Thermionic emission
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Thermionic emission (archaicly 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.
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History
It was initially reported in 1873 by Guthrie in Britain. While doing work on charged objects, Professor Guthrie discovered that a red-hot iron sphere with a negative charge would lose its charge (discharging the extra electrons into the vacuum). He also found that this did not happen if the sphere had a positive charge. Other early contributors included Hittorf (1869-1883), Goldstein (1885), and Elster and Geitel (1882-1889).
Owen Richardson worked with thermionic emission and received a Nobel prize in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him".
Details
In any metal, there are one or two electrons per atom that are free to move from atom to atom. This is sometimes referred to as a "sea of electrons". Their velocities follow a statistical distribution, rather than being uniform, and occasionally an electron will have enough velocity to exit the metal without being pulled back in. The minimum amount of energy needed for an electron to leave the surface is called the work function, and varies from metal to metal. A thin oxide coating is often applied to metal surfaces in vacuum tubes to give a lower work function, as it is easier for electrons to leave the surface of the oxide.
The Richardson-Dushmann equation states that the emitted current density J (A/m2) is related to temperature T by the equation:
- <math>J = A T^2 e^{-W \over k T}<math>
where T is the metal temperature in kelvins, W is the work function of the metal, k is the Boltzmann constant, and A is Richardson's constant.
The exponent in the equation shows that the amount of current emitted increases drastically with temperature.
The thermionic emission equations are important in semiconductor design.
The Edison effect
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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.
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.
The vacuum diode tube
The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that the Edison Effect could be used to detect radio waves. Fleming went on to develop the two-element vacuum tube known as the diode, which he patented on November 16, 1904.
The thermionic diode can also be configured as a device that converts a heat difference to electric power directly without moving parts (a heat engine).
See also
External links
- How vacuum tubes really work (http://www.john-a-harper.com/tubes201/) - Has a good section on thermionic emission, with equations
- Owen Richardson's Nobel lecture on thermionics (http://www.nobel.se/physics/laureates/1928/richardson-lecture.pdf), December 12, 1929. (PDF)
- History of the electron tube (http://www.radio-electronics.com/info/radio_history/valve/hov.php)de:Edison-Effekt