In Bell's spaceship paradox a thread between equally accelerating spaceships breaks, because length contraction shortens the spaceships and the thread but not the distance between them. Wikipedia does not explain what happens when the observer accelerates instead of the spaceships, but I assume the thread does not break then.
Compare this with Purcell's description of electromagnetism replicated here and linked from here. The magnetic force on a (positive) test charge moving along a live wire is nothing but relativistic electrostatic force — the latter created due to a relativistically reduced distance between (negative) charges.
Moving the test charge past the wire is similar to the spaceship situation except the observer accelerates not the spaceships and the thread does not break. But why then should the distance between the negative charges in the wire shrink, but not the distances between the positive ones? Relativistically, the whole wire gets shorter in the frame of the moving test charge, but this would then decrease the average distance between all charges, not only the (in this case) negative charges.
Puzzled! Can someone explain this?