An instrument that measures electric current. The main types are the moving-coil ammeter, the moving-iron ammeter, and the thermoammeter. The moving-coil instrument is a moving-coil galvanometer fitted with a shunt to reduce its sensitivity. It can only be used for d.c., but can be adapted for a.c. by using a rectifier. In moving-iron instruments, a piece of soft iron moves in the magnetic field created when the current to be measured flows through a fixed coil. They can be used with a.c. or d.c. but are less accurate (though more robust) than the moving-coil instruments. In thermoammeters, which can also be used with a.c. or d.c., the current is passed through a resistor, which heats up as the current passes. This is in contact with a thermocouple, which is connected to a galvanometer. This indirect system is mainly used for measuring high frequency a.c. In the hot-wire instrument the wire is clamped at its ends and its elongation as it is heated causes a pointer to move over a scale.