English: Beam of cathode rays (electrons) in a demonstration vacuum tube moving in a circle in a magnetic field (cyclotron motion). The electrons are produced by an electron gun at bottom, consisting of a hot cathode, a metal plate heated by a filament so it emits electrons, and a cylindrical metal anode at a high voltage which accelerates the electrons into a beam. The red circular Helmholtz coil partially visible behind the tube generates a horizontal magnetic field that exerts a Lorentz force on the electrons. The force on the electrons is perpendicular to their direction of motion, causing them to bend into a circular path. Cathode rays are normally invisible, but enough air has been left in the tube so that the air molecules glow pink from fluorescence when struck by the fast-moving electrons.