This type of instrument is based
on the principle that when a current carrying conductor is placed in a magnetic
field, a mechanical force acts on the conductor (Electrodynamic effect).
Controlling torque is provided by two hair springs placed
above and below the coil.
Damping torque is provided by eddy currents induced in a
metal former or core on which the coil is wound or in the circuit of the coil
itself.
When a current is passing through the coil windings, a
torque in developed on the coil by the interaction of the magnetic field and
the field set up by current in the coil. The aluminium pointer attached to the
moving coil moves over a graduated scale.
The pointer returns to its original position when there is no current flowing
through the coil. If the current in the coil is reversed, the deflecting torque
also gets reversed since the direction of the field of the permanent magnet is
same.
- Since the deflecting torque reverses with reversal of
current in the coil, PMMC instruments are used to measure direct current
and voltage only.
