The Aharonov–Bohm effect is a quantum-mechanical phenomenon in which an electrically charged particle is influenced by the vector potential in regions in which the magnetic field is zero. A beam of monoenergetic electrons passes through a double slit on opposite sides of a solenoid. The expected interference pattern of the waves going through the two slits is shifted by an additional phase difference when the solenoid encloses a magnetic field, despite the magnetic field being zero in the regions through which the electrons pass. This can be observed experimentally by the horizontal displacement of the interference fringes.