The Moving Coil Galvanometer: Principle, Current Sensitivity and Construction

Moving coil galvanometer is an electromagnetic instrument used to measure an electric current of a small circuit and is conducted to detect the magnitude of the current. It is constructed with coil, permanent horse-shoe magnet, soft iron core, pointer, pivoted spring, non-metallic frame, scale. A galvanometer is a type of ammeter. A current when passing through the circuit, makes the magnetic needle to turn at right angles to the circuit for its direction is collateral to the lines of induction around the circuit and its north pole points in the direction in which these lines of induction flow. 

Read More: Moving Charges and Magnetism Important Questions

Key Takeaways: Moving Coil Galvanometer, Ammeter, Electromagnetism, Current Coil, Magnetic field, Electric current, Induction, Magnet, Induction flow

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Principle of Galvanometer

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Galvanometer transforms electrical energy into mechanical energy. When current passes through the circuit, it produces a magnetic field then it experiences a magnetic torque or twisting force. A basic galvanometer is a coil consisting of two leads coming out to a copper plate and a Horseshoe magnet over the top, when a voltage is connected to the plates the coil becomes an electromagnetic field. This magnetic field will be attracted to the magnet. A galvanometer produces a magnetic field which experiences a torque.

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Parts of Galvanometer

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• Horseshoe magnet: A powerful cylindrical magnet used to provide a radial magnetic field.
• Coil: The coil consists of a large number of turns of thinly insulated copper wire wound over a light metallic frame.
• Spring: The spring develops a restoring torque while the coil deflects.
• Pointer: The pointer moves along the arc of the scale.

Read More: Earth’s Magnetic field

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Construction of Galvanometer

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Galvanometer is comprised of rectangular coil of a large number of turns of thinly insulated copper wire wound over a light metallic frame. The coil is ceded between the pole pieces of a horseshoe magnet by a fine phosphor – bronze strip from a movable circumambulation head. The lower end of the coil is attached to a hairspring of phosphor bronze having only a few turns and the other end of the spring is attached to a binding screw. The current enters and leaves the coil through the spring fibre.

For settling the coil in the position that we want, the upper part of the fibre is attached to a rotating screw. Concerning the shape of the plane the magnet generates a radical magnetic field for which the coil rotates in any position and the plane is parallel to the direction of the magnetic field. Inside the coil there is a soft iron cylinder that is symmetrically placed. The horse-shoe magnet has hemispherical magnetic poles which produce a magnetic field. A concave mirror is placed to the suspension wire to measure the deflection of the coil using a lamp and scale arrangement.

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Principle of Galvanometer

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Consider a single turn of the coil is PQRS whose length is l and breadth is b. Current I is flowing through the rectangular coil. The permanent horse-shoe magnet generates a magnetic field. As the coil is always parallel to the magnetic field in a radial magnetic field then QR and SP are always parallel to the field. They don’t experience any force and the PQ and RS are up force to the magnetic field.

PQ=RS=l (l = length of the rectangular coil)

PS = QR = b (b= breadth of the rectangular coil)

Force experienced on each side is, F=BII

According to Fleming's left-hand rule, force are equal though opposite in the direction, work normal to the plane and act externally. As these forces are equal and opposite, they produce torque.

Torque = Force × upright distance between the forces

τ = F × b

τ = BI l × b

τ = BI A ( as l b = A area of the coil)

τ = n BIA ( If the coil has ‘n’ turns)

As the torque works then the coil rotates through an angle θ. The coil’s rotation produces a twist which generates a torque and that is proportional to the deflection of θ.

τ = θ

τ = k θ (k is the restoring torque per unit twist)

When the coil comes to equilibrium the restoring torque will balance the deflecting torque.

Deflecting torque = restoring torque

n BIA = k θ

I = ( k / nBA) θ

The amount is constant in the bracket,

I = θ

Therefore in a moving coil galvanometer current is directly proportional to the angle of the deflection of the coil.

Read More: Magnetic Field due to current element

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Advantages and Disadvantages of Galvanometer

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Advantages:

• When the value of K decreases, the value of n, B, A increases so the sensitivity increases because of this.
• Very accurate and reliable.
• Not affected by the magnetic field.

Disadvantages:

• Sensitivity scale can not be changed by will.
• Change in temperature causes change in restoring torque.

Read More: Electrostatic Potential

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Things to Remember

• Moving coil galvanometer is an electromagnetic instrument used to measure an electric current of a small circuit and is conducted to detect the magnitude of the current.
• Galvanometer transforms electrical energy into mechanical energy.
• The different parts of a galvanometer are Horseshoe magnet, Coil, Spring and Pointer.
• A basic galvanometer is a coil consisting of two leads coming out to a copper plate and a Horseshoe magnet over the top, when a voltage is connected to the plates the coil becomes an electromagnetic field.
• A galvanometer produces a magnetic field which experiences a torque.

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