English: Aharonov–Bohm effect apparatus showing barrier, X; slots S₁ and S₂; electron paths e₁ and e₂; magnetic whisker, W; screen, P; interference pattern, I; magnetic flux density, B (pointing out of figure); and magnetic vector potential, A. B is essentially nil outside the whisker. In some experiments, the whisker is replaced by a solenoid. The electrons in path 1 are shifted with respect to the electrons in path 2 by the vector potential even though the flux density is nil.