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 the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius.^{ [1] }^{ [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). The **Kelvin scale** is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its null (zero) point.^{ [2] }^{ [5] }

- History
- Precursors
- Lord Kelvin
- Triple point standard
- 2019 redefinition
- Practical uses
- Colour temperature
- Kelvin as a unit of noise temperature
- Derived units and SI multiples
- Unicode character
- See also
- References
- Bibliography
- External links

Historically, the Kelvin scale was developed by shifting the starting point of the much-older Celsius scale down from the melting point of water to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius, but since 2019 the scale has been defined by fixing the Boltzmann constant k to be exactly 1.380649×10^{−23} J⋅K^{−1}.^{ [1] } Hence, one kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy *kT* by 1.380649×10^{−23} J. The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15,^{ [2] } meaning that a *change* or *difference* in temperature has the same value when expressed in degrees Celsius as in kelvins, and that 0 °C is equal to 273.15 K.

The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries Celsius remains the dominant scale outside of these fields. In the United States, outside of the physical sciences the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature. Those are defined using the kelvin.^{ [6] }^{ [7] }

The kelvin is never referred to nor written as a *degree*. The word *kelvin* is not capitalised, but is pluralised as appropriate. The unit symbol K is a capital letter. For example, "It is 50 degrees Fahrenheit outside" vs "It is 10 degrees Celsius outside" vs "It is 283 kelvins outside".^{ [8] }

During the 18th century, multiple temperature scales were developed,^{ [9] } 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. This temperature interval would go on to become the template for the kelvin.^{[ citation needed ]}

In 1848, William Thomson, who was later ennobled as Lord Kelvin, published a paper *On an Absolute Thermometric Scale*.^{ [10] }^{ [11] }^{ [12] } 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.^{[ citation needed ]}

Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one^{ [11] }^{ [13] } 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.^{ [14] }

In 1873 William Thomson's older brother James coined the term * triple point *^{ [15] } 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 1 pressure and only at 1 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 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.^{ [16] }

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"^{ [17] }^{ [18] }

In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K.^{ [8] }^{ [19] } 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] }^{ [20] }^{ [21] }

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] }^{ [22] }^{ [23] }

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.^{ [24] } 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.^{ [25] }

The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition,^{ [26] } but was finally adopted at the 26th CGPM in late 2018, with a value of k = 1.380649×10^{−23} J⋅K^{−1}.^{ [27] }^{ [24] }^{ [1] }^{ [2] }^{ [4] }^{ [28] }

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] }^{ [29] } 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}.^{ [30] } 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.^{ [28] }^{ [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 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.

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 |

The symbol 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."^{ [31] }

**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 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 **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,** known by the international abbreviation **SI** in all languages and sometimes pleonastically as the **SI system**, is the modern form of the metric system and based on the metre as the unit of length and either the kilogram as the unit of mass or the kilogram-force as the unit of force.</ref> and the world's most widely used system of measurement. Established and maintained by the General Conference on Weights and Measures (CGPM), 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 curve, fusion curve and the vaporisation curve 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 kinetic 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, i.e. the pressure–volume product, 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 **standard atmosphere** is a unit of pressure defined as 101325 Pa. It is sometimes used as a *reference pressure* or *standard pressure*. It is approximately equal to Earth's average atmospheric pressure at sea level.

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. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency in 1968, and since 1993 continues to be evaluated and studied by the IAEA along with the European Institute for Reference Materials and Measurements and the American National Institute of Standards and Technology. The standard includes both the established values of stable isotopes found in waters and calibration materials provided for standardization and interlaboratory comparisons of instruments used to measure these values in experimental materials.

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).

A **conventional electrical unit** is a unit of measurement in the field of electricity which is based on the so-called "conventional values" of the Josephson constant, the von Klitzing constant agreed by the International Committee for Weights and Measures (CIPM) in 1988, as well as Δ*ν*_{Cs} used to define the second. These units are very similar in scale to their corresponding SI units, but are not identical because of the different values used for the constants. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol *V*_{90} – as they came into international use on 1 January 1990.

The **degree Celsius** is the unit of temperature on the **Celsius scale**, one of 2 temperature scales used in the International System of Units (SI), alongside the Kelvin scale. The degree Celsius can refer to a specific temperature on the Celsius scale or a unit to indicate a difference or range between two temperatures. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale in 1742. Before being renamed in 1948 to honour Anders Celsius, the unit was called *centigrade*, from the Latin *centum*, which means 100, and *gradus*, which means steps. Most major countries use this scale; the other major scale, Fahrenheit, is still used in the United States, some island territories, and Liberia. The Kelvin scale is of use in the sciences, with 0 K (−273.15 °C) representing absolute zero.

**Temperature** is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.

**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, 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 (20 May 2019). "Mise en pratique for the definition of the kelvin in the SI".
*BIPM.org*. Retrieved 18 February 2022. - 1 2 3 4 5 6 7 "SI Brochure: The International System of Units (SI) – 9th edition". BIPM. Retrieved 21 February 2022.
- ↑ "SI base unit: kelvin (K)".
*bipm.org*. BIPM. Retrieved 5 March 2022. - 1 2 3 4 5 "A Turning Point for Humanity: Redefining the World's Measurement System".
*Nist*. 12 May 2018. Retrieved 21 February 2022. - ↑ "Kelvin: Introduction".
*NIST*. 14 May 2018. Retrieved 2 September 2022. - ↑ Benham, Elizabeth (6 October 2020). "Busting Myths about the Metric System".
*Nist*. Taking Measure (official blog of the NIST). Retrieved 21 February 2022. - ↑ "Handbook 44 – 2022 – Appendix C – General Tables of Units of Measurement" (PDF).
*nist.gov*. NIST. Retrieved 21 February 2022. - 1 2 "Resolution 3 of the 13th CGPM (1967)".
*bipm.org*. BIPM. Retrieved 21 February 2022. - ↑ "Kelvin: History".
*Nist*. 14 May 2018. Retrieved 21 February 2022. - ↑ 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 21 February 2022. - 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 21 February 2022.
- ↑ Lord Kelvin, William (October 1848). "On an Absolute Thermometric Scale".
*Philosophical Magazine*. Archived from the original on 1 February 2008. Retrieved 6 February 2008. - ↑ 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 21 February 2022. - ↑ For example,
*Encyclopaedia Britannica*editions from the 1920s and 1950s, one example being the article "Planets". - ↑ James Thomson (1873) "A quantitative investigation of certain relations between the gaseous, the liquid, and the solid states of water-substance",
*Proceedings of the Royal Society*,**22**: 27–36. From a footnote on page 28: " … the three curves would meet or cross each other in one point, which I have called the*triple point*". - ↑ "Resolution 3 of the 9th CGPM (1948)".
*bipm.org*. BIPM. Retrieved 21 February 2022. - ↑ "Resolution 3 of the 10th CGPM (1954)".
*bipm.org*. BIPM. Retrieved 21 February 2022. - ↑ "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. - ↑ "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. - ↑ "Resolution 4 of the 13th CGPM (1967)".
*bipm.org*. BIPM. Retrieved 21 February 2022. - ↑ "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. - ↑ "Resolution 10 of the 23rd CGPM (2007)".
*bipm.org*. BIPM. Retrieved 21 February 2022. - ↑ "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 26 September 2007. Retrieved 6 February 2008. - 1 2 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.
- ↑ "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.
- ↑ 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.

- ↑ "2018 CODATA Value: Boltzmann constant".
*The NIST Reference on Constants, Units, and Uncertainty*. NIST. 20 May 2019. Retrieved 20 May 2019. - 1 2 "Resolution 1 of the 26th CGPM (2018)".
*bipm.org*. BIPM. Retrieved 21 February 2022. - ↑ "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.
- ↑ 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*N*_{A}for the revision of the SI".*Metrologia*.**55**(1): L13–L16. Bibcode:2018Metro..55L..13N. doi: 10.1088/1681-7575/aa950a . - ↑ "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.

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

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