Question

Draw a labelled diagram of a moving coil galvanometer and explain its working. What is the function of radial magnetic field inside the coil?

Answer 1

Moving coil Galvanometer:

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 coil PQRS is suspended freely in the magnetic field.

Let, l = Length PQ or RS of the coil

b = Breadth QR or SP of the coil

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 coil is suspended

I = Current passing through the coil in the direction PQRS

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 = nIBA sinα

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

Let θ be the twist produced in the phosphor bronze strip due to rotation of the coil and K be 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,I = k/(NBA) theta = Gtheta`

Where, `k/(NBA) = G=a` (Constant for a galvanometer)

It is known as galvanometer constant.

• Current sensitivity of the galvanometer is the deflection per unit current. `therefore phi/I = (NAB)/k`

• Voltage sensitivity is the deflection per unit voltage.

`therefore phi/V =(NAB)/k(1/V) = (NAB)/K 1/R        (thereforeV= IR)`

The uniform radial magnetic field keeps the plane of the coil always parallel to the direction of the magnetic field. That is, the angle between the plane of the coil and the magnetic field is zero in all the orientations of the coil.