Which instrument is not appropriate for use with a live circuit?

An ohmmeter is not appropriate for use with a live circuit because it is designed to measure resistance. When you connect an ohmmeter to a circuit that is powered on, it can send a small current through the circuit to determine resistance values, which could potentially damage the circuit, create unsafe conditions, or lead to inaccurate readings. Ohmmeters operate under the assumption that the circuit is de-energized to accurately measure resistance.

In contrast, ammeters, voltmeters, and galvanometers can be designed to measure current and voltage in live circuits without inherently risking damage to the circuit, as long as they are used within the correct parameters and with appropriate safety precautions. Ammeters measure current flowing through a circuit, voltmeters measure the voltage across components, and galvanometers show the presence of small currents. Each of these instruments has its own specific applications, but their design allows for safe use in live situations, unlike the ohmmeter.