CBSE Class 12-science Answered

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 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 = 90^o 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

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