Kelvin

The kelvin, symbol K, is the base unit of thermodynamic temperature in the International System of Units (SI). It measures temperature on an absolute scale whose zero corresponds to absolute zero, rather than to an arbitrarily selected material reference point. A temperature interval of one kelvin has the same magnitude as one degree Celsius. Named after William Thomson, Lord Kelvin, the unit is defined through an exact value of the Boltzmann constant, which connects temperature with a microscopic energy scale. (bipm.org)

Definition and physical meaning

Since May 20, 2019, the kelvin has been defined by fixing the Boltzmann constant, $k$, at exactly

Here J denotes the joule, the SI unit of energy. Equivalently, a change in thermodynamic temperature of 1 K produces a change of exactly $1.380649\times10^{-23}$ J in the associated energy scale $kT$. This is a statement about the temperature–energy relationship, not an assertion that every particle gains that amount of total energy whenever its temperature rises by one kelvin. (bipm.org)

The definition draws on statistical mechanics while preserving temperature as a separate base quantity. In SI base units, the Boltzmann constant has dimensions $\mathrm{kg\,m^2\,s^{-2}\,K^{-1}}$. The kilogram, metre, and second are themselves established through defining constants, including the Planck constant, the speed of light, and the caesium transition frequency. Thus the kelvin belongs to a coordinated system of constant-based definitions rather than depending on one particular substance or thermometer. (bipm.org)

Relationship to Celsius and Fahrenheit

The Celsius temperature $t$ is defined relative to thermodynamic temperature $T$ by subtracting the reference temperature 273.15 K. In terms of numerical values,

Consequently, 0 °C corresponds exactly to 273.15 K, and 25 °C corresponds exactly to 298.15 K. The offset is exact; it does not represent the uncertainty of a measured freezing point. Although the Celsius scale developed historically around properties of water, its modern definition follows from the kelvin. (nist.gov)

A temperature value must be distinguished from a temperature difference. An increase from 20 °C to 30 °C is an increase of 10 °C or 10 K: no offset is added to an interval. On the Fahrenheit scale, an interval of 1 K corresponds to 1.8 °F. Absolute zero is therefore 0 K, −273.15 °C, or −459.67 °F. These relationships allow conversion between scales without changing the physical temperature being described. (nist.gov)

Historical development

In 1848, Thomson published On an Absolute Thermometric Scale, proposing an absolute approach to temperature within the developing science of thermodynamics. His work placed the lower limit near −273 °C. The modern value of −273.15 °C reflects the subsequently standardized relationship between Celsius and thermodynamic temperature, rather than the precision of that nineteenth-century estimate. (nist.gov)

In 1954, the General Conference on Weights and Measures selected the triple point of water as the fundamental fixed point and assigned it a temperature of 273.16 K. At the triple point, solid, liquid, and vapor coexist in thermodynamic equilibrium. The resulting definition made the unit $1/273.16$ of that temperature. In 1967, the name “kelvin” and symbol K replaced “degree kelvin” and °K. (bipm.org)

The constant-based redefinition was adopted in 2018 and became effective in 2019. It removed the definition’s dependence on preparing pure water with a specified isotopic composition. The selected value of the Boltzmann constant preserved continuity with the previous unit. Water’s triple-point thermodynamic temperature consequently became a measured quantity rather than the exact defining value of the kelvin. (bipm.org)

Realization and practical measurement

In metrology, defining a unit and realizing it experimentally are distinct tasks. Primary thermometry determines thermodynamic temperature through a physical relationship whose relevant parameters can be measured independently. Methods include acoustic gas thermometry, dielectric-constant gas thermometry, and refractive-index gas thermometry. Their experimental uncertainties remain even though the defining constant is exact. (bipm.org)

Many calibrations instead use the International Temperature Scale of 1990 (ITS-90). It prescribes reproducible fixed points, instruments, and interpolation procedures, providing a practical approximation to thermodynamic temperature from 0.65 K upward. Its temperature, $T_{90}$, is not identical by definition to $T$; differences can be determined experimentally. The Provisional Low Temperature Scale of 2000 covers approximately 0.9 mK to 1 K. (bipm.org)

These scales support metrological traceability through calibrated instruments. Platinum resistance thermometers are important for contact measurements, while radiation thermometry uses blackbody radiation for non-contact measurements, particularly at high temperatures. Calibration connects an instrument’s response to an established temperature scale; the instrument does not itself define the kelvin. (nist.gov)

Names and notation

The unit name is written kelvin, normally with a lowercase initial, while its symbol is the uppercase K. It is not preceded by a degree sign: 300 K, not 300 °K. A space separates the numerical value and unit symbol. In prose, the plural is “kelvins”; the symbol remains K. SI prefixes provide convenient subdivisions, such as the millikelvin (mK), equal to $10^{-3}$ K. These conventions distinguish the unit symbol from the italic quantity symbol $T$. (nist.gov)