Consider a current-carrying straight wire with a magnetic field of lines oriented anticlockwise. The direction of the current is then defined by

When the current-carrying conductor is held in the right hand, the thumb points in the current direction, and the fingers wrap the wire in the magnetic field direction.

The Right-hand thumb rule may be used to detect the direction of current since the magnetic field is anti-clockwise. If we position our right hand so that the curl of our figure is in the direction of the magnetic field, i.e., anti-clockwise, our thumb will point upward.