Paralix

Meet Paralix, a pair of wires who lean in whenever their currents flow the same way.

Every current makes a magnetic field that circles around its wire. A second wire nearby sits inside that field, and the moving charges in it feel a magnetic force. When the two currents run in the same direction the force pulls the wires together. When they run in opposite directions it pushes them apart.

The force on each metre of wire is F/L = μ₀I₁I₂/(2πd). Two wires 10 cm apart, each carrying 10 A, feel about 2 × 10⁻⁴ N on every metre. That is tiny, but in a power station busbar carrying thousands of amps it becomes strong enough to bend metal during a fault.

Double either current and the force doubles. Move the wires twice as far apart and it halves.

For decades this effect defined the ampere itself: one amp was the current that gave a force of 2 × 10⁻⁷ N per metre between wires 1 m apart.