kelvin | |
---|---|

General information | |

Unit system | SI |

Unit of | temperature |

Symbol | K |

Named after | William Thomson, 1st Baron Kelvin |

Conversions | |

x K in ... | ... corresponds to ... |

Celsius | (x − 273.15) °C |

Fahrenheit | (1.8 x − 459.67) °F |

Rankine | 1.8 x °Ra |

The **kelvin**, symbol **K**, is a unit of measurement for temperature.^{ [1] } The **Kelvin scale** is an absolute scale, which is defined such that 0 K is absolute zero and a change of thermodynamic temperature T by 1 kelvin corresponds to a change of thermal energy *kT* by 1.380649×10^{−23} J. The Boltzmann constant *k* = 1.380649×10^{−23} J⋅K^{−1} was exactly defined in the 2019 redefinition of the SI base units such that the triple point of water is 273.16±0.0001 K.^{ [2] } The kelvin is the base unit of temperature in the International System of Units (SI), used alongside its prefixed forms.^{ [2] }^{ [3] }^{ [4] } It is named after the Belfast-born and University of Glasgow-based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).^{ [5] }

- History
- Precursors
- Charles's law
- Lord Kelvin
- Triple point standard
- 2019 redefinition
- Practical uses
- Colour temperature
- Kelvin as a unit of noise temperature
- Derived units and SI multiples
- Orthography
- See also
- Notes
- References
- Bibliography
- External links

Historically, the Kelvin scale was developed from the Celsius scale, such that 273.15 K was 0 °C (the approximate melting point of ice) and a change of one kelvin was exactly equal to a change of one degree Celsius.^{ [1] }^{ [5] } This relationship remains accurate, but the Celsius, Fahrenheit, and Rankine scales are now defined in terms of the Kelvin scale.^{ [2] }^{ [6] }^{ [7] } The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries the Celsius scale remains the dominant scale outside of these fields.^{ [5] } In the United States, outside of the physical sciences, the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature.^{ [6] }

During the 18th century, multiple temperature scales were developed,^{ [8] } notably Fahrenheit and centigrade (later Celsius). These scales predated much of the modern science of thermodynamics, including atomic theory and the kinetic theory of gases which underpin the concept of absolute zero. Instead, they chose defining points within the range of human experience that could be reproduced easily and with reasonable accuracy, but lacked any deep significance in thermal physics. In the case of the Celsius scale (and the long since defunct Newton scale and Réaumur scale) the melting point of water served as such a starting point, with Celsius being defined, from the 1740s up until the 1940s, by calibrating a thermometer such that

- The freezing point of water is 0 degrees.
- The boiling point of water is 100 degrees.

This definition assumes pure water at a specific pressure chosen to approximate the natural air pressure at sea level. Thus an increment of 1 °C equals 1/100 of the temperature difference between the melting and boiling points. The same temperature interval was later used for the Kelvin scale.

From 1787 to 1802, it was determined by Jacques Charles (unpublished), John Dalton,^{ [9] }^{ [10] } and Joseph Louis Gay-Lussac ^{ [11] } that, at constant pressure, ideal gases expanded or contracted their volume linearly (Charles's law) by about 1/273 parts per degree Celsius of temperature's change up or down, between 0 °C and 100 °C. This suggested that the volume of a gas cooled at about −273 °C would reach zero.

In 1848, William Thomson, who was later ennobled as Lord Kelvin, published a paper *On an Absolute Thermometric Scale*.^{ [12] }^{ [13] }^{ [14] } Using the soon-to-be-defunct caloric theory, he proposed an "absolute" scale based on the following parameters:

- The melting point of water is 0 degrees.
- The boiling point of water is 100 degrees.

"The arbitrary points which coincide on the two scales are 0° and 100°"

- Any two heat engines whose heat source and heat sink are both separated by the same number of degrees will, per Carnot's theorem, be capable of producing the same amount of mechanical work per unit of "caloric" passing through.

"The characteristic property of the scale which I now propose is, that all degrees have the same value; that is, that a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T − 1)°, would give out the same mechanical effect, whatever be the number T. This may justly be termed an absolute scale, since its characteristic is quite independent of the physical properties of any specific substance."

As Carnot's theorem is understood in modern thermodynamics to simply describe the maximum efficiency with which thermal energy can be converted to mechanical energy and the predicted maximum efficiency is a function of the *ratio* between the absolute temperatures of the heat source and heat sink:

- Efficiency ≤ 1 − absolute temperate of heat sink/absolute temperature of heat source

It follows that increments of equal numbers of degrees on this scale must always represent equal *proportional* increases in absolute temperature. The numerical value of an absolute temperature, *T*, on the 1848 scale is related to the absolute temperature of the melting point of water, *T*_{mpw}, and the absolute temperature of the boiling point of water, *T*_{bpw}, by

*T*(1848 scale) = 100 (ln*T*/*T*_{mpw}) / (ln*T*_{bpw}/*T*_{mpw})

On this scale, an increase of 222 degrees always means an approximate doubling of absolute temperature regardless of the starting temperature.

In a footnote Thomson calculated that "infinite cold" (absolute zero, which would have a numerical value of negative infinity on this scale) was equivalent to −273 °C using the air thermometers of the time. This value of "−273" was the negative reciprocal of 0.00366—the accepted coefficient of thermal expansion of an ideal gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.^{ [15] }

Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one^{ [13] }^{ [16] } based on the 2 features that would characterise all future versions of the Kelvin scale:

- Absolute zero is the null point.
- Increments have the same magnitude as they do in the Celsius scale.

In 1892, Thomson was awarded the noble title 1st Baron Kelvin of Largs, or more succinctly Lord Kelvin. This name was a reference to the River Kelvin which flows through the grounds of Glasgow University.

In the early decades of the 20th century, the Kelvin scale was often called the "absolute Celsius" scale, indicating Celsius degrees counted from absolute zero rather than the freezing point of water, and using the same symbol for regular Celsius degrees, °C.^{ [lower-alpha 1] }

In 1873, William Thomson's older brother James coined the term * triple point *^{ [17] } to describe the combination of temperature and pressure at which the solid, liquid, and gas phases of a substance were capable of coexisting in thermodynamic equilibrium. While any two phases could coexist along a range of temperature-pressure combinations (e.g. the boiling point of water can be affected quite dramatically by raising or lowering the pressure), the triple point condition for a given substance can occur only at a single pressure and only at a single temperature. By the 1940s, the triple point of water had been experimentally measured to be about 0.6% of standard atmospheric pressure and very close to 0.01 °C per the historical definition of Celsius then in use.

In 1948, the Celsius scale was recalibrated by assigning the triple point temperature of water the value of 0.01 °C exactly^{ [18] } and allowing the melting point at standard atmospheric pressure to have an empirically determined value (and the actual melting point at ambient pressure to have a fluctuating value) close to 0 °C. This was justified on the grounds that the triple point was judged to give a more accurately reproducible reference temperature than the melting point.^{ [19] } The triple point could be measured with ±0.0001 °C accuracy, while the melting point just to ±0.001 °C.^{ [18] }

In 1954, with absolute zero having been experimentally determined to be about −273.15 °C per the definition of °C then in use, Resolution 3 of the 10th General Conference on Weights and Measures (CGPM) introduced a new internationally standardised Kelvin scale which defined the triple point as exactly 273.15 + 0.01 = 273.16 degrees Kelvin.^{ [20] }^{ [21] }

In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K.^{ [22] }^{ [23] }^{ [24] } The 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."^{ [4] }^{ [25] }^{ [26] }

After the 1983 redefinition of the metre, this left the kelvin, the second, and the kilogram as the only SI units not defined with reference to any other unit.

In 2005, noting that the triple point could be influenced by the isotopic ratio of the hydrogen and oxygen making up a water sample and that this was "now one of the major sources of the observed variability between different realizations of the water triple point", the International Committee for Weights and Measures (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 would refer to water having the isotopic composition specified for Vienna Standard Mean Ocean Water.^{ [4] }^{ [27] }^{ [28] }

In 2005, the CIPM began a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous method. In particular, the committee proposed redefining the kelvin such that the Boltzmann constant takes the exact value 1.3806505×10^{−23} J/K.^{ [29] } The committee had hoped that the program 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 program.^{ [30] }

The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition,^{ [31] } but was finally adopted at the 26th CGPM in late 2018, with a value of k = 1.380649×10^{−23} J⋅K^{−1}.^{ [32] }^{ [29] }^{ [1] }^{ [2] }^{ [4] }^{ [33] }

For scientific purposes, the main advantage is that this allows 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 unit J/K is equal to kg⋅m^{2}⋅s^{−2}⋅K^{−1}, where the kilogram, metre and second are defined in terms of the Planck constant, the speed of light, and the duration of the caesium-133 ground-state hyperfine transition respectively.^{ [2] } Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously. The challenge was to avoid degrading the accuracy of measurements close to the triple point. For practical purposes, the redefinition was unnoticed; water still freezes at 273.15 K (0 °C),^{ [2] }^{ [34] } and the triple point of water continues 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}.^{ [35] } Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now 273.1600(1) K.

The new definition officially came into force on 20 May 2019, the 144th anniversary of the Metre Convention.^{ [33] }^{ [1] }^{ [2] }^{ [4] }

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 5772 K as adopted by IAU 2015 Resolution B3.

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.

For 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.

The only SI derived unit with a special name derived from the kelvin is the degree Celsius. Like other SI units, the kelvin can also be modified by adding a metric prefix that multiplies it by a power of 10:

Submultiples | Multiples | ||||
---|---|---|---|---|---|

Value | SI symbol | Name | Value | SI symbol | Name |

10^{−1} K | dK | decikelvin | 10^{1} K | daK | decakelvin |

10^{−2} K | cK | centikelvin | 10^{2} K | hK | hectokelvin |

10^{−3} K | mK | millikelvin | 10^{3} K | kK | kilokelvin |

10^{−6} K | μK | microkelvin | 10^{6} K | MK | megakelvin |

10^{−9} K | nK | nanokelvin | 10^{9} K | GK | gigakelvin |

10^{−12} K | pK | picokelvin | 10^{12} K | TK | terakelvin |

10^{−15} K | fK | femtokelvin | 10^{15} K | PK | petakelvin |

10^{−18} K | aK | attokelvin | 10^{18} K | EK | exakelvin |

10^{−21} K | zK | zeptokelvin | 10^{21} K | ZK | zettakelvin |

10^{−24} K | yK | yoctokelvin | 10^{24} K | YK | yottakelvin |

10^{−27} K | rK | rontokelvin | 10^{27} K | RK | ronnakelvin |

10^{−30} K | qK | quectokelvin | 10^{30} K | QK | quettakelvin |

According to SI convention, the kelvin is never referred to nor written as a *degree*. The word "kelvin" is not capitalised when used as a unit. It is pluralised as appropriate (for example, "it is 283 kelvins outside", in contrast with "it is 50 degrees Fahrenheit" or "10 degrees Celsius").^{ [36] }^{ [37] }^{ [38] } The unit symbol K is a capital letter.^{ [22] } It is common convention to capitalize Kelvin when referring to Lord Kelvin^{ [5] } or the Kelvin scale.^{ [39] }

The unit symbol K is encoded in Unicode at code point U+212AKKELVIN SIGN. However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using U+004BKLATIN CAPITAL LETTER K instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: U+2126ΩOHM SIGN, U+212AKKELVIN SIGN, and U+212BÅANGSTROM SIGN. In all three instances, the regular letter should be used."^{ [40] }

- ↑ For example,
*Encyclopaedia Britannica*editions from the 1920s and 1950s, one example being the article "Planets".

**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 zero kelvin. 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 degrees on the Celsius scale, which equals −459.67 degrees on the Fahrenheit scale. The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition.

The **Fahrenheit scale** is a temperature scale based on one proposed in 1724 by the European physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the **degree Fahrenheit** as the unit. Several accounts of how he originally defined his scale exist, but the original paper suggests the lower defining point, 0 °F, was established as the freezing temperature of a solution of brine made from a mixture of water, ice, and ammonium chloride. The other limit established was his best estimate of the average human body temperature, originally set at 90 °F, then 96 °F.

The **kilogram** is the base unit of mass in the International System of Units (SI), having the unit symbol **kg**. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a **kilo** colloquially. It means 'one thousand grams'.

The **Metre Convention**, also known as the **Treaty of the Metre**, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States of America, and Venezuela.

The **Rankine scale** is an absolute scale of thermodynamic temperature named after the University of Glasgow engineer and physicist Macquorn Rankine, who proposed it in 1859.

The **International System of Units**, internationally known by the abbreviation **SI**, is the modern form of the metric system and the world's most widely used system of measurement. Coordinated by the International Bureau of Weights and Measures it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

The **SI base units** are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.

In thermodynamics, the **triple point** of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium. It is that temperature and pressure at which the sublimation, fusion, and vaporisation curves meet. For example, the triple point of mercury occurs at a temperature of −38.8 °C (−37.8 °F) and a pressure of 0.165 mPa.

The **caesium standard** is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.

**Thermodynamic temperature** is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.

The **Boltzmann constant** is the proportionality factor that relates the average relative thermal energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, and in Planck's law of black-body radiation and Boltzmann's entropy formula, and is used in calculating thermal noise in resistors. The Boltzmann constant has dimensions of energy divided by temperature, the same as entropy. It is named after the Austrian scientist Ludwig Boltzmann.

The **molar gas constant** is denoted by the symbol *R* or *R*. It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per amount of substance, rather than energy per temperature increment per *particle*. The constant is also a combination of the constants from Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law. It is a physical constant that is featured in many fundamental equations in the physical sciences, such as the ideal gas law, the Arrhenius equation, and the Nernst equation.

The term * degree* is used in several scales of temperature, with the notable exception of kelvin, primary unit of temperature for engineering and the physical sciences. The degree symbol

**Vienna Standard Mean Ocean Water** (**VSMOW**) is an isotopic standard for water, that is, a particular sample of water whose proportions of different isotopes of hydrogen and oxygen are accurately known. VSMOW is distilled from ocean water and does not contain salt or other impurities. Published and distributed by the Vienna-based International Atomic Energy Agency in 1968, the standard and its essentially identical successor, VSMOW2, continue to be used as a reference material.

The **International Temperature Scale of 1990** (**ITS-90**) is an equipment calibration standard specified by the International Committee of Weights and Measures (CIPM) for making measurements on the Kelvin and Celsius temperature scales. It is an approximation of thermodynamic temperature that facilitates the comparability and compatibility of temperature measurements internationally. It defines fourteen calibration points ranging from 0.65 K to 1357.77 K and is subdivided into multiple temperature ranges which overlap in some instances. ITS-90 is the most recent of a series of International Temperature Scales adopted by the CIPM since 1927. Adopted at the 1989 General Conference on Weights and Measures, it supersedes the International Practical Temperature Scale of 1968 and the 1976 "Provisional 0.5 K to 30 K Temperature Scale". The CCT has also published several online guidebooks to aid realisations of the ITS-90. The lowest temperature covered by the ITS-90 is 0.65 K. In 2000, the temperature scale was extended further, to 0.9 mK, by the adoption of a supplemental scale, known as the Provisional Low Temperature Scale of 2000 (PLTS-2000).

The **degree Celsius** is the unit of temperature on the **Celsius scale**, one of two temperature scales used in the International System of Units (SI), the other being the closely related Kelvin scale. The degree Celsius can refer to a specific temperature on the Celsius scale or to a difference or range between two temperatures. It is named after the Swedish astronomer Anders Celsius (1701–1744), who proposed the first version of it in 1742. The unit was called *centigrade* in several languages for many years. In 1948, the International Committee for Weights and Measures renamed it to honor Celsius and also to remove confusion with the term for one hundredth of a gradian in some languages. Most countries use this scale.

**Temperature** is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making up a substance.

**Scale of temperature** is a methodology of calibrating the physical quantity temperature in metrology. Empirical scales measure temperature in relation to convenient and stable parameters or **reference points**, such as the freezing and boiling point of water. Absolute temperature is based on thermodynamic principles: using the lowest possible temperature as the zero point, and selecting a convenient incremental unit.

In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.

The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.

- 1 2 3 4 BIPM (2019-05-20). "Mise en pratique for the definition of the kelvin in the SI".
*BIPM.org*. Retrieved 2022-02-18. - 1 2 3 4 5 6 7 "SI Brochure: The International System of Units (SI) – 9th edition (updated in 2022)". BIPM. Retrieved 2022-09-07.
- ↑ "SI base unit: kelvin (K)".
*bipm.org*. BIPM. Retrieved 2022-03-05. - 1 2 3 4 5 "A Turning Point for Humanity: Redefining the World's Measurement System".
*NIST*. 2018-05-12. Retrieved 2022-02-21. - 1 2 3 4 "Kelvin: Introduction".
*NIST*. 2018-05-14. Retrieved 2022-09-02. - 1 2 Benham, Elizabeth (2020-10-06). "Busting Myths about the Metric System".
*NIST*. Taking Measure (official blog of the NIST). Retrieved 2022-02-21. - ↑ "Handbook 44 – 2022 – Appendix C – General Tables of Units of Measurement" (PDF).
*nist.gov*. NIST. Retrieved 2022-02-21. - ↑ "Kelvin: History".
*NIST*. 2018-05-14. Retrieved 2022-02-21. - ↑ Dalton, John (1801). "Essay II. On the force of steam or vapour from water and various other liquids, both in vacuum and in air".
*Memoirs of the Literary and Philosophical Society of Manchester*. 5 part 2: 550–574. - ↑ Dalton, John (1801). "Essay IV. On the expansion of elastic fluids by heat".
*Memoirs of the Literary and Philosophical Society of Manchester*. 5 part 2: 595–602. - ↑ Gay-Lussac, Joseph Louis (1802), "Recherches sur la dilatation des gaz et des vapeurs",
*Annales de Chimie*,**XLIII**: 137. English translation (extract). - ↑ Thomson, William. "On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations".
*zapatopi.net*. Philosophical Magazine. Retrieved 2022-02-21. - 1 2 Thomson, William. "On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations (1881 reprint)" (PDF). Philosophical Magazine. Retrieved 2022-02-21.
- ↑ Kelvin, William (October 1848). "On an Absolute Thermometric Scale".
*Philosophical Magazine*. Archived from the original on 2008-02-01. Retrieved 2008-02-06. - ↑ Thomson, William. "On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations (1881 reprint)" (PDF). Philosophical Magazine. Retrieved 2022-02-21.
If we push the strict principle of graduation, stated above, sufficiently far, we should arrive at a point corresponding to the volume of air being reduced to nothing, which would be marked as -273° of the scale (-100/·366, if ·366 be the coefficient of expansion); and therefore -273° of the air-thermometer is a point which cannot be reached at any finite temperature, however low

- ↑ Thomson, William. "On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule's equivalent of a Thermal Unit, and M. Regnault's Observations on Steam (Excerpts)".
*Zapatopi.net*. Transactions of the Royal Society of Edinburgh and Philosophical Magazine. Retrieved 2022-02-21. - ↑ Thomson, James (1873). "A quantitative investigation of certain relations between the gaseous, the liquid, and the solid states of water-substance".
*Proceedings of the Royal Society of London*.**22**: 28. Bibcode:1873RSPS...22...27T. ISSN 0370-1662.and consequently that the three curves would meet or cross each other in one point, which I have called the

*triple point*. - 1 2 Swinton, F. L. (September 1967). "The triplet point of water".
*Journal of Chemical Education*.**44**(9): 541. Bibcode:1967JChEd..44..541S. doi:10.1021/ed044p541. ISSN 0021-9584. - ↑ "Resolution 3 of the 9th CGPM (1948)".
*bipm.org*. BIPM. Retrieved 2022-02-21. - ↑ "Resolution 3 of the 10th CGPM (1954)".
*bipm.org*. BIPM. Retrieved 2022-02-21. - ↑ "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 2007-06-23. Retrieved 2008-02-06. - 1 2 "Resolution 3 of the 13th CGPM (1967)".
*bipm.org*. BIPM. Retrieved 2022-02-21. - ↑ "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 2007-04-21. Retrieved 2008-02-06. - ↑ Westphal, Wilhelm Heinrich (1952). "Nox, Dunkelleuchtdichte, Skot". In Westphal, Wilhelm H. (ed.).
*Physikalisches Wörterbuch*(in German) (1 ed.). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag OHG. pp. 125, 271, 389. doi:10.1007/978-3-662-12706-3. ISBN 978-3-662-12707-0 . Retrieved 2023-03-16. pp. 271, 389:Dunkelleuchtdichte. […] Unter Zugrundelegung dieser Empfindlichkeitskurve hat man 1940 in Deutschland die Dunkelleuchtdichte mit der Einheit Skot (sk) so festgesetzt, daß bei einem Licht der Farbtemperatur 2360 °K 1 sk = 10

^{−3}asb gilt. 1948 ist von der Internationalen Beleuchtungskommission (IBK) die Bezugstemperatur auf 2046 °K, die Erstarrungstemperatur des Platins, festgesetzt worden. Die Bezeichnung Skot wurde von der IBK nicht übernommen, dafür soll "skotopisches Stilb" gesagt werden. Als höchstzulässiger Grenzwert für die Dunkelleuchtdichte ist in Deutschland 10 Skot festgesetzt worden, um eine Verwendung der Dunkelleuchtdichte im Gebiet des gemischten Zapfen- und Stäbchensehens zu vermeiden, da in diesem Bereich die photometrischen Maßgrößen wegen der allmählich gleitenden Augenempfindlichkeitskurve ihren Sinn verlieren. […] Skot, abgek[ürzt] sk, Einheit für die Dunkelleuchtdichte, welche für zahlenmäßige Angaben und zum Anschluß der Dunkelleuchtdichte an die normale Leuchtdichte 1940 von der Deutschen Lichttechnischen Gesellschaft geschaffen wurde. Für diesen Anschluß wurde die Strahlung des schwarzen Körpers bei*T*= 2360 °K, d.h. eine Strahlung der Farbtemperatur*T*_{1}= 2360 °K vereinbart. Eine Lichtquelle strahlt mit der Dunkelleuchtdichte 1 sk, wenn sie photometrisch gleich einer Strahlung der Farbtemperatur*T*_{2}= 2360 °K und der Leuchtdichte von 10^{−3}asb (Apostilb) ist. Bei der Farbtemperatur*T*_{1}= 2360 °K gilt also die Relation: 1 sk = 10^{−3}asb = 10^{−7}/π sb. - ↑ "Resolution 4 of the 13th CGPM (1967)".
*bipm.org*. BIPM. Retrieved 2022-02-21. - ↑ "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 2007-06-15. Retrieved 2008-02-06. - ↑ "Resolution 10 of the 23rd CGPM (2007)".
*bipm.org*. BIPM. Retrieved 2022-02-21. - ↑ "Unit of thermodynamic temperature (kelvin)".
*SI Brochure, 8th edition*. Bureau International des Poids et Mesures. 1967. pp. Section 2.1.1.5. Archived from the original on 2007-09-26. Retrieved 2008-02-06. - 1 2 Ian Mills (2010-09-29). "Draft Chapter 2 for SI Brochure, following redefinitions of the base units" (PDF). CCU. Archived (PDF) from the original on 2011-01-10. Retrieved 2011-01-01.
- ↑ "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. 2011-10-23. Archived (PDF) from the original on 2012-02-09. Retrieved 2011-10-25.
- ↑ 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 2015-10-13.
[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.

- ↑ "2018 CODATA Value: Boltzmann constant".
*The NIST Reference on Constants, Units, and Uncertainty*. NIST. 2019-05-20. Retrieved 2019-05-20. - 1 2 "Resolution 1 of the 26th CGPM (2018)".
*bipm.org*. BIPM. Retrieved 2022-02-21. - ↑ "Updating the definition of the kelvin" (PDF). International Bureau for Weights and Measures (BIPM). Archived (PDF) from the original on 2008-11-23. Retrieved 2010-02-23.
- ↑ 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) (2018-01-29). "The CODATA 2017 values of
*h*,*e*,*k*, and*N*_{A}for the revision of the SI".*Metrologia*.**55**(1): L13–L16. Bibcode:2018Metro..55L..13N. doi: 10.1088/1681-7575/aa950a . - ↑ "Kelvin: Introduction".
*www.nist.gov*. 2018-05-14. Retrieved 2023-08-21. - ↑ "Definition of KELVIN".
*www.merriam-webster.com*. Retrieved 2023-08-21. - ↑
*CERN English Language Style Guide*(PDF). CERN. 2022. p. 64. - ↑ Brady, James E.; Senese, Fred (2008-01-28).
*Chemistry, Student Study Guide: The Study of Matter and Its Changes*. John Wiley & Sons. p. 15. ISBN 978-0-470-18464-6. - ↑ "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 2016-12-06. Retrieved 2015-09-06.

- Bureau International des Poids et Mesures (2019). "The International System of Units (SI) Brochure" (PDF). 9th Edition. International Committee for Weights and Measures. Retrieved 2022-04-28.

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