What does an ammeter measure?

An ammeter is a device specifically designed to measure the rate of current flow in an electrical circuit. Current, measured in amperes (often abbreviated as amps), reflects the flow of electric charge over a given period of time. An ammeter is connected in series within the circuit so that it can measure all the current flowing through that part of the circuit. This measurement provides essential information for troubleshooting and designing electrical systems, ensuring they operate safely and efficiently.

The other options refer to different electrical concepts: voltage pertains to the potential energy per unit charge (measured by a voltmeter), resistance pertains to the opposition to current flow in a circuit (measured by an ohmmeter), and magnetism relates to magnetic fields which are distinct from electrical measurements. Therefore, the focus of the ammeter's function makes it clear that the correct answer is the rate of current flow.