Measurements & Instruments MCQ – 18 previous year questions with solutions

B — Moving-iron instrument. Its deflecting torque is proportional to the square of current (T ∝ I²), so it responds the same way to AC and DC.

B — T_d = ½ I² (dL/dθ). The stored magnetic energy is ½LI²; differentiating it with respect to the deflection angle gives the torque.

A — Two. A repulsion-type MI instrument has two soft-iron pieces — one fixed and one moving — magnetised with the same polarity so they repel.

Moving-iron instruments use air-friction damping. Eddy-current damping needs a permanent magnet, whose field would disturb the MI instrument’s own operating field.

A control spring produces restoring torque proportional to deflection: T_C = Kθ. At the final reading, deflecting torque equals controlling torque.

C — A stationary coil with many turns. The measured current flows through this fixed multi-turn coil and creates the magnetic field that acts on the soft-iron vane.

A — Provides controlling torque. The spring produces a restoring torque that opposes the deflecting torque, so the pointer settles where the two balance.

A — The reed tuned to 50 Hz. The reed whose natural frequency equals the applied frequency goes into resonance and vibrates the most.

A — Two inductors with mutual coupling. Mutual inductance exists only when the two coils are magnetically coupled, which the Carey Foster bridge balances against a standard capacitor and resistors.

Hay’s bridge is designed for high-Q inductors (Q = ωL/R large, typically above about 10).

B — Non-inductive resistor. Hay's bridge uses non-inductive resistors in its standard arms along with a standard capacitor, so no stray inductance disturbs the balance.

A frequency counter estimates frequency by counting input cycles during a fixed gate/sampling interval. A longer interval counts more cycles and therefore improves frequency resolution and precision; the basic count-to-frequency relation is f = N/T_g.

D — Impedance. An LCR meter measures impedance and the related inductance, capacitance and resistance (often with Q and D factors).

C — Damping System. In the conventional single-phase induction energy meter, the four named systems are driving, moving, braking and registering. A separate damping system is not included in this classification.

A — Power factor. A tri-vector meter records kWh, kVARh and kVAh/maximum demand; in this PYQ's framing, power factor is a derived quantity rather than a directly metered vector.

Rise times combine by root-sum-square, so t_signal = √(10² − 6²) = √64 = 8 ns.

D — “Sensitivity is inversely proportional to the gain” is the incorrect statement under the question’s deflection-per-volt definition of sensitivity. Increasing vertical gain increases screen deflection for the same input voltage.

For a balanced three-phase load, W₁+W₂=P=10 kW and tanφ=√3(W₁−W₂)/(W₁+W₂). With cosφ=0.9, φ≈25.84°; solving gives W₁≈6.40 kW and W₂≈3.60 kW, nearest to 6350 W and 3650 W.