The kelvin, symbol K, is the primary unit of
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
in the
International System of Units (SI), used alongside its
prefixed forms and the
degree Celsius
Degree may refer to:
As a unit of measurement
* Degree (angle), a unit of angle measurement
** Degree of geographical latitude
** Degree of geographical longitude
* Degree symbol (°), a notation used in science, engineering, and mathematics
...
.
It is named after the
Belfast
Belfast ( , ; from ga, Béal Feirste , meaning 'mouth of the sand-bank ford') is the capital and largest city of Northern Ireland, standing on the banks of the River Lagan on the east coast. It is the 12th-largest city in the United Kingdom ...
-born and
University of Glasgow
, image = UofG Coat of Arms.png
, image_size = 150px
, caption = Coat of arms
Flag
, latin_name = Universitas Glasguensis
, motto = la, Via, Veritas, Vita
, ...
-based
engineer
Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the limit ...
and
physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicists generally are interested in the root or ultimate cau ...
William Thomson, 1st Baron Kelvin
William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, Mathematical physics, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy (Glasgow), Professor of Natural Philoso ...
(1824–1907). The Kelvin scale is an
absolute thermodynamic temperature
Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...
scale, meaning it uses
absolute zero as its null (zero) point.
Historically, the Kelvin scale was developed by shifting the starting point of the much-older Celsius scale down from the
melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
of
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
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
The Boltzmann constant ( or ) 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, ...
to be exactly .
Hence, one kelvin is equal to a change in the thermodynamic temperature that results in a change of thermal energy by . The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15,
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
Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
and the
physical sciences
Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "physical sciences".
Definition
Phy ...
, 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
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 (symbol: °F) as the unit. Several accounts of how he originally defined hi ...
predominates, with the kelvin or
Rankine scale
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.
History
Similar to the Kelvin scale, which was first proposed in 1848 ...
employed for absolute temperature. Those are defined using the kelvin.
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, 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".
It is common convention to capitalize Kelvin when referring to the Kelvin scale.
History
Precursors
During the
18th century,
multiple temperature scales were developed,
notably
Fahrenheit
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 (symbol: °F) as the unit. Several accounts of how he originally defined hi ...
and centigrade (later
Celsius). These scales predated much of the modern science of
thermodynamics
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
, including
atomic theory
Atomic theory is the scientific theory that matter is composed of particles called atoms. Atomic theory traces its origins to an ancient philosophical tradition known as atomism. According to this idea, if one were to take a lump of matter ...
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
The Newton scale is a temperature scale devised by Isaac Newton in 1701.
He called his device a "thermometer", but he did not use the term "temperature", speaking of "degrees of heat" (''gradus caloris'') instead.
Newton's publication represents ...
and
Réaumur scale
__NOTOC__
The Réaumur scale (; °Ré, °Re, °r), also known as the "octogesimal division", is a temperature scale for which the melting and boiling points of water are defined as 0 and 80 degrees respectively. The scale is named for René An ...
) the melting point of
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
served as such a starting point, with Celsius being defined, from the
1740s
File:1740s montage.jpg, 420x420px, From top left, clockwise: The War of Jenkins' Ear, a conflict between the British and Spanish Empires lasting from 1739 to 1748. The War of the Austrian Succession from 1740 to 1748, caused by the death of Emperor ...
up until the
1940s
File:1940s decade montage.png, Above title bar: events during World War II (1939–1945): From left to right: Troops in an LCVP landing craft approaching Omaha Beach on D-Day; Adolf Hitler visits Paris, soon after the Battle of France; The Ho ...
, by calibrating a thermometer such that:
* The
freezing point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depend ...
of water is 0 degrees.
* The
boiling point of water is 100 degrees.
This definition assumes pure water at a specific
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
chosen to approximate the natural air pressure at
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
. Thus an increment of 1 °C equals of the temperature difference between the melting and boiling points. This temperature interval would go on to become the template for the kelvin.
Lord Kelvin
In 1848, William Thomson, who was later ennobled as
Lord Kelvin
William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy at the University of Glasgow for 53 years, he did important ...
, published a paper ''On an Absolute Thermometric Scale''.
Using the soon-to-be-defunct
caloric theory
The caloric theory is an obsolete scientific theory that heat consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores i ...
, 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
In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...
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
In physics, work is the energy transferred to or from an object via the application of force along a displacement. In its simplest form, for a constant force aligned with the direction of motion, the work equals the product of the force stre ...
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 at the temperature ° of this scale, to a body at the temperature ( − 1)°, would give out the same mechanical effect, whatever be the number . 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
The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, de ...
can be converted to mechanical
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
and the predicted maximum efficiency is a function of the ''ratio'' between the absolute temperatures of the heat source and heat sink:
*Efficiency ≤ 1 −
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, , on the 1848 scale is related to the absolute temperature of the melting point of water, , and the absolute temperature of the boiling point of water, , by:
* (1848 scale) = 100 () / ()
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
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kineti ...
of an ideal gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.
Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one
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
Traditional rank amongst European royalty, peers, and nobility is rooted in Late Antiquity and the Middle Ages. Although they vary over time and among geographic regions (for example, one region's prince might be equal to another's grand duk ...
1st
Baron Kelvin of
Largs
Largs ( gd, An Leargaidh Ghallda) is a town on the Firth of Clyde in North Ayrshire, Scotland, about from Glasgow. The original name means "the slopes" (''An Leargaidh'') in Scottish Gaelic.
A popular seaside resort with a pier, the town mark ...
, or more succinctly Lord Kelvin. This name was a reference to the
River Kelvin
The River Kelvin (Scottish Gaelic: ''Abhainn Cheilbhinn'') is a tributary of the River Clyde in northern and northeastern Glasgow, Scotland. It rises on the moor south east of the village of Banton, east of Kilsyth. At almost long, it init ...
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.
Triple point standard
In 1873, William Thomson's older brother
James
James is a common English language surname and given name:
*James (name), the typically masculine first name James
* James (surname), various people with the last name James
James or James City may also refer to:
People
* King James (disambiguati ...
coined the term ''
triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the sub ...
'' to describe the combination of temperature and
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
at which the solid, liquid, and gas
phases of a substance were capable of coexisting in
thermodynamic equilibrium
Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In the ...
. While any two phases could coexist along a range of temperature-pressure combinations (e.g. the
boiling point of
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
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
The standard atmosphere (symbol: atm) is a unit of pressure defined as 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.
History
The s ...
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
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
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.
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".
In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K.
The 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction of the thermodynamic temperature of the triple point of water."
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
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
(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
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 ( ...
.
2019 redefinition
In 2005, the
CIPM
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
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
The Boltzmann constant ( or ) 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, ...
takes the exact value .
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.
The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition,
but was finally adopted at the 26th CGPM in late 2018, with a value of =
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
The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its prefi ...
and
second are defined in terms of the
Planck constant
The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivale ...
, the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
, and the duration of the
caesium-133 ground-state
hyperfine transition
In atomic physics, hyperfine structure is defined by small shifts in otherwise degenerate energy levels and the resulting splittings in those energy levels of atoms, molecules, and ions, due to electromagnetic multipole interaction between the nuc ...
respectively.
Thus, this definition depends only on
universal constants
A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is contrasted with a mathematical constant, ...
, 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),
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 , with a relative standard uncertainty of .
Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now .
The new definition officially came into force on 20 May 2019, the 144th anniversary of the
Metre Convention
The Metre Convention (french: link=no, Convention du Mètre), 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, Brazi ...
.
Practical uses
Colour temperature
The kelvin is often used as a measure of the
colour temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
of light sources. Colour temperature is based upon the principle that a
black body
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about appear reddish, whereas those above about appear bluish. Colour temperature is important in the fields of image projection and
photography
Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employe ...
, where a colour temperature of approximately is required to match "daylight" film emulsions. In
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
, 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
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, for instance, has an effective temperature of .
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
For electronics, the kelvin is used as an indicator of how
noisy a circuit is in relation to an ultimate
noise floor
In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored.
In radio com ...
, i.e. the
noise temperature
In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. (This is to be distinguished from Temperature Noise in Thermodynamics or Principal Interferrometric Analysis Over C ...
. The so-called
Johnson–Nyquist noise
Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens reg ...
of discrete resistors and capacitors is a type of thermal noise derived from the
Boltzmann constant
The Boltzmann constant ( or ) 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 can be used to determine the noise temperature of a circuit using the
Friis formulas for noise
Friis formula or Friis's formula (sometimes Friis' formula), named after Danish-American electrical engineer Harald T. Friis, is either of two formulas used in telecommunications engineering to calculate the signal-to-noise ratio of a multistage a ...
.
Derived units and SI multiples
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
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pr ...
that multiplies it by a
power of 10
A power of 10 is any of the integer powers of the number ten; in other words, ten multiplied by itself a certain number of times (when the power is a positive integer). By definition, the number one is a power (the zeroth power) of ten. The f ...
:
Unicode character
The symbol is encoded in
Unicode
Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, wh ...
at code point . However, this is a
compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using instead; that is, a normal capital
K. "Three letterlike symbols have been given canonical equivalence to regular letters: , , and . In all three instances, the regular letter should be used."
See also
*
Comparison of temperature scales
This is a collection of temperature conversion formulas and comparisons among eight different temperature scales, several of which have long been obsolete.
Temperatures on scales that either do not share a numeric zero or are nonlinearly related c ...
*
International Temperature Scale of 1990
*
Negative temperature
Certain systems can achieve negative thermodynamic temperature; that is, their temperature can be expressed as a negative quantity on the Kelvin or Rankine scales. This should be distinguished from temperatures expressed as negative numbers ...
References
Bibliography
*
External links
{{CGS units
1848 introductions
Scottish inventions
SI base units
William Thomson, 1st Baron Kelvin
Scales of temperature
Scales in meteorology