English: Beam of cathode rays (electrons) 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 metal anode (right) at a high voltage with a hole which accelerates the electrons into a beam. 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.