working principal of pmmc instrument with a neat sketch

pmmc(permanent magnet moving coil)

pmmc instrument is used only for D.C measurements.the working of a pmmc instrument is similar to the working of a D'arsonal galvanometer but the difference is that direct reading can be made in pmmc instruments with the help of a pointer and scale.the schematic of pmmc is shown in the following figure

as it consists of a permanent magnet and coil which is moving. it is termed as permanent magnet moving coil(pmmc)

working

this principal of working is same as that of working of a d.c motor. whenever, a current carrying coil is kept in the magnetic field,it experiences a force,and it tends to move.when the current to be measured is made to flow through the coil,a deflecting torque proportional to the flux density and dimensions of the coil is produced which makes the coil to move.as the coil moves,the pointer shows deflection on the scale .the deflection torqueis proportional to the coils current and the magnitude ofthe current flowing through the coil is indicated by a pointer which moves over a graduated scale.when the deflecting torque is equal to the controlling torque,a balance is attained and the pointers shows zero deflection.here,as the deflection is directly proportional to the current in the coil,uniform scale is obtained