The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International
System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic
temperature scale using as its null point absolute zero, the temperature at which all
thermal motion ceases in the classical description of thermodynamics. The kelvin is defined as
the fraction 1⁄273.16 of the thermodynamic temperature of the triple point of water.
In other words, it is defined such that the triple point of water is exactly 273.16 K.
The Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist
William Thomson, 1st Baron Kelvin, who wrote of the need for an "absolute thermometric
scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or
typeset as a degree. The kelvin is the primary unit of measurement in the physical sciences,
but is often used in conjunction with the degree Celsius, which has the same magnitude.
Subtracting 273.16 K from the temperature of the triple point of water makes absolute
zero equivalent to −273.15 °C.
History
In 1848 Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need
for a scale whereby "infinite cold" was the scale's null point, and which used the degree
Celsius for its unit increment. Thomson calculated that absolute zero was equivalent to −273 °C
on the air thermometers of the time. This absolute scale is known today as the Kelvin
thermodynamic temperature scale. Thomson's value of "−273" was the reciprocal of 0.00366—the
accepted expansion coefficient of gas per degree Celsius relative to the ice point,
giving a remarkable consistency to the currently accepted value.
In 1954, the Resolution 3 of the 10th CGPM gave the Kelvin scale its modern definition
by designating the triple point of water as its second defining point and assigned its
temperature to exactly 273.16 kelvins. In 1967/1968 Resolution 3 of the 13th CGPM
renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing
"degree Kelvin", symbol °K. Furthermore, feeling it useful to more explicitly define
the magnitude of the unit increment, the 13th CGPM also held in Resolution 4 that "The kelvin,
unit of thermodynamic temperature, is equal to the fraction 1⁄273.16 of the thermodynamic
temperature of the triple point of water." In 2005 The Comité International des Poids
et Mesures, a committee of the CGPM, affirmed that for the purposes of delineating the temperature
of the triple point of water, the definition of the Kelvin thermodynamic temperature scale
would refer to water having an isotopic composition specified as VSMOW.
Usage conventions This SI unit is named after William Thomson,
1st Baron Kelvin. As with every International System of Units unit whose name is derived
from the proper name of a person, the first letter of its symbol is upper case. However,
when an SI unit is spelled out in English, it should always begin with a lower case letter,
except in a situation where any word in that position would be capitalized, such as at
the beginning of a sentence or in capitalized material such as a title. Note that "degree
Celsius" conforms to this rule because the "d" is lowercase.— Based on The International
System of Units, section 5.2. When spelled out or spoken, the unit is pluralised using
the same grammatical rules as for other SI units such as the volt or ohm. When reference
is made to the "Kelvin scale", the word "kelvin"—which is normally a noun—functions adjectivally
to modify the noun "scale" and is capitalized. As with most other SI unit symbols there is
a space between the numeric value and the kelvin symbol.
Before the 13th General Conference on Weights and Measures in 1967–1968, the unit kelvin
was called a "degree", the same as with the other temperature scales at the time. It was
distinguished from the other scales with either the adjective suffix "Kelvin" or with "absolute"
and its symbol was °K. The latter, which was the unit's official name from 1948 until
1954, was rather ambiguous since it could also be interpreted as referring to the Rankine
scale. Before the 13th CGPM, the plural form was "degrees absolute". The 13th CGPM changed
the unit name to simply "kelvin". The omission of "degree" indicates that it is not relative
to an arbitrary reference point like the Celsius and Fahrenheit scales, but rather an absolute
unit of measure which can be manipulated algebraically. Use in conjunction with Celsius
In science and in engineering, degrees Celsius and kelvins are often used simultaneously
in the same article. This practice is permissible because the degree Celsius is a special name
for the kelvin for use in expressing Celsius temperatures and the magnitude of the degree
Celsius is exactly equal to that of the kelvin. Notwithstanding that the official endorsement
provided by Resolution 3 of the 13th CGPM states, "a temperature interval may also be
expressed in degrees Celsius", the practice of simultaneously using both "°C" and "K"
remains widespread throughout the scientific world as the use of SI prefixed forms of the
degree Celsius to express a temperature interval has not been widely adopted.
Proposed redefinition
In 2005 the CIPM embarked on a program to redefine, amongst others, the kelvin using
a more rigorous basis than was in use. The current definition is unsatisfactory for temperatures
below 20 K and above 1300 K. The committee proposes defining the kelvin as the temperature
scale for which Boltzmann's constant is 1.3806505×10−23 J/K exactly. The committee 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. From a scientific point of view, this will
link temperature to the rest of SI and result in a stable definition that is independent
of any particular substance. From a practical point of view, the redefinition will pass
unnoticed; water will still freeze at 0 °C. Practical uses
Colour temperature
The kelvin is often used in the measure of the colour temperature of light sources. Colour
temperature is based upon the principle that a black body radiator emits light of which
the colour depends on the temperature of the radiator. Black bodies with 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.
Kelvin as a measure of noise
In electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an
ultimate noise floor, i.e. the noise temperature. The so-called Johnson–Nyquist noise of discrete
resistors and capacitors is a type of thermal noise derived from the Boltzmann constant
and can be used to determine the noise temperature of a circuit using the Friis formulas for
noise. Temperature conversion between units
Unicode character The symbol is encoded in Unicode, in the Letterlike
Symbols range, at codepoint U+212A K kelvin sign.
See also Comparison of temperature scales
International Temperature Scale of 1990 Negative temperature
Rankine scale Thermodynamic temperature
Triple point Notes and references
External links Bureau International des Poids et Mesures.
The International System of Units Brochure. 8th Edition. International Committee for Weights
and Measures. Retrieved 2008-02-06.