The device used to measure electric current is an ammeter. Its name is short for “ampere meter,” because current is measured in amperes, symbol A.
For a direct measurement, an ammeter is connected in series with the circuit so the same current passes through both the circuit and the instrument. It is designed to have very low resistance, minimizing the voltage drop it introduces. Connecting a conventional ammeter directly across a power source can create a dangerous short circuit.
Current-measuring instruments may be built into a multimeter, which can also measure voltage and resistance. A galvanometer is a sensitive instrument that detects or measures small currents, while an ohmmeter measures resistance and a voltmeter measures potential difference.
The history of ammeters rests on electromagnetism. In 1820, Hans Christian Ørsted observed that a current-carrying wire deflected a compass needle, revealing the link between electric current and magnetic effects. Later instruments used that relationship to display current readings.