Working of a Moving Coil Galvanometer

A galvanometer is an electromechanical instrument used to measure electric current. Galvanometers were previously uncalibrated; however, we now have improvised versions of the same known as ammeters. We can measure the flow of current very precisely with the help of modern galvanometers.

In a galvanometer, a deflecting pointer works in response to an electric current flowing through a coil in the presence of a constant magnetic field. Hans Christian Oersted is credited as the first person to observe that a needle can deflect in the presence of a wire having an electric current. 

Over time, galvanometers have proved to be essential in the development of science and technology. They have enabled long-distance communication, and in the biomedical field, allowed for the discovery of electrical activity of the heart and the brain.  

Objectives and Functions

The most commonly observed galvanometer is the D’Arsonval galvanometer. In this instrument, the indicating system comprises a light wire coil suspended from a metallic ribbon located between poles of a permanent magnet. Here, the magnetic field produced by the current flowing through the coil interacts with the magnetic field of the permanent magnet and produces a twisting force (torque). 

The coil is attached to an indicating needle that rotates because of the twisting force. The angle at which the needle rotates to balance helps measure the amount of current flowing through the coil. The angle can be measured by the movement of the needle or by the deflection of a beam of light reflected from a mirror. 

Principle of a Moving Coil Galvanometer

The working of a moving coil galvanometer is dependent on the fact that the current-carrying wire coils experience torque when placed in a uniform magnetic field. This causes the wire placed in the system to rotate. Hence, one can conclude that the deflection in the moving coil galvanometer directly corresponds to the current flowing inside the coil. 

Construction of a Moving Coil Galvanometer

Usually, a moving coil galvanometer system comprises a rectangular coil with numerous turns. These copper wires are adequately insulated and wound over a light metallic frame or ribbon. This copper coil is then suspended between the poles of a horseshoe magnet. 

A fine phosphor-bronze strip is used to suspend the coil between poles of a permanent magnet. This is done by connecting the lower end of the copper coil to a hairspring of phosphor and bronze. On the other hand, the other end of the spring is connected to a binding screw. 

A soft cylinder made of iron is placed inside the copper coil. The magnetic poles of the horseshoe magnet produce a radial magnetic field. The plane of the coil will align parallelly with the magnetic field. We need to place a small plane mirror on the suspension wire along with a lamp and scale to measure the deflection of the coil.  

Working of a Moving Coil Galvanometer

Let us consider PQRS to be a single turn of the coil. Current (I) flows through the coil. In the radial magnetic field, the plane of the coil should always be parallel to the magnetic field. Hence, sides QR and SP are always parallel to the field and do not experience any force. The sides PQ and RS should always be perpendicular to the magnetic field.  

RS = PQ = Length of the coil (l) and PS = QR = Breadth of the coil (b). Force on PQ, F will be equal to BI (PQ) = BI(l). As per Fleming’s left-hand rule, this force will be normal to the plane of the coil and shall act outwards. 

Force on RS, F = BI(RS) = BI(l). This force will be normal to the plane of the coil and will act inwards. These two equal and opposing forces will make a couple and eventually deflect the coil. 

So, if there are n turns in the coil, the moment of force (couple) will be = n BIl – b

Hence, the moment of the deflecting couple will be = nBIA

The suspension wire in the system will twist when the coil deflects. Due to the wire’s elasticity, it will experience a restoring couple force proportional to the twist. If the angular twist is θ , the moment of the restoring couple will be Cθ, where C = restoring couple per unit twist. 

At equilibrium, the deflecting couple = restoring couple

nBIA = Cθ

I = (C/nBA) × θ (here C is the torsional constant of the spring)

Conclusion

A galvanometer is one of the most useful inventions in science and technology. It not only helps identify the flow of current but can also help measure the current flowing inside the wire. Galvanometers are used for positioning and controlling systems in modern automobiles and heavy mechanical equipment.