The circuit used to measure electric current is called an ammeter circuit. An ammeter is an electrical instrument that measures the flow of electric current in a circuit, typically in amperes (A). When connected in series to the circuit, it allows for accurate readings of current as it flows through. Proper configuration ensures minimal impact on the circuit's operation while providing precise current measurements. Ammeters can be analog or digital, with digital ammeters providing easier reading and enhanced accuracy. This tool is essential in electrical engineering and various applications to diagnose circuit performance and safety.