Two flexible wires are suspended vertically. The wires are conected in series or parallel to a 12V storage battery. When the wires are connected in series and power is applied they will repel each other; when they are connected in parallel they weill attract one another.

This effect is due to the magnetic fields created by the charge flowing through the wires. When the wires are in parallel, the currents in each are going in the same direction and thus attract. In series the currents are going in opposite directions and repel.