Complete Lorentz transformation of a charge-current density
  It is generally assumed in the literature that a Lorentz transformation on a neutral current loop results in a moving current loop with a nonvanishing charge distribution and an electric dipole moment. We show in this paper that this is not, in fact, correct. The derivation that leads to the charge distribution was based on an incomplete Lorentz transformation, which transforms the charge-current four-vector $j^\mu=[\rho({\bf r},t),{\bf j(r},t)]$, but not the space-time four-vector $x^\mu=(t,{\bf r})$.   We show that completing the Lorentz transformation by using the variable $t'$ in the moving frame, rather than keeping the rest frame time variable $t$, results in there being no induced charge density and no resulting electric dipole moment. 
Jerrold Franklin
physics.gen-ph