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.

Answer 2

Measuring instruments like galvanometers and ammeters rely on a moving pointer, and different physical quantities influence different aspects of its behaviour. Let's examine what each option in the question is actually responsible for.

1. Mass of the moving part: The mass, or more precisely, the moment of inertia, of the moving system affects how quickly the pointer responds and whether it oscillates, but it does not create any force that pulls the pointer back to zero. A heavier pointer with no other torque acting on it would simply stay wherever it was left.
2. Balancing weight: Balancing weights are used to counter the effect of gravity on the pointer assembly so that the instrument reads correctly regardless of its orientation. This ensures the deflection is not distorted by gravity, but it plays no role in actively restoring the pointer to zero when current is removed.
3. Controlling torque: This torque, usually produced by a spring or by gravity acting on a separate control weight, always acts opposite to the deflecting torque. When the current, and hence the deflecting torque, is switched off, the controlling torque is the only one left acting on the system, and it pulls the pointer back to its zero position.
4. Damping torque: Damping torque opposes the motion of the pointer, not its position, which prevents oscillation and lets the pointer settle quickly. It affects how smoothly the pointer returns, but it is not the reason the pointer heads back to zero in the first place; without a controlling torque, damping alone would just let the pointer settle wherever it happened to be.

Only the controlling torque provides an active restoring force directed at zero deflection once the deflecting torque disappears.

Therefore, the correct answer is controlling torque.