The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin (symbol: K) is the base unit of temperature in the International System of Units (SI).

Until 2018, the kelvin was defined as the fraction ​1273.16 of the thermodynamic temperature of the triple point of water (exactly 0.01 °C or 32.018 °F).[1] In other words, it was defined such that the triple point of water is exactly 273.16 K.

On 16 November 2018, a new definition was adopted, in terms of a fixed value of the Boltzmann constant. For legal metrology purposes, the new definition will officially come into force on 20 May 2019 (the 130th anniversary of the Metre Convention).[2]

The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907), who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or written as a degree. The kelvin is the primary unit of temperature measurement in the physical sciences, but is often used in conjunction with the degree Celsius, which has the same magnitude. The definition implies that absolute zero (0 K) is equivalent to −273.15 °C (−459.67 °F).

Unit systemSI base unit
Unit ofTemperature
Named afterWilliam Thomson, 1st Baron Kelvin
Official definition
DefinitionThe kelvin, unit of thermodynamic temperature, is 1/273.16 of the thermodynamic temperature of the triple point of water.
2019 kelvin
Unit systemSI base unit
Unit ofTemperature
Named afterWilliam Thomson, 1st Baron Kelvin
Official definition
Defining authorityInternational Committee for Weights and Measures
Defining event2018 General Conference on Weights and Measures
Effective since20 May 2019
DefinitionThe kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10−23 when expressed in the unit J⋅K−1, which is equal to kg⋅m2⋅s−2⋅K−1, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.


Lord Kelvin photograph
Lord Kelvin, the namesake of the unit

In 1848, William Thomson, who later was made Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby "infinite cold" (absolute zero) was the scale's null point, and which used the degree Celsius for its unit increment. Kelvin calculated that absolute zero was equivalent to −273 °C on the air thermometers of the time.[3] This absolute scale is known today as the Kelvin thermodynamic temperature scale. Kelvin's value of "−273" was the negative reciprocal of 0.00366—the accepted expansion coefficient of gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.

In 1954, Resolution 3 of the 10th General Conference on Weights and Measures (CGPM) gave the Kelvin scale its modern definition by designating the triple point of water as its second defining point and assigned its temperature to exactly 273.16 kelvins.[4]

In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K.[5] Furthermore, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction ​1273.16 of the thermodynamic temperature of the triple point of water."[1]

In 2005, the Comité International des Poids et Mesures (CIPM), a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition specified as Vienna Standard Mean Ocean Water.[6]

In 2018, Resolution A of the 26th CGPM adopted a significant redefinition of SI base units which included redefining the Kelvin in terms of a fixed value for the Boltzmann constant of 1.380649×10−23 J/K.

Usage conventions

When spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm (e.g. "the triple point of water is exactly 273.16 kelvins"[7]). When reference is made to the "Kelvin scale", the word "kelvin"—which is normally a noun—functions adjectivally to modify the noun "scale" and is capitalized. As with most other SI unit symbols (angle symbols, e.g. 45° 3′ 4″, are the exception) there is a space between the numeric value and the kelvin symbol (e.g. "99.987 K").[8][9]

Before the 13th CGPM in 1967–1968, the unit kelvin was called a "degree", the same as with the other temperature scales at the time. It was distinguished from the other scales with either the adjective suffix "Kelvin" ("degree Kelvin") or with "absolute" ("degree absolute") and its symbol was °K. The latter term (degree absolute), which was the unit's official name from 1948 until 1954, was ambiguous since it could also be interpreted as referring to the Rankine scale. Before the 13th CGPM, the plural form was "degrees absolute". The 13th CGPM changed the unit name to simply "kelvin" (symbol: K).[10] The omission of "degree" indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales (although the Rankine scale continued to use "degree Rankine"), but rather an absolute unit of measure which can be manipulated algebraically (e.g. multiplied by two to indicate twice the amount of "mean energy" available among elementary degrees of freedom of the system).

Use in conjunction with degrees Celsius

A thermometer calibrated in degrees Celsius (left) and kelvins (right).

In science and engineering, degrees Celsius and kelvins are often used simultaneously in the same article, where absolute temperatures are given in degrees Celsius, but temperature intervals are given in kelvins. E.g. "its measured value was 0.01028 °C with an uncertainty of 60 µK."

This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing relative temperatures, and the magnitude of the degree Celsius is exactly equal to that of the kelvin.[11] Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states "a temperature interval may also be expressed in degrees Celsius",[5] the practice of simultaneously using both °C and K is widespread throughout the scientific world. The use of SI prefixed forms of the degree Celsius (such as µ°C or microdegree Celsius) to express a temperature interval has not been widely adopted.

2018 redefinition

In 2005 the CIPM embarked on a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous methodology. In particular, the committee proposed redefining the kelvin such that Boltzmann's constant takes the exact value 1.3806505×10−23 J/K.[12] The committee had hoped that the programme would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger programme.[13]

The redefinition was further postponed in 2014, pending more accurate measurements of Boltzmann's constant in terms of the current definition,[14] but was finally adopted at the 26th CGPM in late 2018, with a value of k = 1.380649×10−23 J/K.[12][15]

From a scientific point of view, the main advantage is that this will allow measurements at very low and very high temperatures to be made more accurately, as the techniques used depend on the Boltzmann constant. It also has the philosophical advantage of being independent of any particular substance. The challenge was to avoid degrading the accuracy of measurements close to the triple point. From a practical point of view, the redefinition will pass unnoticed; water will still freeze at 273.15 K (0 °C),[16] and the triple point of water will continue to be a commonly used laboratory reference temperature.

The difference is that, before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of 1.38064903(51)×10−23 J/K, with a relative standard uncertainty of 3.7×10−7.[15] Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now 273.1600(1) K.

Practical uses

Kelvin temperature conversion formulae
from kelvins to kelvins
Celsius [°C] = [K] − 273.15 [K] = [°C] + 273.15
Fahrenheit [°F] = [K] × ​95 − 459.67 [K] = ([°F] + 459.67) × ​59
Rankine [°R] = [K] × ​95 [K] = [°R] × ​59
For temperature intervals rather than specific temperatures,
1 K = 1 °C = ​95 °F = ​95 °R
Comparisons among various temperature scales

Colour temperature

The kelvin is often used as a measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about 4000 K appear reddish, whereas those above about 7500 K appear bluish. Colour temperature is important in the fields of image projection and photography, where a colour temperature of approximately 5600 K is required to match "daylight" film emulsions. In astronomy, the stellar classification of stars and their place on the Hertzsprung–Russell diagram are based, in part, upon their surface temperature, known as effective temperature. The photosphere of the Sun, for instance, has an effective temperature of 5778 K.

Digital cameras and photographic software often use colour temperature in K in edit and setup menus. The simple guide is that higher colour temperature produces an image with enhanced white and blue hues. The reduction in colour temperature produces an image more dominated by reddish, "warmer" colours.

Kelvin as a unit of noise temperature

In electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate noise floor, i.e. the noise temperature. The so-called Johnson–Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit using the Friis formulas for noise.

Unicode character

The symbol is encoded in Unicode at code point U+212A KELVIN SIGN. However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using U+004B K LATIN CAPITAL LETTER K instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: U+2126 OHM SIGN, U+212A KELVIN SIGN, and U+212B ANGSTROM SIGN. In all three instances, the regular letter should be used."[17]

See also

Notes and references

  1. ^ a b "Resolution 4: Definition of the SI unit of thermodynamic temperature (kelvin)". Resolutions of the 13th CGPM. Bureau International des Poids et Mesures. 1967. Archived from the original on 15 June 2007. Retrieved 6 February 2008.
  2. ^ Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018) (PDF)
  3. ^ Lord Kelvin, William (October 1848). "On an Absolute Thermometric Scale". Philosophical Magazine. Archived from the original on 1 February 2008. Retrieved 6 February 2008.
  4. ^ "Resolution 3: Definition of the thermodynamic temperature scale". Resolutions of the 10th CGPM. Bureau International des Poids et Mesures. 1954. Archived from the original on 23 June 2007. Retrieved 6 February 2008.
  5. ^ a b "Resolution 3: SI unit of thermodynamic temperature (kelvin)". Resolutions of the 13th CGPM. Bureau International des Poids et Mesures. 1967. Archived from the original on 21 April 2007. Retrieved 6 February 2008.
  6. ^ "Unit of thermodynamic temperature (kelvin)". SI Brochure, 8th edition. Bureau International des Poids et Mesures. 1967. pp. Section Archived from the original on 26 September 2007. Retrieved 2008-02-06.
  7. ^ "Rules and style conventions for expressing values of quantities". SI Brochure, 8th edition. Bureau International des Poids et Mesures. 1967. pp. Section Archived from the original on 16 July 2012. Retrieved 27 August 2012.
  8. ^ "SI Unit rules and style conventions". National Institute of Standards and Technology. September 2004. Archived from the original on 5 February 2008. Retrieved 6 February 2008.
  9. ^ "Rules and style conventions for expressing values of quantities". SI Brochure, 8th edition. Bureau International des Poids et Mesures. 1967. pp. Section 5.3.3. Archived from the original on 23 September 2015. Retrieved 13 December 2015.
  10. ^ Barry N. Taylor (2008). "Guide for the Use of the International System of Units (SI)" (.PDF). Special Publication 811. National Institute of Standards and Technology. Archived (PDF) from the original on 3 June 2016. Retrieved 5 March 2011.
  11. ^ "Units with special names and symbols; units that incorporate special names and symbols". SI Brochure, 8th edition. Bureau International des Poids et Mesures. 2006. pp. Section 2.2.2, Table 3. Archived from the original on 18 June 2007. Retrieved 27 June 2016.
  12. ^ a b Ian Mills (29 September 2010). "Draft Chapter 2 for SI Brochure, following redefinitions of the base units" (PDF). CCU. Archived (PDF) from the original on 10 January 2011. Retrieved 1 January 2011.
  13. ^ "General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram" (PDF) (Press release). Sèvres, France: General Conference on Weights and Measures. 23 October 2011. Archived (PDF) from the original on 9 February 2012. Retrieved 25 October 2011.
  14. ^ Wood, B. (3–4 November 2014). "Report on the Meeting of the CODATA Task Group on Fundamental Constants" (PDF). BIPM. p. 7. Archived (PDF) from the original on 13 October 2015. [BIPM director Martin] Milton responded to a question about what would happen if ... the CIPM or the CGPM voted not to move forward with the redefinition of the SI. He responded that he felt that by that time the decision to move forward should be seen as a foregone conclusion.
  15. ^ a b Newell, D B; Cabiati, F; Fischer, J; Fujii, K; Karshenboim, S G; Margolis, H S; de Mirandés, E; Mohr, P J; Nez, F; Pachucki, K; Quinn, T J; Taylor, B N; Wang, M; Wood, B M; Zhang, Z; et al. (Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants) (29 January 2018). "The CODATA 2017 values of h, e, k, and NA for the revision of the SI". Metrologia. 55 (1). doi:10.1088/1681-7575/aa950a.
  16. ^ "Updating the definition of the kelvin" (PDF). International Bureau for Weights and Measures (BIPM). Archived (PDF) from the original on 23 November 2008. Retrieved 23 February 2010.
  17. ^ "22.2". The Unicode Standard, Version 8.0 (PDF). Mountain View, CA, USA: The Unicode Consortium. August 2015. ISBN 978-1-936213-10-8. Archived (PDF) from the original on 6 December 2016. Retrieved 6 September 2015.

External links

Absolute zero

Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as 0. The fundamental particles of nature have minimum vibrational motion, retaining only quantum mechanical, zero-point energy-induced particle motion. The theoretical temperature is determined by extrapolating the ideal gas law; by international agreement, absolute zero is taken as −273.15° on the Celsius scale (International System of Units), which equals −459.67° on the Fahrenheit scale (United States customary units or Imperial units). The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition.

It is commonly thought of as the lowest temperature possible, but it is not the lowest enthalpy state possible, because all real substances begin to depart from the ideal gas when cooled as they approach the change of state to liquid, and then to solid; and the sum of the enthalpy of vaporization (gas to liquid) and enthalpy of fusion (liquid to solid) exceeds the ideal gas's change in enthalpy to absolute zero. In the quantum-mechanical description, matter (solid) at absolute zero is in its ground state, the point of lowest internal energy.

The laws of thermodynamics indicate that absolute zero cannot be reached using only thermodynamic means, because the temperature of the substance being cooled approaches the temperature of the cooling agent asymptotically, and a system at absolute zero still possesses quantum mechanical zero-point energy, the energy of its ground state at absolute zero. The kinetic energy of the ground state cannot be removed.

Scientists and technologists routinely achieve temperatures close to absolute zero, where matter exhibits quantum effects such as superconductivity and superfluidity.


The Celsius scale, also known as the centigrade scale, is a temperature scale used by the International System of Units (SI). As an SI derived unit, it is used by all countries except the United States, the Bahamas, Belize, the Cayman Islands and Liberia. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty. Before being renamed to honor Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps.

From 1743, the Celsius scale is based on 0 °C for the freezing point of water and 100 °C for the boiling point of water at 1 atm pressure. Prior to 1743, the scale was also based on the boiling and melting points of water, but the values were reversed (i.e. the boiling point was at 0 degrees and the melting point was at 100 degrees). The 1743 scale reversal was proposed by Jean-Pierre Christin.

By international agreement, since 1954 the unit degree Celsius and the Celsius scale are defined by absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW), a specially purified water. This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible, is defined as being exactly 0 K and −273.15 °C. The temperature of the triple point of water is defined as exactly 273.16 K (0.01 °C). This means that a temperature difference of one degree Celsius and that of one kelvin are exactly the same.On May 20, 2019, the degree Kelvin, and along with it the degree Celsius, will again be re-defined so that its value will be determined by definition of the Boltzmann constant.

Joule–Thomson effect

In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin effect, Kelvin–Joule effect) describes the temperature change of a real gas or liquid (as differentiated from an ideal gas) when it is forced through a valve or porous plug while keeping them insulated so that no heat is exchanged with the environment. This procedure is called a throttling process or Joule–Thomson process. At room temperature, all gases except hydrogen, helium and neon cool upon expansion by the Joule–Thomson process when being throttled through an orifice; these three gases experience the same effect but only at lower temperatures. Most liquids such as hydraulic oils will be warmed by the Joule-Thomson throttling process.

The gas-cooling throttling process is commonly exploited in refrigeration processes such as air conditioners, heat pumps, and liquefiers. In hydraulics, the warming effect from Joule-Thomson throttling can be used to find internally leaking valves as these will produce heat which can be detected by thermocouple or thermal-imaging camera. Throttling is a fundamentally irreversible process. The throttling due to the flow resistance in supply lines, heat exchangers, regenerators, and other components of (thermal) machines is a source of losses that limits the performance.

Kelvin Anderson

Kelvin Anderson (born February 4, 1972) is a former professional gridiron football player. As a running back, he rushed for over 1,000 yards in eight consecutive seasons in the Canadian Football League, a league record. He won the CFL's Grey Cup championship twice, as a member of the Calgary Stampeders, in 1998 and 2001.

Anderson played football at New Madrid County Central High School. He played college football at Southeast Missouri Statewhere he earned the nickname "Earthquake". Anderson's eight consecutive 1,000 yard seasons began in 1996 with the Calgary Stampeders. In 2000, he played in 15 games for Calgary, and ran for 1,048 yards and scored six touchdowns. He caught 34 passes for 283 yards and two touchdowns, and captured the Eddie James Memorial Trophy as the Western Division’s top rusher. In 2001 he was named a Western All-Star for the fifth consecutive season, and was named a CFL All-Star for the third time in his career, and ran for a career-high 1,383 yards and six touchdowns. Anderson also caught 48 passes for 433 yards and nine touchdowns. Anderson also played in the only XFL season in 2001 for the San Francisco Demons, leading the team in rushing with 228 yards.

Anderson had played all of his CFL career with the Calgary Stampeders when he was released by them prior to the 2003 season in favor of Lawrence Phillips. Anderson signed with the BC Lions, following head coach and general manager Wally Buono who had also been let go by Calgary. With the Lions in 2003, he rushed for over 1,000 yards for the eighth consecutive season, setting a league record. He retired just prior the 2004 CFL season. [1] Antonio Warren succeeded him as the Lions' starting running back.

Anderson was inducted to the Canadian Football Hall of Fame in 2017.

Kelvin Benjamin

Kelvin Benjamin (born February 5, 1991) is an American football wide receiver for the Kansas City Chiefs of the National Football League (NFL). He played college football at Florida State and was drafted by the Carolina Panthers in the first round of the 2014 NFL Draft. He has also played for the Buffalo Bills.

Kelvin Carpenter

Kelvin Carpenter is a fictional character from the BBC soap opera EastEnders, played by Paul J. Medford from 12 March 1985 to 3 September 1987.

Kelvin is a bright spark and full of initiative. He opens several businesses in Albert Square and even forms a band. He is a bit of a heartbreaker in the early years, but eventually he has his heart broken in return, when his middle-aged girlfriend jilts him. Always a bit too intelligent for Walford, Kelvin eventually leaves for university.

Kelvin Gastelum

Kelvin Gastelum (born October 24, 1991) is an American professional mixed martial artist who is currently signed to the Ultimate Fighting Championship (UFC). He has competed for the company since he won The Ultimate Fighter 17 As of February 9, 2019, he is #4 in the official UFC middleweight rankings.

Kelvin Grove, Queensland

Kelvin Grove is an inner northern suburb of Brisbane, Queensland, Australia approximately 3 kilometres (1.9 mi) from the CBD. This hilly suburb takes its name from Kelvingrove Park in Glasgow, Scotland. It is primarily residential with tree-lined streets and some commercial and light industrial activities along its main thoroughfare, Kelvin Grove Road. La Boite Theatre Company, Queensland's second largest theatre company, operates from the Roundhouse Theatre on the Kelvin Grove campus of the Queensland University of Technology.

According to the 2016 Census there were 7,927 people living in the suburb with a median age of 27.In common with many inner suburbs of Brisbane, the suburb has a mixture of traditional "Queenslander" homes, some post-war worker's cottages and more modern apartment blocks.

Kelvin Hayden

Kelvin Darnell Hayden, Jr. (born July 24, 1983) is a former American football cornerback. He was drafted by the Indianapolis Colts in the second round of the 2005 NFL Draft. He played college football at Illinois.

Hayden has also played for the Atlanta Falcons and Chicago Bears. He returned a Rex Grossman interception 56 yards for a touchdown in Super Bowl XLI against the Bears. It was his first career interception.

Kelvin MacKenzie

Kelvin Calder MacKenzie (born 22 October 1946) is an English media executive and a former newspaper editor. Best known for being editor of The Sun from 1981, the publication was by then established as the Britain's largest circulation newspaper. After leaving The Sun in 1994, he was appointed to executive roles in satellite television and other broadcasting outlets, as well as being involved in a number of publishing enterprises.

After short periods as a columnist at the Daily Mail and The Daily Telegraph, MacKenzie returned to his old paper in the same role. His contract was terminated in May 2017 after being suspended.

Kelvin Sampson

Kelvin Dale Sampson (born October 5, 1955) is an American basketball coach who is currently the head coach of the Houston Cougars men's basketball team. He was the head coach at Montana Tech from 1981 to 1985, Washington State University from 1987 to 1994, the University of Oklahoma from 1994 to 2006, and Indiana University 2006 to 2008. He has also been an assistant coach for National Basketball Association (NBA) teams including the Milwaukee Bucks and Houston Rockets.

Kelvin wave

A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., the phase speed of the wave crests is equal to the group speed of the wave energy for all frequencies. This means that it retains its shape as it moves in the alongshore direction over time.

A Kelvin wave (fluid dynamics) is also a long scale perturbation mode of a vortex in superfluid dynamics; in terms of the meteorological or oceanographical derivation, one may assume that the meridional velocity component vanishes (i.e. there is no flow in the north–south direction, thus making the momentum and continuity equations much simpler). This wave is named after the discoverer, Lord Kelvin (1879).

Kelvin–Helmholtz mechanism

The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats the core of the star/planet. This mechanism is evident on Jupiter and Saturn and on brown dwarfs whose central temperatures are not high enough to undergo nuclear fusion. It is estimated that Jupiter radiates more energy through this mechanism than it receives from the Sun, but Saturn might not. The latter process causes Jupiter to shrink at a rate of two centimetres each year.The mechanism was originally proposed by Kelvin and Helmholtz in the late nineteenth century to explain the source of energy of the Sun. By the mid-nineteenth century, conservation of energy had been accepted, and one consequence of this law of physics is that the Sun must have some energy source to continue to shine. Because nuclear reactions were unknown, the main candidate for the source of solar energy was gravitational contraction.

However, it soon was recognized by Sir Arthur Eddington and others that the total amount of energy available through this mechanism only allowed the Sun to shine for millions of years rather than the billions of years that the geological and biological evidence suggested for the age of the Earth. (Kelvin himself had argued that the Earth was millions, not billions, of years old.) The true source of the Sun's energy remained uncertain until the 1930s, when it was shown by Hans Bethe to be nuclear fusion.

Lockport, Illinois

Lockport is a city in Will County, Illinois, United States, located 30 miles southwest of Chicago. The city was incorporated in 1853. It is situated along the Illinois and Michigan (I&M) Canal, and was the headquarters of the canal when the canal was operating. A section of the canal runs through Lockport, including the remains of the canal's Lock No. 1 from which the town received its name. The canal right-of-way is now the Illinois and Michigan National Heritage Corridor. Because of proactive efforts dating back several years, the city of Lockport is one of the best-preserved canal sites in existence today.

Queensland University of Technology

The Queensland University of Technology (QUT) is a public research university located in the urban coastal city of Brisbane, Queensland, Australia. QUT is located on two campuses in the Brisbane area: Gardens Point and Kelvin Grove. The university in its current form was founded in 1989, when the then Queensland Institute of Technology (QIT) was granted university status by the 'Queensland University of Technology Act' passed in 1988 and also by the subsequent merger of Brisbane College of Advanced Education with QUT in 1990. QUT was a member of the Australian Technology Network of universities and had withdrawn participation since 28 September 2018 onwards.In 2015, QUT had enrolled 48,503 students, including 8,218 international students from more than 100 countries, 35,304 undergraduate students, 12,035 postgraduate students and 1,164 non-award students. The university had a total of 4,872 full-time equivalent (FTE) staff members (academic, professional, full-time or fractional full-time and casual) in 2015, and an annual budget of over $900 million. In 2016, the total revenue generated by QUT from its continuing operations was $992.519 million.

SI base unit

The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the meter for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity, and the mole for amount of substance.

The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology.

The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the metre (US English: meter) has the symbol m, but the kelvin has symbol K, because it is named after Lord Kelvin and the ampere with symbol A is named after André-Marie Ampère.

Several other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI.


Temperature is a physical quantity expressing hot and cold. It is measured with a thermometer calibrated in one or more temperature scales. The most commonly used scales are the Celsius scale (formerly called centigrade) (denoted °C), Fahrenheit scale (denoted °F), and Kelvin scale (denoted K). The kelvin (the word is spelled with a lower-case k) is the unit of temperature in the International System of Units (SI), in which temperature is one of the seven fundamental base quantities. The Kelvin scale is widely used in science and technology.

Theoretically, the coldest a system can be is when its temperature is absolute zero, at which point the thermal motion in matter would be zero. However, an actual physical system or object can never attain a temperature of absolute zero. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, and −459.67 °F on the Fahrenheit scale.

For an ideal gas, temperature is proportional to the average kinetic energy of the random microscopic motions of the constituent microscopic particles.

Temperature is important in all fields of natural science, including physics, chemistry, Earth science, medicine, and biology, as well as most aspects of daily life.

Thermal conductivity

The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by , , or .

Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, while the opposite is true for insulating materials like Styrofoam. Correspondingly, materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation. The reciprocal of thermal conductivity is called thermal resistivity.

The defining equation for thermal conductivity is , where is the heat flux, is the thermal conductivity, and is the temperature gradient. This is known as Fourier's Law for heat conduction. Although commonly expressed as a scalar, the most general form of thermal conductivity is a second-rank tensor. However, the tensorial description only becomes necessary in materials which are anisotropic.

William Thomson, 1st Baron Kelvin

William Thomson, 1st Baron Kelvin, (26 June 1824 – 17 December 1907) was a Scots-Irish mathematical physicist and engineer who was born in Belfast in 1824. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging discipline of physics in its modern form. He worked closely with mathematics professor Hugh Blackburn in his work. He also had a career as an electric telegraph engineer and inventor, which propelled him into the public eye and ensured his wealth, fame and honour. For his work on the transatlantic telegraph project he was knighted in 1866 by Queen Victoria, becoming Sir William Thomson. He had extensive maritime interests and was most noted for his work on the mariner's compass, which previously had limited reliability.

Absolute temperatures are stated in units of kelvin in his honour. While the existence of a lower limit to temperature (absolute zero) was known prior to his work, Lord Kelvin is known for determining its correct value as approximately −273.15 degree Celsius or −459.67 degree Fahrenheit.

He was ennobled in 1892 in recognition of his achievements in thermodynamics, and of his opposition to Irish Home Rule, becoming Baron Kelvin, of Largs in the County of Ayr. He was the first British scientist to be elevated to the House of Lords. The title refers to the River Kelvin, which flows near his laboratory at the University of Glasgow. His home was the red sandstone mansion Netherhall, in Largs. Despite offers of elevated posts from several world-renowned universities, Kelvin refused to leave Glasgow, remaining professor of Natural Philosophy for over 50 years, until his eventual retirement from that post. The Hunterian Museum at the University of Glasgow has a permanent exhibition on the work of Lord Kelvin including many of his original papers, instruments, and other artifacts, such as his smoking pipe.

Active in industrial research and development, he was recruited around 1899 by George Eastman to serve as vice-chairman of the board of the British company Kodak Limited, affiliated with Eastman Kodak.

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