In what frame is a current-carrying conductor neutral?, P.C. Peters, Am. J. Phys. 53(12), December 1985
A current-carrying conductor is often used as an example to illustrate the transformation laws of the electric and magnetic fields. TGhe conductor is generally assumed to be neutral in the rest frame of the lattice of positive ions, in which frame the free-electrons have a drift velocity v. A reexamination of this question, taking into account a self induced Hall effect, shows that the bulk of the conductor is negatively charged in this frame. The bulk of the conductor is neutral, however, in the frame in which the free-electrons are at rest, just the opposite of what is usually assumed for this system. A simple ring circuit driven by an induced emf is used to understand the role of the surface charge densities on the conductor, and implications for more general circuits are discussed.