All popular expositions ([e.g. these ones][1]) of [relativistic electromagnetism][2] claim univocally that electrons in motion become more dense due to the speed. They teach that Lorentz contraction of charges causes charge imbalance and wire with current charged. Thereby, no teacher says that the wire becomes negatively charged (because electrons move) in the lab frame, where we had it originally neutral, when no current was flowing. That is a question: is wire neutrality conserved, once we create a current of electrons in it? 

It must be the case because, as Feynman points out, the lighter electrons better react on temperature changes and would charge the wire when heated. But, then answer how neutrality is persisted after you say that electron contraction takes place and it increases their density? Might be "some light" positive charges start moving in opposite direction, to compensate the growth of negative charge?.

I see that all discussions of "magnetism as electricity+relativism" avoid concerning this, most basic and most interesting case. Instead, they jump immediately to the case where you have an electric balance for a test charge in motion. Once finished with this, you are encouraged to answer [how it is possible that density of charges is increased whereas volume of the loop is intact][4]?


  [1]: http://notepad.cc/share/vF0szTUYns
  [2]: http://en.wikipedia.org/wiki/Relativistic_electromagnetism
  [3]: http://notepad.cc/share/vF0szTUYns
  [4]: https://physics.stackexchange.com/questions/63008/relativistic-charge-density-contraction-in-a-closed-loop