Okay, I'm sure I must have overlooked something very trivial, so please help me with this: Two parallel infinite line currents of equal magnitude attract each other. The current can be thought of as consisting of positive and negative particles with equal linear densities, but each moving with a different velocity (wlog let's assume the positive ones are static in some frame of reference K). Let us now move to the frame of reference K' which moves with the current so that the current in K' is zero. The force which was purely magnetic in K is now replaced with a purely electrical one - the linear densities of the positive and negative particles undergo Lorentz transformation, so that the total density in K' is non-zero. Here's my problem: if linear densities and the velocities of particles (and hence the currents) have the same magnitude in K, then they must also have the same magnitude in K' (the transformation respects the symmetry between the two lines). But like charges repel each other. So it would seem that in K' (hence also in K), the currents actually repel each other.