Elementary motor: Electron beam revolves in magnetic field.

Elementary motor: Electron beam revolves in magnetic field. The glass tube in this apparatus is evacuated, with a trace of helium added. A large D.C. voltage is placed across the tube terminals, creating an arc of glowing ionized gas. When a key-switch is pressed, a current flows through a coil at the base of the tube, creating a magnetic field in an iron rod that extends up into the tube. The magnetic field is at an angle to the current in the arc of glowing gas, causing the glowing arc to rotate.