Moving Coil Galvanometer

Have you ever thought about how the utility company knows about the power your house has used? The technique behind knowing power consumption is that it uses an electric metre. 

The galvanometer is an instrument used to determine the presence, direction, and strength of an electric current in a conductor. A galvanometer is a type of ammeter which determines current by detecting and measuring electric current.

Moving coil galvanometer

A moving coil galvanometer is an electromagnetic device that can measure even very small values of current. 

Essentially, a galvanometer consists of such a needle attached to a coil set up so that the coil is allowed to pivot freely within a magnetic field. These magnetic fields are created by the poles of one or more permanent magnets.

Principle of moving coil galvanometer

A moving coil galvanometer toils on the principle that a current-carrying coil placed in a magnetic field experiences a torque. The coil springs down through the radial field, ensuring the deflection is proportional to the strength of the current.

Function of galvanometer

We take a coil with many turns and place it in a very strong magnetic field. As more turns mean the current is large, and due to this, the torque will be high. 

Similarly, the stronger the magnetic field, the more torque produced.

The small current that needs to be detected by the galvanometer is sent to the coil. A torque acting on the coil would rotate the coil.

Now, this coil keeps on rotating.

How do we count the number of deflections it makes? With the use of a pointer and a scale, we would be able to get the deflection of the coil.

Hence, the more the current, the more is the torque. This way, the deflection (rotation) would be more, and a galvanometer would show high deflection.

Construction of galvanometer

It consists of an oblong coil of an outsized number of turns of thinly insulated copper wire wound over a lightweight metallic frame. 

The coil is hung between the pole pieces of a horseshoe magnet. The lower end of the coil is connected to a hairspring of bronze, having only a couple of turns.

The other part of the spring is attached to a binding screw. An iron cylinder in shape is placed symmetrically inside the coil.

The hemispherical magnetic poles produce a radial magnetic flux during which the plane of the coil is parallel to the magnetic flux altogether its positions. 

A plane mirror attached to the suspension wire is working alongside a lamp and scale arrangement to live the deflection of the coil.

Working of a galvanometer

Suppose PQRS to be a single turn of the coil. A current flows through the coil. In a radial magnetic flux, the plane of the coil is usually parallel to the magnetic flux. 

Thus, the sides QR and SP are parallel to the field. Because of this, they do not experience any force. The sides PQ and RS are always perpendicular to the sector.

PQ = RS = l, length of the coil 

and PS = QR = b, breadth of the coil.

Force on PQ, 

F = BI (PQ) = BIl 

By Fleming’s left-hand rule, this force is normal to the plane of the coil and acts outwards.

Force on RS, 

F = BI (RS) = BIl

The force acting over here is normal to the plane of the coil and acts inwards. 

If there are n turns within the coil, the instant of the deflecting couple = n×BIl × b

Hence the instant of the deflecting couple = nBIA

On account of elasticity, a restoring couple is about up within the wire. 

This couple is proportional to the twist. If θ is that the angular twist, then, the instant of the restoring couple = Cθ, 

where C is the restoring couple per unit twist.

At equilibrium, 

nBIA=CӨ

Hence we can write, 

nBIA= CӨ

I = (CnBA)Ө

where C is the torsional constant of the spring, i.e. the restoring torque per unit twist.

A pointer attached to the spring indicates the deflection θ on the size.

Sensitivity of moving coil galvanometer

The sensitivity of a galvanometer can be defined as the ratio of the change in deflection of the galvanometer to the change in current. Therefore, 

Sensitivity =dӨdi

Even for a small current, if a galvanometer shows deflection, then it is a sensitive galvanometer.

The current in moving coil galvanometer is

 I = (CnBA) × θ

Therefore, 

θ = (nBAC) × I

Differentiating on both sides wrt I, 

we get,

 dӨdi = (nBAC)

Conclusion:

This article explains the moving cell galvanometer, its functioning, construction and working. A galvanometer is an instrument used to determine the presence, direction, and strength of an electric current in a conductor. Moving coil galvanometer is an electromagnetic device that can measure even very small values of current. A moving coil galvanometer toils on the principle that a current-carrying coil placed in a magnetic field experiences a torque. The coil springs down through the radial field, ensuring the deflection to be proportional to the strength of the current. The sensitivity of a galvanometer can be defined as the ratio of the change in deflection of the galvanometer to the change in current.