An
ammeter is a
measuring instrument used to measure the
current in a
circuit. Electric currents are measured in
amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as
milliammeters or
microammeters. Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in
electric power systems.