When the pointer of an indicating instrument is in the final deflection position, at that time

The pointer of an indicating instrument is in the final deflected position, the damping torque is zero at the steady state position only deflecting torque and controlling torque will be acting and they will be equal and opposite.

Torque in indicating Instrument

For satisfactory operation of an indicating instrument three torque are essentially required, these are as follows:

1.) Deflecting Torque

• This torque produces the required amount of force in the pointer to move from its `zero’ position when the instrument is connected to the supply.
• To obtain this force in an instrument, different effects of electric current are used such as magnetic effect, heating effect, chemical effect, etc.
• Without deflecting torque pointer of an instrument cannot be moved for any value.

2.) Controlling Torque

• Controlling torque is essential to control the movement of the pointer and to ensure that the magnitude of the deflection of the pointer is always the same as the given value of the quantity to be measured.
• The controlling force always acts in opposite direction to the deflecting force. And also return the pointer to its initial zero position when the instrument is disconnected from the supply.
• The controlling force could be produced by any one of the following methods: Gravity control or spring control.

3.) Damping Torque

• This force is necessary to bring the pointer to rest in its final deflected position quickly.
• Without such damping, the combination of deflecting torque and controlling torque makes the pointer oscillate about its final deflected position for some time. It results in a waste of time while taking the reading.
  
  NOTE:
  When the pointer of an indicating instrument is in the final deflection position, the damping torque is zero and only the deflecting and controlling torques are acting. These torques are equal and opposite.