An amp meter or ammeter is a device that measures current in a circuit in amperes which is a measurement of the movement of electrons over a point through time.
The first ammeters were galvanometers, which exploit the deflection of a needle by a current through a coil (magnetic field), accomplished via spring action; galvanometers can measure only direct current (DC). Moving iron ammeters can measure both DC and alternating current (AC) and replace the needle with a piece of iron which is acted on by deflection across the magnetic field. To measure larger currents, a shunt (which acts as a resistor) is added to the system; most of the current is redirected through the shunt and, because the resistance across the shunt is known, it remains possible to measure the current.
A clamp-on ammeter, also known as a current clamp or current probe, can be clamped around a conductor via its two jaws, allowing the monitor to get a reading of amperage. Generally speaking, clamp-on ammeters use their jaws to detect the conductor's magnetic field, which acts on an iron vane, or a sensitive cylinder of iron, which provides a reading of current.
A clamp-on ammeter may be part of an electrician's or other technician's multimeter, which is an instrument capable of taking a number of electrical readings, such as current, resistance and voltage.