The following instrument is used to detect a smallest amounts of flow of electric current in a circuit:

Galvanometer: A galvanometer is a device that detects and measures tiny currents in a conducting circuit.

A voltmeter is a device that measures the potential difference between two points in a circuit.

The rheostat is a device that helps to adjust the current in a circuit by altering the resistance.

The ammeter is a device that measures the amount of electricity flowing through a circuit.

The existence of a current may be detected using a galvanometer. The direction of current in that circuit is shown by the deflection on the galvanometer.

A galvanometer is a device that can determine the direction of current flow.