Question

With the help of a neat and labelled diagram, explain the principle and working of a moving coil galvanometer ?

Answer 1

Principle:

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

Working:

Suppose that a coil PQRS is suspended freely in a magnetic field.

Let l = length PQ or RS of the coil;

b = breadth QR or SP of the coil and

n = number of turns in the coil

Area of each turn of the coil, A = l × b

Let B = strength of the magnetic field in which the coil is suspended.

and I = current passing through the coil in the direction PQRS

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

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

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

Let θ be the twist produced in the phosphor bronze stripe due to the rotation of the coil and let k be the restoring torque per unit twist of the phosphor bronze stripe.
Then total restoring torque produced = kθ

In equilibrium position of the coil, the deflecting torque is equal to the restoring torque.

∴ NIBA = kθ

⇒ `I = k/(NBA) Θ` 0r GΘ

Here,`k/(NBA) = G= a`

 [constant for a galvanometer]

It is known as the galvanometer constant.