Relative Charge Density
From the perspective of the theory of relativity, the length of a segment of wire depends on velocity of observer, so charge density is a relative concept. Anthony French has described how the magnetic field force of a current-bearing wire arises from this relative charge density. He used (p 260) a Minkowski diagram to show "how a neutral current-bearing wire appears to carry a net charge density as observed in a moving frame." The approach to electromagnetism through spacetime symmetry is called relativistic electromagnetism.
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