What instrument measures the amperage in a circuit?

The instrument that measures the amperage in a circuit is the ammeter. An ammeter is specifically designed to measure the flow of electric current in amperes through a circuit. It must be connected in series with the circuit to allow the current to pass through it, thereby providing a direct measurement of the current flowing in that particular circuit.

While the voltmeter, ohmmeter, and multimeter serve important functions, they do not measure current directly. A voltmeter measures the voltage or potential difference across two points in a circuit, which is essential for understanding voltage levels but does not provide information about current flow. An ohmmeter measures resistance in ohms, which is crucial for assessing components in a circuit, but again, it does not indicate how much current is flowing. A multimeter can perform multiple functions, including measuring current, but it is important to appropriately set it to the amperage measurement mode for accurate readings. However, when specifically asking for the instrument that exclusively measures amperage, the ammeter is the definitive answer.