How does an ohmmeter measure the resistance of a circuit or component?

An ohmmeter determines the resistance of a component by relying on Ohm's Law (Resistance = Voltage / Current). It uses an internal battery to apply a known, very small test current through the component, and then it measures the resulting voltage drop across it. By knowing both the applied current and the measured voltage, the meter's internal circuitry easily calculates and displays the resistance. Because the meter supplies its own power to make this measurement, you must always disconnect power from the circuit being tested to avoid damaging the ohmmeter or getting a wildly inaccurate reading.