Before measuring resistance in an unknown resistor in a circuit, what must you do?

To accurately measure resistance in an unknown resistor within a circuit, it is essential to isolate the resistor from the circuit. This process ensures that the measurement reflects only the resistance of the resistor itself without interference from other components in the circuit or any parallel paths that might affect the reading. When the resistor is isolated, it will not have current flowing through it, which is crucial since resistance measurements typically require the component to be de-energized.

Performing this isolation correctly allows for a precise measurement, ensuring that any voltages or currents present in the rest of the circuit do not influence the measurements taken, thus leading to reliable and valid results. Measurements taken on live circuits can give misleading or inaccurate readings due to the presence of voltage, making isolation a fundamental step before testing resistance.