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.
Erik Steel is a graduate of the University of Michigan, earning his bachelor's degree in Russian. Steel has worked as writer for more than four years and has contributed content to eHow and Pluck on Demand. His work recently appeared in the literary journal "Arsenic Lobster."