Question

The pointer in a current measuring instrument returns to zero position when it is disconnected. This is due to ____.

• A. mass of the moving part
• B. balancing weight
• C. controlling torque
• D. damping torque

Hint:

Think of it like a spring-loaded door. The "deflecting torque" is you pushing the door open, while the "controlling torque" is the spring that pulls the door shut once you let go.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Measuring instruments require specific torques to function correctly: deflecting torque (to move the pointer), controlling torque (to oppose motion and return to zero), and damping torque (to prevent oscillations).

Step 2: Detailed Explanation:
The controlling torque (also called restoring torque) is usually provided by a spring or gravity. Its primary functions are:
• To oppose the deflecting torque so that the pointer stops at a position proportional to the magnitude of the quantity being measured.
• To bring the moving system (pointer) back to the zero position when the deflecting torque is removed (i.e., when the instrument is disconnected from the circuit). Without controlling torque, the pointer would not return to zero and would instead stay at its last deflected position.

Step 3: Final Answer:
The pointer returns to zero due to the controlling torque.