ammeters

An ammeter is a device that measures the electric current in a circuit. It is designed to be connected in series with the circuit so that the same current flows through the meter as through the load. Ammeters are rated in amperes and often in milliamperes for small currents, with precision indicated by accuracy classes.

Historically, ammeters are electromechanical instruments. The moving-coil type uses a coil carrying the measured current placed in a fixed magnetic field; the resulting torque is proportional to the current, producing a deflection on a calibrated scale. Moving-iron meters use soft iron elements whose attraction depends on the magnetic field and can measure both AC and DC, though with lower sensitivity. For high currents, shunt ammeters employ a low-resistance shunt in parallel with the meter movement; most of the current flows through the shunt, while the meter reads the voltage drop and infers total current.

Modern ammeters include digital and clamp-on devices. Digital ammeters use electronic sensing, often with a precision shunt or current sensor (Hall effect or current transformer) and display the reading in amperes. Clamp meters measure current without breaking the circuit by surrounding a conductor with a magnetic core that senses the field and outputs a proportional signal.

AC meters may be true RMS or calibrated for average-responding signals; accuracy and burden (voltage drop across the meter) are important considerations because the meter’s insertion can alter the circuit.

Safety and calibration are essential, with meters rated for the expected voltage and current, sometimes incorporating fuses and isolation. Ammeters are essential tools in electrical testing, power distribution, and laboratory measurements, named after Andre-Marie Ampere.