(sorry my bad english) Let's imagine two frames of reference, one of wich is a imaginary room containing two charges. Now let's accelerate this room from 0 to v and keep that speed during some time interval t and then we "pause" the situation again. An observer inside the box would measure the electric force acting on the charges and the observer in the other frame would measure a greater electric force. But the final positions of the two charges after time t should be the same as seen by all observers. So, there must be a force acting on the opposite direction as seen by the observer outside the room, that decreases the net force, and that force is what we call magnetic force. Is it correct to think so?