Kelvin (K)
Definition, realization and temperature measurement in the International System of Units
Temperature is a fundamental physical quantity that describes the thermal state of a system. It affects chemical reactions, material properties, biological processes and technical systems. The modern definition of the kelvin, and how it is realized, is less well known.
The modern kelvin is not tied to the properties of any particular material or to a specific reference point, but is defined through a fundamental constant of nature. To understand this it is, as with the other SI base units, necessary to distinguish between definition, realization and practical use.
The definition of the kelvin in the SI system
Since 2019, the kelvin has been defined within the International System of Units (SI) according to the following principle:
The kelvin is the SI unit of thermodynamic temperature. It is defined by giving the Boltzmann constant k the exact numerical value
1.380 649 × 10⁻²³ J·K⁻¹.
Since the joule can be expressed in terms of the kilogram, the meter and the second (J = kg·m²·s⁻²), this definition means that the kelvin is directly linked to energy at the microscopic level.
Temperature can thus be interpreted as a measure of the average thermal energy of the particles of a system.
Historical background and the 2019 SI reform
Before 2019, the kelvin was defined through the triple point of water, which was assigned the temperature of exactly 273.16 K. This definition was practically useful but tied the measure of temperature to the properties and isotopic composition of a specific material.
The 2019 SI reform brought a shift to a constant-based definition, in which the kelvin was instead tied to the Boltzmann constant. Temperature thereby became a fully universal quantity, independent of specific substances or phase transitions.
Continuity was also ensured: temperature values before and after the redefinition stayed the same within the measurement uncertainty.
Definition, realization and use
For temperature measurement too, it is important to distinguish between three levels:
The definition states what the kelvin is in principle and is tied to the Boltzmann constant.
The realization refers to the experimental methods by which this definition is turned into practical temperature standards.
The use refers to temperature measurement in laboratories, industry, climate research and everyday life.
Realizing the kelvin
The definition of the kelvin prescribes no specific method of temperature measurement. Instead there are several primary thermometric methods that can be used to realize temperature directly from the definition.
Common to these methods is that they link temperature to energy, motion or spectral properties of particles.
Primary thermometric methods
Examples of primary methods for realizing the kelvin are:
- Acoustic gas thermometry, in which temperature is determined from the speed of sound in a gas.
- Johnson noise thermometry, in which temperature is related to the thermal noise in an electrical resistor.
- Doppler broadening thermometry, in which the width of spectral lines is used to determine the velocity distribution of particles.
- Radiation and optical thermometry, in which temperature is determined from the electromagnetic spectrum of thermal radiation.
These methods can realize temperature without calibration against fixed reference points.
Secondary temperature standards
Practical applications often use secondary temperature standards, such as platinum resistance thermometers and thermocouples. These are not primary realizations of the kelvin, but are calibrated against primary methods or established temperature points and offer high stability and reproducibility.
International temperature scales, for example ITS-90, are used to ensure consistent temperature measurement over a wide temperature range.
The role of the kelvin in derived units
The kelvin appears in several derived SI units and physical relationships, including:
- joule per kelvin (heat capacity)
- pascal per kelvin (thermal expansion effects)
- watt per square meter per kelvin (heat transfer)
A stable, universal definition of the kelvin therefore matters for thermodynamics, materials science and climate research.
Practical temperature measurement
In everyday life and industry, temperature is almost always measured indirectly, for example through changes in electrical resistance, voltage or radiation. These measurements are traceable to the SI definition through calibration chains that begin in primary realizations of the kelvin.
Summary
The kelvin is today defined through the Boltzmann constant and is thus a unit of temperature directly linked to energy at the microscopic level. This definition makes the kelvin independent of specific materials and enables high-precision temperature measurement over very wide temperature ranges.
The transition to a constant-based definition has strengthened the foundations of temperature measurement and made the kelvin a fully integrated part of the modern, coherent SI system.