Thermionic emission is the discharge of electrons from heated materials, widely used as a source of electrons in conventional electron tubes (e.g., television picture tubes) in the fields of electronics and communications.
The phenomenon was first observed (1883) by Thomas A. Edison as a passage of electricity from a filament to a plate of metal inside an incandescent lamp. The classical example of thermionic emission is the emission of electrons from a hot cathode into a vacuum (also known as thermal electron emission or the Edison effect) in a vacuum tube. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals.
Vacuum emission from metals tends to become significant only for temperatures over 1000 K. The science dealing with this phenomenon has been known as "thermionics," but this name seems to be gradually falling into disuse.