Essential Requirements of an instrument

       In case of measuring instruments, the effect of unknown quantity is converted into a mechanical force which is transmitted to the pointer which moves over a calibrated scale. The moving system of such instrument is mounted on a pivoted spindle. For satisfactory operation of any indicating instrument, following system must be present in an instrument.

1) Deflecting system producing deflecting torque T_d  

2) Controlling system producing damping torque T_c  

3) Damping system producing damping torque.