Match the following instrument types with their characteristic scales or uses. List I (Instrument) List II (Characteristic) A. PMMC (Permanent-Magnet Moving-Coil) 1. Square-law type scale B. Moving-iron meter 2. Very good frequency response (wideband thermal sensing) C. Thermocouple meter 3. Linear scale (uniform deflection) D. Electrostatic instrument 4. Voltmeter (high-voltage measurement)
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AA-4, B-1, C-2, D-3
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BA-3, B-2, C-1, D-4
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CA-4, B-2, C-1, D-3
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DA-3, B-1, C-2, D-4
Answer
Correct Answer: A-3, B-1, C-2, D-4
Explanation
Introduction / Context:This matching question checks core instrumentation knowledge: how common meters behave and which scales they naturally produce. Recognizing these pairings helps you select the right instrument in labs and exams.
Given Data / Assumptions:
- PMMC meters respond to average of rectified current and feature linear torque with current in their coils.
- Moving-iron meters exhibit a force proportional to I^2, leading to a non-linear (square-law) scale.
- Thermocouple (thermal) meters convert RF power to heat, then to DC via a thermocouple, enabling wideband response.
- Electrostatic instruments use electrostatic attraction, ideal for high-voltage measurements (as voltmeters).
Concept / Approach:
Map each physical principle to its signature measurement behavior: linearity for PMMC, square-law for moving-iron, wide frequency response for thermocouple, and voltage measurement for electrostatic devices.
Step-by-Step Solution:
PMMC → torque ∝ I, hence a linear scale ⇒ A-3.Moving-iron → deflecting force ∝ I^2, giving a square-law scale ⇒ B-1.Thermocouple meter → responds to heating effect of current, excellent RF response ⇒ C-2.Electrostatic instrument → measures voltage via electrostatic force ⇒ D-4.Verification / Alternative check:
Lab manuals list PMMC as the preferred DC ammeter/voltmeter for linear scales; moving-iron meters are recommended for AC with non-linear scales; thermal meters are rated for RF; electrostatic voltmeters are standard for high-voltage DC/AC.
Why Other Options Are Wrong:
- Any mapping that assigns linear scale to moving-iron conflicts with I^2 dependence.
- Assigning “wide response” to PMMC ignores frequency limits due to coil inductance and inertia.
- Electrostatic instruments as current meters are impractical; they are voltage-oriented.
Common Pitfalls:
Confusing thermocouple meters with PMMC because both involve DC readout; the key difference is the thermal sensing element that enables RF measurement.
Final Answer:
A-3, B-1, C-2, D-4