Within the electron source, current is passed through a thin wire (filament) until it becomes so hot that its electrons can almost "boil out" of the metal.
A voltage is then applied between the filament and the outer shell of the electron gun. The voltage creates an electric field that pulls electrons out of the finely pointed tip of the filament in a process known as "field emission."
A small hole in the outer cover (Wehnelt cup) then allows some of these electrons to spray downward.
