Meter for measuring the flow of current in an electrical circuit. Meters are available to measure AC or DC current. Low current measurements are often expressed in milliamperes (0.001 ampere) and higher currents are usually expressed in amperes. AC ammeters come in two main types: a “true rms ammeter” which measures rms current, and an “average responding, rms indicating ammeter” which only provides correct rms results when the current waveform is a sine wave. (see: Alternating Current; Direct Current, rms, fundamental.)