Ohmmeter principle of operation: Evaluate the statement: “An ohmmeter uses an internal battery to measure the resistance of a resistor.”

Introduction / Context:Resistance cannot be measured directly without applying a stimulus. Both analog and digital ohmmeters contain an internal source to drive a small current through the device under test, then infer resistance from the measured voltage or current. Recognizing this principle explains common cautions such as “never measure resistance on a live circuit.”

Given Data / Assumptions:

• Device: ohmmeter function, whether stand-alone or inside a digital multimeter.
• Unknown resistor is passive and disconnected from external power.
• Standard two-wire measurement method is used.

Concept / Approach:An ohmmeter sources a known current I_test (or applies a known voltage V_test) from its internal battery. It then measures the corresponding voltage V_DUT across the resistor or the current through it and computes R using R = V_DUT / I_test or R = V_test / I_DUT. The internal battery is essential for this stimulus. Consequently, if the battery is weak, resistance readings drift or the meter indicates “low battery.”

Step-by-Step Solution:

Verification / Alternative check:User manuals specify open-circuit test voltages and test currents for various ranges (for example, 0.3 V test on sensitive ranges). This confirms the use of an internal source derived from the meter’s battery.

Why Other Options Are Wrong:

• Incorrect: Conflicts with the fundamental measurement method.
• Only true for analog / only above 1 kΩ / depends on lead capacitance: Modern digital multimeters also use internal sources; lead capacitance is irrelevant to static resistance measurement.

Common Pitfalls:Attempting to measure resistance in-circuit with power applied (backfeeds the meter); measuring components in parallel without isolating them, which lowers the apparent resistance.

Final Answer:Correct