This force names the dual Yang-Mills theory with strong coupling in Montonen-Olive duality. A gravitational analog of this force is often used to explain the frame-dragging caused by rotating objects. The components of this force are the spacelike components of the Faraday tensor. What appears to be this force in the observer frame can be derived as a consequence of length contraction (15[1])of a (*) charge density in a moving particle's rest frame, which is why (10[1])this force is unified with a counterpart in special relativity. This force can do no work, since the direction of this force on a particle is always perpendicular to its motion and equals ■END■