Space charge
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.
Space charge is an inherent property of all vacuum tubes which, at times, has made life harder or easier for electrical engineers who used tubes in their designs. For example, space charge significantly limited the practical aplication of triode amplifiers because it impedes the flow of electrons from cathode to anode, thus reducing the level of gain that could be achieved in such tubes.
On the other hand, space charge came in quite handy in some tube applications because it generates a negative EMF within the tube's envelope, which could be utilized to create a negative bias on the tube's grid. This could improve the the engineer's control and fidelity of amplification.
