Difficulty: Easy
Correct Answer: All frequencies up to a certain maximum value
Explanation:
Introduction / Context:Moving-iron (MI) instruments operate by the deflection caused due to the magnetic field created by current. Their response depends on inductance and coil impedance, making them frequency-dependent for AC measurement.
Given Data / Assumptions:
Concept / Approach:At low frequencies (including DC), the impedance is nearly resistive. As frequency increases, inductive reactance grows and the current for a given applied RMS voltage decreases, causing errors. Thus, MI instruments remain accurate only up to a certain limiting frequency.
Step-by-Step Solution:
Impedance = √(R^2 + (ωL)^2).At small ω, Z ≈ R, hence accurate.At higher ω, inductive effect increases → error grows.Verification / Alternative check:
Practical MI voltmeters are specified up to ~100–200 Hz for accuracy.Why Other Options Are Wrong:
Low or high frequencies alone are misleading; ”one frequency” is irrelevant; resonance does not apply here.Common Pitfalls:
Assuming MI meters are broadband instruments; they are not.Final Answer:
All frequencies up to a certain maximum value
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