Final answer:
Ohmmeters must be used on circuits that are de-energized to measure resistance accurately and to prevent damage to the meter or safety hazards.
Step-by-step explanation:
It is true that ohmmeters should only be used on circuits that are de-energized. An ohmmeter is designed to measure the resistance of a component within a circuit. This requires that the component be isolated or removed from the circuit before taking a measurement. Ohmmeters, like other measurement devices, operate according to Ohm's law (R = V/I), which relates voltage (V), current (I), and resistance (R). When working with these instruments, it is essential to ensure the circuit is not live, meaning it should not be connected to a power source with current flowing through it. Performing measurements on a live circuit with an ohmmeter can lead to inaccurate readings and could potentially damage the meter or create a safety hazard.