Thermionic emission

Thermionic emission (archaically known as the Edison effect) is the flow of electrons from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. The effect increases dramatically with increasing temperature (1000-3000 K), but is always present at temperatures above absolute zero. The science dealing with this phenomenon is thermionics. The charged particles are called thermions.

The Edison effect

The effect was accidentally rediscovered by Thomas Edison on February 13, 1880, while trying to discover the reason for breakage of lamp filaments and uneven blackening (darkest near one terminal of the filament) of the bulbs in his incandescent lamps.

Related Topics:
Thomas Edison - February 13 - 1880 - Incandescent lamp

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Edison built a bulb with the inside surface covered with metal foil. He connected the foil to the lamp filament through a galvanometer. When the foil was given a more negative charge than the filament, no current flowed between the foil and the filament because the cool foil emitted few electrons. However, when the foil was given a more positive charge than the filament, the many electrons emitted from the hot filament were attracted to the foil causing current to flow. This one-way flow of current was called the Edison effect (although the term is occasionally used to refer to thermionic emission itself).

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Edison saw no use for this effect, and although he patented it in 1883, he did not study it any further.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~