I am giving a little lecture on the conceptual aspect of relativity tomorrow and was wondering something. Consider a wire carying positive charges moving to the right and negative charges moving to the left. Say that a positive charge is placed above the wire and this positive charge moves at the some velocity as the positive charges within the wire. Now I know that what happens is from the frame of the negative charges there is a magnetic force and from the frame of the positive charges there is an electric force due to length contraction acting on the free positive charge but my question is this: why doesnt length also contract for the frame of the negative charges? It seems to me that the positive charges should contract and therefore create an electric force which opposes the magnetic force. Does it have something to due with the fact that the charge on top is moving too? Any help is GREATLY appreciated.