Kelvin

The kelvin, symbol K, is the SI base unit of temperature,^[1][2][3][4] named after the Belfast-born and University of Glasgow based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907). The Kelvin scale is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its null point. It was developed by shifting the starting point of the much older Celsius scale down from the melting point of water to absolute zero and its increments still closely approximate the historic definition of a degree Celsius, but as of 2019 the scale is defined by fixing the numerical value of the Boltzmann constant k to 1.380649×10⁻²³ J⋅K⁻¹.^[1] Hence, one kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380649×10⁻²³ J. The temperature in Celsius is now defined as the temperature in kelvins minus 273.15,^[2] meaning that a change or difference in temperature has exactly the same value when expressed in kelvins as in Celsius, that absolute zero is exactly 0 K and −273.15 °C, and that pure water freezes very close to 273.15 K and boils close to 373.15 K at standard atmospheric pressure.

The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries Celsius remains the dominant scale outside of these fields and in the United States the Fahrenheit scale (also now defined using the kelvin^[5][6]) is still widespread.

Unlike the degree Fahrenheit and degree Celsius, the kelvin dates to the mid 19th century rather than the early to mid 18th and is never referred to nor written as a degree, nor typically capitalised except when abbreviated to K, e.g. "It's 50 degrees Fahrenheit outside" vs "It's 10 degrees Celsius outside" vs "It's 283 kelvins outside".^[7]

During the 18th century multiple temperature scales were developed,^[8] most notably Fahrenheit and centigrade (later Celsius). These scales predated much of the modern science of thermodynamics, including the scientific consensus in favour of atomic theory and the kinetic theory of gases that underpin the concept of absolute zero. Instead, they chose defining points inside the range of human experience at the time that could be easily and reasonably accurately reproduced, but did not necessarily have any deep significance in thermal physics. In the case of the Celsius scale (and the long since defunct Newton scale and Réaumur scale) the melting point of water served as such a semi-arbitrary starting point, with Celsius being defined, from the 1740s up until the 1940s, by calibrating a thermometer such that:

This referring to pure water at a set pressure designed to approximate the natural air pressure at sea level. An implication of this was that an increment of 1 °C was equal to an increment of 1/100 of the temperature difference between the melting and boiling points. This temperature interval would go on to become the template for the kelvin.

In October 1848, William Thomson, a 24 year old professor of Natural Philosophy at the University of Glasgow, published a paper On an Absolute Thermometric Scale.^[9][10][11] Using the soon-to-be-defunct caloric theory, he proposed an "absolute" scale based on the following parameters: