Question
Wed January 19, 2011 By: Jaspreet Singh

can you plz explain the working and theory of moving coil galvanometer

Expert Reply
Wed January 19, 2011
Dear student

Principle

A current carrying conductor placed in a magnetic field experiences a torque.

Construction

It consists of N turns of coil PQRS wound over a non-magnetic metallic frame usually rectangular or circular. The coil is suspended from a torsion head H by means of phosphor bronze strip in a magnetic pole by pieces N and S. A mirror M is attached to the phosphor bronze strip. The other end of the coil is connected to a hairspring, which in turn is connected to an external terminal.

The concave or cylindrical pole pieces ensure that the plane of coil is parallel to B thus (cosq = 1 and sina = 1) in the expression t = NIBA cosq and t = NIBA sina .

The whole arrangement is enclosed in non-magnetic case, which is provided with leveling screws. The torsion head is connected to terminal T1. The galvanometer can be connected to the circuit through terminals T1 and T2.

Working

When a current is passed through the coil in the direction PQRS, then the coil experiences a torque

t = NIBA sina

Since the magnified field is radial the plane of the coil is parallel to the magnetic field such that a = 90o and hence t = NIBA.

The coil rotates and the phosphor bronze strip gets twisted. As a result a restoring torque comes into play trying to restore the coil back to original position.

If f be the twist produced in the strip and C be the restoring torque per unit twist then the restoring torque = Cf.

In equilibrium,

 

 

where G is the galvanometer constant.

This shows that galvanometer has a linear scale and detects the presence of current.

 

Hope this helps

Regards

Team

topperlearning

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