What instrument should be used to measure the current in an electric circuit?

The appropriate instrument for measuring current in an electric circuit is an ammeter. This device is specifically designed to measure the flow of electric charge in amperes. An ammeter is connected in series with the circuit, allowing it to capture the total current flowing through the circuit without affecting the operation of the circuit significantly.

Other instruments listed serve different purposes. For example, an oscilloscope is utilized for observing the varying signal voltages in a circuit over time, making it ideal for analyzing waveforms rather than measuring current directly. A wattmeter measures the power consumption in a circuit, indicating how much energy is used over time, which again does not provide a direct measurement of current alone. Lastly, a micrometer is a precision measuring tool used for dimensions and is unrelated to electrical measurements. Thus, choosing the ammeter provides the correct means to measure current effectively in a circuit.