Closeup of the filament on a low pressure mercury gas discharge lamp showing white thermionic emission mix coating on the central portion of the coil. Typically made of a mixture of barium, strontium and calcium oxides, the coating is sputtered away through normal use, often eventually resulting in lamp failure.

Thermionic emission (archaically known as the Edison effect) is the flow of charged particles called thermions from a charged metal or a charged metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. The charge of the thermions (either positive or negative) will be the same as the charge of the metal/metal oxide. The effect increases dramatically with increasing temperature (1000–3000 K). The science dealing with this phenomenon is thermionics. Image File history File links Download high-resolution version (813x758, 148 KB) Closeup of a filament in a germicidal lamp showing thermionic coating. ... Image File history File links Download high-resolution version (813x758, 148 KB) Closeup of a filament in a germicidal lamp showing thermionic coating. ... Filaments surrounding a solar flare, caused by the interaction of the plasma in the Suns atmopshere with its magnetic field. ... It has been suggested that this article or section be merged with Fluorescent lamp. ... General Name, Symbol, Number barium2, Ba, 56 Chemical series alkaline earth metals Group, Period, Block 2, 6, s Appearance silvery white Standard atomic weight 137. ... General Name, Symbol, Number strontium, Sr, 38 Chemical series alkaline earth metals Group, Period, Block 2, 5, s Appearance silvery white metallic Standard atomic weight 87. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Standard atomic weight 40. ... In language, an archaism is the deliberate use of an older form that has fallen out of current use. ... A student demonstrating the effects of electrostatics. ... Thermionic emission 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. ...

History

The phenomenon was initially reported in 1873 by Daniel Lordan in Britain. While doing work on charged objects, Lordi discovered that a red-hot iron sphere with a negative charge would lose its charge (discharging electrons into vacuum). He also found that this did not happen if the sphere had a positive charge. He didn't understand what any of this meant. ^{[citation needed]} Other early contributors included Hittorf (1869–1883), Goldstein (1885), and Elster and Geitel (1882–1889). 1873 (MDCCCLXXIII) was a common year starting on Wednesday (see link for calendar). ...

The Edison effect in a diode tube. A diode tube is connected in two configurations, one has a flow of electrons and the other does not. Note that the arrows represent electron current, not conventional current.

The effect was 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. The Edison effect. ... In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... Thomas Alva Edison (February 11, 1847 – October 18, 1931) was an American inventor and businessman who developed many devices that greatly influenced life around the world, including the phonograph and a long lasting light bulb. ... is the 44th day of the year in the Gregorian calendar. ... Year 1880 (MDCCCLXXX) was a leap year starting on Thursday (link will display the full calendar) of the Gregorian calendar (or a leap year starting on Tuesday of the 12-day slower Julian calendar). ... The incandescent light bulb uses a glowing wire filament heated to white-hot by electrical resistance, to generate light (a process known as thermal radiation). ...

Edison built several experiment bulbs, some with an extra wire, a metal plate, or foil inside the bulb which was electrically separate from the filament. He connected the extra metal electrode 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). He found that the current emitted by the hot filament increased rapidly with increasing voltage, and filed a patent application for a voltage regulating device using the effect on November 15, 1883 (U.S. patent 307,031, the first US patent for an electronic device). He found that sufficient current would pass through the device to operate a telegraph sounder. This was exhibited at the International Electrical Exposition in Philadelphia in September 1884. William Preece, a British scientist took back with him several of the Edison Effect bulbs, and presented a paper on them in 1885, where he referred to thermionic emission as the "Edison Effect." ^[1] 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. It has been suggested that Tangent galvanometer be merged into this article or section. ... William Henry Preece Sir William Henry Preece (1834-1913) was a Welsh electrical engineer and inventor. ... Sir John Ambrose Fleming (), (November 29, 1849 - April 18, 1945) was an English electrical engineer and physicist. ... Structure of a vacuum tube diode Structure of a vacuum tube triode In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. ... Types of diodes. ... November 16 is the 320th day of the year (321st in leap years) in the Gregorian calendar, with 45 days remaining. ... 1904 (MCMIV) was a leap year starting on a Friday (see link for calendar). ...

The thermionic diode can also be configured as a device that converts a heat difference to electric power directly without moving parts (a thermionic converter, a type of heat engine). A thermionic converter, consists of a hot electrode which thermionically emits electrons over a potential energy barrier to a cooler electrode, producing a useful electric power output. ... A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. ...

Owen Willans 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". Owen Willans Richardson (down) Solvay conference 1927 Sir Owen Willans Richardson (April 26, 1879 - February 15, 1959) was a British physicist, a professor at Princeton University from 1906 to 1913, and a recipient of the Nobel Prize in Physics in 1928 for his work on the thermionic phenomenon and especially... The Nobel Prizes (Swedish: ) are awarded for Physics, Chemistry, Literature, Peace, and Physiology or Medicine. ...

Richardson's Law

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. The work function is characteristic of the material and for most metals is on the order of several electronvolt. Thermionic currents can be increased by decreasing the work function. This often-desired goal can be achieved by applying various oxide coatings to the wire. The work function is the minimum energy (usually measured in electron volts) needed to remove an electron from a solid to a point immediately outside the solid surface. ... The electronvolt (symbol eV) is a unit of energy. ...

In 1901 Owen Willans Richardson published the results of his experiments: the current from a heated wire seemed to depend exponentially on the temperature of the wire with a mathematical form similar to the Arrhenius equation. The modern form of this law (demonstrated by Saul Dushman in 1923, and hence sometimes called the Richardson-Dushman equation) states that the emitted current density J (A/m²) is related to temperature T by the equation: Owen Willans Richardson (down) Solvay conference 1927 Sir Owen Willans Richardson (April 26, 1879 - February 15, 1959) was a British physicist, a professor at Princeton University from 1906 to 1913, and a recipient of the Nobel Prize in Physics in 1928 for his work on the thermionic phenomenon and especially... The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of a chemical reaction rate, more correctly, of a rate coefficient, as this coefficient includes all magnitudes that affect reaction rate except for concentration. ... In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... Fig. ...

where T is the metal temperature in kelvin, W is the work function of the metal, k is the Boltzmann constant. The proportionality constant A, known as Richardson's constant, given by The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. ... The work function is the minimum energy (usually measured in electron volts) needed to remove an electron from a solid to a point immediately outside the solid surface. ... Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ...

where m and -e are the mass and charge of an electron, and h is Planck's constant. A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ...

Because of the exponential function, the current increases rapidly with temperature when kT is less than W. (For essentially every material, melting occurs well before kT=W.)

The thermionic emission equations are of fundamental importance in electronics, significantly affecting both older vacuum tube technology (e.g. CRT applications, like television picture tubes and computer monitors, as well as high end radio and microwave applications requiring the high power intrinsic to tube technology), and more modern semiconductor designs. Structure of a vacuum tube diode Structure of a vacuum tube triode In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. ... Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT: 1. ... Nineteen inch (48 cm) CRT computer monitor A computer display, monitor or screen is a computer peripheral device capable of showing still or moving images generated by a computer and processed by a graphics card. ... Microwaves are electromagnetic waves with wavelengths longer than those of terahertz (THz) frequencies, but relatively short for radio waves. ... A semiconductor is a fuckin solid whose electrical conductivity is in between that of a metal and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ...

While A theoretically has a value of 1.20.10⁶ A m^-2 K^-2, in practice it strongly depends on material used. See work function for some practical values for A and W for some commonly used materials. The work function is the minimum energy (usually measured in electron volts) needed to remove an electron from a solid to a point immediately outside the solid surface. ...

Field-enhanced thermionic emission

The Richardson-Dushman equation must be corrected for the Schottky Effect; the current emitted from the metal cathode into the vacuum depends on the metal's thermionic work function, and that this function is lowered from its normal value by the presence of image forces and by the electric field at this cathode. This enhancement is given by the Field-enhanced thermionic emission (FEE) equation: The Schottky diode (named after German physicist Walter H. Schottky) is a semiconductor diode with a low forward voltage drop and a very fast switching action. ...

Where E_c is the electric field strength at the cathode spot, ε₀ is the vacuum permittivity. Permittivity is a physical quantity that describes how an electric field affects and is affected by a dielectric medium and is determined by the ability of a material to polarize in response to an applied electric field, and thereby to cancel, partially, the field inside the material. ...

This equation is relatively accurate for electric field strengths lower than about 10⁸ V m⁻¹. For electric field strengths higher than 10⁸ V m⁻¹ the use of the Murphy and Good equation for thermo-field (T-F) emission is more appropriate.

References

1. ^ "Edison" by Matthew Josephson. McGraw Hill, New York, 1959, ISBN 07-033046-8

See also

• Cathode ray tube
• Space charge
• Thermoelectric effect
• Vacuum tube
• Work function
• X-ray tube

Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT: 1. ... Space charge is the electrical current that results when a metal object is heated to incandescence in a vacuum. ... The Peltier–Seebeck effect, or thermoelectric effect, is the direct conversion of heat differentials to electric voltage and vice versa. ... Structure of a vacuum tube diode Structure of a vacuum tube triode In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. ... The work function is the minimum energy (usually measured in electron volts) needed to remove an electron from a solid to a point immediately outside the solid surface. ... An X-Ray tube is a vacuum tube designed to produce man made X-Ray photons on demand. ...

External links

• How vacuum tubes really work — Has a good section on thermionic emission, with equations
• Owen Richardson's Nobel lecture on thermionics, December 12, 1929. (PDF)
• Derivations of thermionic emission equations from an undergraduate lab