When a metal object is placed in a vacuum, and is heated to
incandescence, the energy is sufficient to cause electrons to "boil" away from the surface atoms and surround the metal object in a cloud of free electrons. The resulting cloud is negatively charged, and can be attracted to any nearby positively charged object, thus producing an electrical current which passes through the vacuum.
This effect was first observed by Thomas Edison in light bulb filaments[?], where it is sometimes called the Edison effect[?].