If two parallel conductors carrying currents I_{1} and I_{2} are at a distance r from each other, the magnetic interaction between them is expressed by the formula F = (μ_{0}I_{1}I_{2})/(2πr) , where F is the magnitude of the force per unit length acting on each of the conductors. If the currents run in the same direction, the conductors attract each other. If the currents run in the opposite direction, they repel each other.

The abovementioned formula is the basis of the official SI definition of the fourth basic unit ? the Ampere.

An Ampere is a unit that measures electric current, and it is defined as that current which, if maintained in each of two parallel conductors separated by one meter in free space, would produce a magnetic force between the two wires of 2e-7 Newtons for each meter of length.