determining current
• C) Finding null points in circuits
• D) Modern uses
• Which component of a galvanometer is used for adjusting the zero setting?
• A) Moving Coil
• B) Suspension
• C) Mirror
• How is the torque due to deflecting couple calculated in a galvanometer?
• A) τ = NIBA cos α
• B) τ = NIBA cos θ
• C) τ = NIBA sin α
• D) τ = NIBA sin θ
• Answer: A) τ = NIBA cos α
• What determines the strength of the wire used in the upper suspension coil of a galvanometer?
• A) Torsional constant
• B) Deflecting torque
• C) Restoring torque
• D) Material and form of the wire
• Answer: D) Material and form of the wire
• What does the mirror in a galvanometer do?
• A) Reflects light for measurement
• B) Holds the coil
• C) Provides a flux path
• D) Adjusts the zero setting
• Answer: A) Reflects light for measurement
• Which physicist named the first galvanometer “multiplier”?
• A) Hans Christian Ørsted
• B) Johann Schweiger
• C) André-Marie Ampère
• D) Luigi Galvani
• What property of the suspension wire ensures that the galvanometer works without jerks?
• A) Torsional constant
• B) Material composition
• C) The strength of the wire
• D) Compliance with Hook’s Law
• Answer: D) Compliance with Hook’s Law
• In a galvanometer, when does the coil come to rest?
• A) When deflecting torque exceeds restoring torque
• B) When restoring torque exceeds deflecting torque
• C) When both torques are equal
• D) When there is no current passing through the coil
• Answer: C) When both torques are equal
• What does the torsion head control in a galvanometer?
• A) Position of the coil
• B) Deflection angle
• C) Restoring torque
• D) Torsional constant
• Answer: A) Position of the coil

Summary:

In this tutorial, we dived into the construction, working principles, and applications of galvanometers. Initially, we defined a galvanometer as an electrical device utilized to detect current flow within a circuit. Its historical name, “moving coil,” highlights its primary function. The tutorial then explored the rich history of the galvanometer, tracing back to pioneers like Hans Christian Ørsted, Johann Schweiger, and André-Marie Ampère, who laid the groundwork for its development.

Moving on to construction, we examined the key components of a galvanometer: the moving coil, suspension system, mirror, and torsion head. These components work in tandem to facilitate accurate current detection. The tutorial elucidated the intricate workings of a galvanometer, elucidating concepts such as deflecting torque and restoring torque, crucial for understanding its functionality. We also discussed the equilibrium state reached when these torques balance each other, allowing the coil to come to rest.

Finally, we explored various applications of galvanometers, ranging from measuring current and voltage in circuits to detecting errors in telecommunication cables and serving modern uses in space systems and imaging technology. This comprehensive tutorial provides a solid foundation for understanding galvanometers and their significance in electrical engineering and technology.

Further Reading:  Forms of Energy & Law of Conservation of Energy