Thermionic Emission

Thermionic emission is the liberation of charged particles, now understood to be ions or electrons, from a hot electrode whose thermal energy gives some particles enough kinetic energy to escape the material's surface. Thomas Edison noticed the resulting current in 1880 while inventing his light bulb, and it became known as the Edison effect, though its explanation in terms of electrons was not understood until after the electron's discovery in 1897. The phenomenon is crucial to a variety of electronic devices and can be used for electricity generation, as in thermionic converters and electrodynamic tethers, or for cooling, and it underlies thermionic vacuum tubes that emit electrons from a hot cathode into a vacuum. Vacuum emission from metals becomes significant only above about 1,000 kelvin, with charge flow increasing sharply as temperature rises further.