3.11 Moving Coil Galvanometer

Moving Coil Galvanometer (MCG) is a device used to detect small current. With slight modifications it can be used to measure current or voltage. MCG was first devised by Kelvin and later modified by D' Arsonval.

The working principle of MCG is that a freely suspended current carrying coil in a uniform magnetic field experiences a torque proportional to the current through the coil. MCG consist of a multi turn coil free to rotate about a vertical axis in a uniform radial magnetic field. There is a cylindrical soft iron core to increase the sensitivity of MCG.

Working

When a current flows through the coil the maximum torque experienced is T =NIAB and as a result the coil gets rotated. The suspension system of the coil supplies a restoring couple proportional to the angle of twist theta. If c is the torsion constant of the suspension system, then T=ctheta.

At equilibrium the external magnetic torque is balanced by the restoring torque.

Tmagnetic=Trestoring

substituting the value of torque

NABI= ctheta

I=(c/NAB)theta.

The bracket in the above expression is a constant for a given arrangement and is known as galvanometer constant k.

Therefore I=k theta.

By measuring theta for a small known current k can be determined. If k is determined once, then any unknown current can be determined by measuring the angle theta alone.

Rectangular Coil in a
Magnetic Field

Conversion of  Galvanometer
into Ammeter