ELECTRODYNAMIC OR DYNAMOMETER TYPE INSTRUMENTS
It is very similar to PMMC instruments except that the permanent magnet field is replaced by coils (usually 2 fixed air cored coils) which carry current to be measured. The coils are usually air cored to avoid hysteresis, eddy currents and other errors.
Deflecting torque is produced by interaction of magnetic fields. One field is due to current in fixed coil and other field is due to current in moving coil.
