Editing Electric current (section)

===Vacuum===
Since a "[[free space|perfect vacuum]]" contains no charged particles, it normally behaves as a perfect insulator. However, metal electrode surfaces can cause a region of the vacuum to become conductive by injecting free electrons or [[ion]]s through either [[field electron emission]] or [[thermionic emission]]. Thermionic emission occurs when the thermal energy exceeds the metal's [[work function]], while [[field electron emission]] occurs when the electric field at the surface of the metal is high enough to cause [[quantum tunneling|tunneling]], which results in the ejection of free electrons from the metal into the vacuum. Externally heated electrodes are often used to generate an [[electron cloud]] as in the [[electrical filament|filament]] or indirectly [[hot cathode|heated cathode]] of [[vacuum tube]]s. [[cold cathode|Cold electrodes]] can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (called ''cathode spots'' or ''anode spots'') are formed. These are incandescent regions of the electrode surface that are created by a localized high current. These regions may be initiated by [[field electron emission]], but are then sustained by localized thermionic emission once a [[vacuum arc]] forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field. [[Vacuum tube]]s and [[Krytron|sprytrons]] are some of the electronic switching and amplifying devices based on vacuum conductivity.