Instrument loading in measurements: On which selectable voltage range will a voltmeter place the minimum load on the circuit under test (i.e., present the highest internal resistance)?

Introduction / Context:
When measuring voltage, the voltmeter's internal resistance should be high to minimize loading and avoid altering the circuit's true operating point. Many meters (especially analog and some digital ranges) change their internal resistance when you change the range. Understanding which range yields the highest input resistance helps preserve measurement accuracy.

Given Data / Assumptions:

• A voltmeter with selectable ranges: 1 V, 50 V, 500 V, and 1,000 V.
• Typical meter behavior: higher voltage ranges correspond to higher internal resistance (or higher ohms-per-volt rating for analog meters).
• Aim: minimize loading on the circuit under test.

Concept / Approach:
Circuit loading is minimized when the meter's input resistance is as large as possible compared to the source/network being measured. On most instruments, the highest range provides the highest input resistance, thus the smallest current draw from the circuit.

Step-by-Step Solution:

Verification / Alternative check:
Analog meters: input resistance ≈ (ohms per volt) * range. Thus, higher range → higher resistance. Many modern DMMs keep roughly constant high input resistance on DC (e.g., 10 MΩ), but on selectable high-voltage ranges certain models effectively apply attenuators, still leading to equal or higher effective input resistance and the least loading at the highest range.

Why Other Options Are Wrong:

• 1 V, 50 V, 500 V: Lower ranges present equal or lower input resistance relative to 1,000 V on many instruments, causing greater loading.

Common Pitfalls:

• Assuming all meters have identical input resistance on every range.
• Confusing current range behavior (low shunt resistance) with voltage range behavior (high input resistance).

Final Answer:
1,000 V