PMMC 

 Working:

                     When instrument connected in circuit to measure current or voltage, the operating current flows through coil. When the current is passes through the coil, a deflecting torque is produced due to the reaction between permanent magnet field and magnetic field of the coil. This torque is proportional to the value of current in the coil.

         Due to this torque the coil moves and the pointer deflects over the calibrated scale. When the coil moves, the springs are twisted, thus the spring offer a controlling torque due to spring is directly proportional to the deflecting of the moving part.

             When the deflecting and controlling torque are equal, the pointer comes to rest at its final position, hence ∅ proportional to I, Since the deflection(∅) is directly
proportional to the current (I), these instruments have uniform scale.