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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 electron s. 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. This has 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 application of triode amplifiers because it impedes the flow of electrons from cathode to anode, thus reducing the level of gainElectronics In electronics, gain is the logarithm (base 10) of the ratio of signal output from a system to signal input to the system. A signal is usually measured in units of voltage or power gain. An audio amplifier changing the 1 volt at the input to 1 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 EMFThe electromagnetic field EMF is composed of two related vectorial fields, the electric field and the magnetic field. This means that the vectors E and B that characterize the field each have a value defined at each point of space and time. If only E the within the tube's envelope, which could be utilized to create a negative bias on the tube's grid. This could improve the engineer's control and fidelity of amplification.