An ammeter is a measuring instrument used to measure the flow of electric current in a circuit. Electric currents are measured in amperes, hence the name. The word "ammeter" is commonly misspelled or mispronounced as "ampmeter" by some.
The earliest design is the galvanometer. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The voltage drop across the coil is kept to a minimum to minimize resistance in any circuit into which the meter is inserted.
A galvanometer can burn out if its tiny, delicate coil overheats. To measure larger currents, a resistor called a shunt is switched across the coil. With this solution, large currents can be measured with only a fraction of the current passing through the meter.
More modern designs use an analog to digital converter to measure the voltage across a resistor. The ADC is read by a microcomputer that performs the calculations to display the current through the resistor.
One problem with the use of an ammeter is the need for the meter to be inserted into the circuit and become part of it. In high voltage circuits an inductive coupling adapter converts the magnetic field around a conductor into a small current that can be easily read by a meter. See clamp meter.