Principle:

Its working is based on the fact that when a current carrying coil is placed in a magnetic field, it experiences a torque.

Working:

Suppose the coilPQRSis suspended freely in the magnetic field.

Let

l= LengthPQorRSof the coil

b= BreadthQRorSPof the coil

n= Number of turns in the coil

Area of each turn of the coil,A=l×b

LetB= Strength of the magnetic field in which coil is suspended

I= Current passing through the coil in the directionPQRS

Let, at any instant, α be the angle which the normal drawn on the plane of the coil makes with the direction of magnetic field.

The rectangular coil carrying current when placed in the magnetic field experiences a torque whose magnitude is given by,

τ=nIBAsinα

Due to deflecting torque, the coil rotates and suspension wire gets twisted. A restoring torque is set up in the suspension wire.

Letθbe the twist produced in the phosphor bronze strip due to rotation of the coil andKbe the restoring torque per unit twist of the phosphor bronze strip. Then,

Total restoring torque produced =kθ

In equilibrium position of the coil,

Deflecting torque = Restoring torque

∴NIBA = kθ

Or,