Some days ago there was a discussion on pf whether eath’s magnetic field was rotating. That got me to think again about magnetism and relativity and I found this article: http://en.wikibooks.org/wiki/Special_Relativity/Simultaneity,_time_dilation_and_length_contraction Including this section: To me there are several points which need clarifying. In the 1st sentence:” ….he sees the wire contracted relative to Bill……” should be: …he sees the +ve ions contracted relative to Bill….. Second and much more interesting why should there be electrical neutrality? Ok I do know that a neutral wire contains the same amount of +ve and –ve charges but I can easily add or subtract some electrons so that there’s no neutrality. Alternatively I can redistribute some electrons so that locally the neutrality is disturbed. Further more in Jim’s rest frame he sees the +ve charges moving. To obtain “electrical neutrality” does he see the +ve ions spread out? To conclude: to me, in the way things are explained here there’s no difference between Jim moving with the electrons looking at Bill or Bill moving with +ve ions looking at Jim. Both have to come to the same conclusion regarding magnetic forces.