Beam of electrons moving in a circle, due to the presence of a magnetic field. Purple light is emitted along the electron path, due to the electrons colliding with gas molecules in the bulb.
(Photo: Marcin Bialek)

Oh, how I love you guys. In our recent post A Fiery Dance on the Sun, Neatoramanaut PlasmaGryphon kindly took the time to explain to us the physics behind solar flares. In the explanation, there was a link to Wikipedia article on Lorentz force, where I found this fascinating image of a circular beam of electrons in a Teltron tube. Neat, huh? (Thanks PlasmaGryphon!)