PMMC-The permanent magnet moving coil instrument is the most accurate type for d.c measurement.The working principal of this instrument is same as that of the d’Arsonval type of galvanometers,the difference being that a direct reading instrument is provide with a pointer and a scale.
Construction of PMMC Instrument- The general constructional features of this instrument are shown
(i) Moving coil- The moving coil is wound with many turns of enamelled or silk covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on fewelled bearings.the coil moves freely in the field of a permanent magnet.
(ii) Magnet systems- In present era developments in materials for permanent magnets and,therefore,magnet assemblies have under gone a lot of change in the recent part. Old style magnet system considered of a relatively long U shaped permanent magnets having soft iron pole pieces. The flux densities used in PMMC,instrument varies from 0.1 wb/meter square to 1 wb/meter square.
(iii) Control- When the coil is supported between two jewel bearings the control torque is provided by the two phosphor bronze hair springs.These springs also serve to lead current in and out of the coil.
(iv) Damping-Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magne.
(v)Pointer and Scale-The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight construction and,apart from those used in some in expensive instrument has the section over the scale twisted to form a fine blade.This help to reduce parallax errors in the reading of the scale.