Question 60 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is A ### Explanation of Why Option A (DC voltmeter) is Correct A DC voltmeter is designed to measure voltage across a load. The fundamental component of many DC voltmeters is a sensitive meter movement (like a d'Arsonval movement), which is essentially a very sensitive DC ammeter designed to handle only small currents. To convert this movement into a practical voltmeter, it must be capable of measuring much higher voltages without drawing excessive current or damaging the movement. This is achieved by placing a **high resistance multiplier (or internal resistance)** in **series** with the meter movement. The purpose of this series resistance is twofold: 1. **Limit Current:** It limits the amount of current flowing through the sensitive meter movement to a value that corresponds to the full-scale voltage being measured, preventing damage. 2. **Voltage Drop:** It ensures that most of the voltage being measured drops across the multiplier resistor, while only a small, proportional voltage drops across the movement itself. ### Explanation of Why Other Options are Incorrect **B) AC frequency meter:** An AC frequency meter (especially a digital or analog type based on tuned circuits or counting) does not use a simple series resistance to modify a basic meter movement for its primary function. Frequency measurement relies on timing circuits, shaping circuits, or resonant components, not a series multiplier resistance attached directly to a movement for scaling voltage. **C) DC ammeter:** A DC ammeter is designed to measure current. To measure large currents using a sensitive meter movement, a **low-value shunt resistance** is placed in **parallel** (not series) with the meter movement. The shunt bypasses most of the current, allowing only a small, proportional fraction to flow through the movement. **D) AC ammeter:** An AC ammeter typically uses a current transformer (CT) to step down the current, or it may use a rectifier circuit followed by a DC meter movement. If it uses a simple meter movement, it will also utilize a **shunt resistor in parallel** (similar to the DC ammeter) to extend the range, or a CT to scale the input, but not a high series resistor for the primary purpose of range extension as found in a voltmeter.