If you had two long straight parallel conductors each with currents (I and I') in the same direction, the magnetic fields generated by each of the currents will interact and cause a force between them. Since the magnetic field of the first conductor is:
(Equation 6-70)
the force on the other conductor must be:
(Equation 6-71)
To find the direction of the force, use the right hand rule, and you will find the the wires attract each other. If the currents in the two wires are going in opposite directions, then the wires will repel (notice that this is one exception to the usual, "opposites attract").
Information provided by: http://library.thinkquest.org