 One postulate of special relativity is length contraction, and because of that, the charge density in the wire increases, so a current-carrying wire viewed in a moving reference frame experiences a length-contracted coulomb force as compared to the wire in a stationary frame.This force is called the magnetic force, and the associated field is the magnetic field.The direction of the magnetic force can be derived from the right-hand rule such that the force is perpendicular to both the direction of motion of the current (or charged particle) and the magnetic field.