How does the force between parallel conductors used in defining the ampere (current SI unit)?

The direction of the net magnetic force is always perpendicular to the plane of current-carrying conductors. The direction of the magnetic force is determined through the right-hand rule. 

The right-hand thumb rule states that if you wrap a wire in a way that the thumb points towards the flow of

The exact formula for the magnetic force per unit length between two parallel current-carrying conductors:

For more information, check: Force between parallel conductors