A pointer of an instrument once deflected returns to zero position, when the current is removed due to
Right Answer is:
When the pointer of an instrument once deflected returns to zero position, when the current is removed due to controlling Torque.
Basically, there are three types of Force or torque act upon the Measuring Instrument
- Deflecting Torque
- Controlling Torque
- Damping Torque
Deflecting Torque: In order to move the pointer from its zero position on the scale deflecting torque is required. The deflecting torque works on the moving system to which the pointer is attached. Obviously, the magnitude of deflecting torque produced is proportional to the magnitude of the quantity being measured, say, the current I flowing through the instrument. A deflecting torque is required to overcome the inertia, damping effect, and controlling effect of the moving system.
This deflecting torque can be produced by any of these effects of current (or of voltage) such as
- Magnetic effect
- Electrostatic effect
- Electromagnetic effect
- Thermal effect
- Chemical Effect
Controlling Torque: With the help of the deflecting torque, the pointer deflection will take place on the calibrated scale but to stop the pointer at the definite position, controlling torque (Tc.) comes into action. As the deflection of the pointer increases, the controlling torque also increases and stops the pointer at the measured value.
Controlling torque is also known as restoring torque i.e., it brings back the pointer to its zero position when deflecting torque is withdrawn. The pointer attains a steady position when controlling torque becomes numerically equal to deflecting torque i.e., Tc = Td
Spring control and gravity control are used for controlling torque.
Damping Torque: At the final deflected position, when the deflecting and controlling torques are equal, the pointer starts oscillating owing to its inertia and therefore, cannot immediately settle at its final deflected position. If no extra force is provided to damp these oscillations, the moving system will take considerable time before coming to settle to the final deflected position. This is especially undesirable if the number of readings to be taken is quite large.