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An ammeter is the instrument specifically designed to measure current in an electrical circuit. It is connected in series with the circuit to ensure that all the current flowing through the circuit also passes through the ammeter, allowing it to effectively measure the amount of electric current flowing in amperes. The design of ammeters allows them to have very low internal resistance, which minimizes their impact on the circuit being measured, ensuring accurate readings without disrupting the normal operation of the circuit.

In contrast, a voltmeter measures voltage across two points in a circuit and is connected in parallel, not suitable for measuring current directly. An ohmmeter measures resistance in ohms, testing how much opposition a component provides to the flow of current, while a wattmeter measures electrical power in watts. Each of these other instruments has distinct roles in electrical measurements, but for measuring current specifically, the ammeter is the correct choice.