An ammeter is a current measuring instrument. In the circuit, it is represented by the symbol A. The ammeter is placed in series with the device or component to measure the current. A series connection is used because objects in series have the same current passing through them. If a circuit has multiple resistors and the current needs to be measured in each resistor, the number of ammeters required depends on whether the circuit is in series or parallel.

When an ammeter is used to measure the current through two resistors connected in series to a battery, a single ammeter is placed in series with the two resistors because the current is the same through the two resistors. Conversely, when two resistors are connected in parallel with a battery, three meters or three separate ammeter readings are necessary to measure the current from the battery and through each resistor. The ammeter is connected in series with the component in question.

Ammeters need to have a negligible resistance, typically a fraction of a milliohm. If the resistance were not negligible, placing the ammeter in the circuit would change the equivalent resistance and modify the current being measured. Since the current in the circuit travels through the meter, ammeters usually contain a fuse to protect the meter from damage from currents that are too high.