The instrument used to measure the amount of electricity flowing through a circuit is called an ammeter.

Here's why:

* Current: The amount of electricity flowing through a circuit is called electric current.

* Amperes: Current is measured in units called amperes (A).

* Ammeter: An ammeter is a device specifically designed to measure the current in amperes.

Therefore, an ammeter is the instrument used to measure the amount of electricity flowing through a circuit.