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)

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:

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