kelvin

The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is 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 (zero) point.

Historically, the Kelvin scale 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 since 2019 the scale has been defined by fixing the Boltzmann constant to be exactly . Hence, one kelvin is equal to a change in the thermodynamic temperature that results in a change of thermal energy by . The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15, meaning that a ''change'' or ''difference'' in temperature has the same value when expressed in degrees Celsius as in kelvins, and that 0 °C is equal to 273.15 K.

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. In the United States, outside of the physical sciences the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature. Those are defined using the kelvin.

According to SI convention, the kelvin is never referred to nor written as a ''degree''. The word ''kelvin'' is not capitalised when used as a unit, but is pluralised as appropriate. The unit symbol K is a capital letter. For example, "It is 50 degrees Fahrenheit outside" vs "It is 10 degrees Celsius outside" vs "It is 283 kelvins outside". It is common convention to capitalize Kelvin when referring to the Kelvin scale.

History

Precursors

During the 18th century, multiple temperature scales were developed, notably Fahrenheit and centigrade (later Celsius). These scales predated much of the modern science of thermodynamics, including atomic theory and the kinetic theory of gases which underpin the concept of absolute zero. Instead, they chose defining points within the range of human experience that could be reproduced easily and with reasonable accuracy, but lacked 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 starting point, with Celsius being defined, from the 1740s up until the 1940s, by calibrating a thermometer such that:

* The freezing point of water is 0 degrees.
* The boiling point of water is 100 degrees.

This definition assumes pure water at a specific pressure chosen to approximate the natural air pressure at sea level. Thus an increment of 1 °C equals of the temperature difference between the melting and boiling points. This temperature interval would go on to become the template for the kelvin.

Lord Kelvin

In 1848, William Thomson, who was later ennobled as Lord Kelvin, published a paper ''On an Absolute Thermometric Scale''. Using the soon-to-be-defunct caloric theory, he proposed an "absolute" scale based on the following parameters:

* The melting point of water is 0 degrees.
* The boiling point of water is 100 degrees.

"The arbitrary points which coincide on the two scales are 0° and 100°"

* Any two heat engines whose heat source and heat sink are both separated by the same number of degrees will, per Carnot's theorem, be capable of producing the same amount of mechanical work per unit of "caloric" passing through.

"The characteristic property of the scale which I now propose is, that all degrees have the same value; that is, that a unit of heat descending from a body at the temperature ° of this scale, to a body at the temperature ( − 1)°, would give out the same mechanical effect, whatever be the number . This may justly be termed an absolute scale, since its characteristic is quite independent of the physical properties of any specific substance."

As Carnot's theorem is understood in modern thermodynamics to simply describe the maximum efficiency with which thermal energy can be converted to mechanical energy and the predicted maximum efficiency is a function of the ''ratio'' between the absolute temperatures of the heat source and heat sink:

*Efficiency ≤ 1 −

It follows that increments of equal numbers of degrees on this scale must always represent equal ''proportional'' increases in absolute temperature. The numerical value of an absolute temperature, , on the 1848 scale is related to the absolute temperature of the melting point of water, , and the absolute temperature of the boiling point of water, , by:

* (1848 scale) = 100 () / ()

On this scale, an increase of 222 degrees always means an approximate doubling of absolute temperature regardless of the starting temperature.

In a footnote Thomson calculated that "infinite cold" ( absolute zero, which would have a numerical value of negative infinity on this scale) was equivalent to −273 °C using the air thermometers of the time. This value of "−273" was the negative reciprocal of 0.00366—the accepted coefficient of thermal expansion of an ideal gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.

Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one based on the 2 features that would characterise all future versions of the kelvin scale:

* Absolute zero is the null point.
* Increments have the same magnitude as they do in the Celsius scale.

In 1892, Thomson was awarded the noble title 1st Baron Kelvin of Largs, or more succinctly Lord Kelvin. This name was a reference to the River Kelvin which flows through the grounds of Glasgow University.

In the early decades of the 20th century, the Kelvin scale was often called the "absolute Celsius" scale, indicating Celsius degrees counted from absolute zero rather than the freezing point of water, and using the same symbol for regular Celsius degrees, °C.

Triple point standard

In 1873, William Thomson's older brother James coined the term '' triple point'' to describe the combination of temperature and pressure at which the solid, liquid, a