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The Kelvin is the base unit of temperature in the International System of Units (SI), having the unit symbol K. It is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).
Since 2019, the Kelvin is defined by fixing the numerical value of the Boltzmann constant k to 1.380 649×10−23 J⋅K−1. This unit is equal to kg⋅m2⋅s−2⋅K−1, where the kilogram, meter 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. Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously, such as the International Prototype of the Kilogram, whose mass diverged over time from the original value. 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.
One Kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380 649×10−23 J.
The Kelvin scale fulfills Thomson's requirements as an absolute thermodynamic temperature scale. It uses absolute zero as its null point.