State principle of moving coil galvanometer?

Answer 1

Principle of Working:

A Moving Coil Galvanometer works on the Electromagnetic effect of current carrying coil (free to move) being placed in an air-gap between the soft iron core and the strong pole pieces of the magnet.

The current following in the moving coil which is the specimen to be sensed or measured results in a generation of a Magnetic Field which is opposite to that of the fixed magnetic pole pieces. And as like poles attract each other results in a generation of small deflecting torque which further deflects the pointer or the mirror attached to the axis of the coil.

Construction:

Moving Coil:

The Moving coil which is free to move is being suspended from a soft metallic (non-magnetic) strip carrying the current specimen which is to be sensed or measured. The Moving coil is being suspended in the air-gap available between the fixed Soft Iron-Core and the fixed pole pieces of the magnet.

The Soft Iron Core is used to provide a path of low reluctance so as to focus the magnetic field generated by the fixed magnetic pole pieces towards the moving coil. The use of low reluctance Soft Iron-Core helps to increase the sensitivity of the Galvanometer as well.

In many of the Moving Coil Galvanometer Construction Soft Iron-Core pieces are removed and this results in the reduction of the sensitivity of the Galvanometer as a whole. In absence of the soft fixed iron core in the middle of the coil the coil then is narrowed in order to reduce the free air space, this narrowing of the coil results in the reduction of the Moment of Inertia(MI) of the moving coil arrangement. This results in having a good near to critical damping with very less oscillations.

The Moving coil consists of a large number of turns wounded on a former which can either be in rectangular or circular in shape. The current carrying coil is made up of a thin non magnetic metal so as to have low resistance and less mass.

Damping:

Damping in Moving Coil Galvanometer is provided by an Eddy current damping mechanism. The damping is made as such to have a nearly critical damping by setting the device to be between under-damped and critical damped case.

Controlling Torque:

Spring Control mechanism is largely utilized to provide the suitable controlling torque.