The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By definition, the units of the Celsius scale (symbol °C) and the Kelvin scale have the same magnitude (that is, a rise in temperature of one kelvin and a rise of 1 °C are both equal to 1 K), and any temperature in degrees Celsius can be converted to the kelvin scale by adding 273.15. The 19th-century British scientist Lord Kelvin first developed and proposed the scale. It was often called the "absolute Celsius" scale in the early 20th century. The kelvin was formally added to the International System of Units in 1954, defining 273.16 K to be the temperature of the triple point of water (0.01 °C). The Celsius, Fahrenheit, and Rankine scales were redefined in terms of the Kelvin scale using this definition. The 2019 revision of the SI now defines the kelvin in terms of energy by setting the Boltzmann constant; every 1 K change of thermodynamic temperature corresponds to a change in the thermal energy, kBT, of exactly 1.380649×10−23 joules.
History
Precursors
During the 18th century, multiple temperature scales were developed, notably Fahrenheit and 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 defunct Newton and Réaumur scales) the melting point of ice served as such a starting point, with Celsius being defined (from the 1740s to the 1940s) by calibrating a thermometer such that:
Water's freezing point is 0 °C. Water's boiling point is 100 °C. 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 1/100 of the temperature difference between the melting and boiling points. The same temperature interval was later used for the Kelvin scale.
Charles's law From 1787 to 1802, it was determined by Jacques Charles (unpublished), John Dalton, and Joseph Louis Gay-Lussac that, at constant pressure, ideal gases expanded or contracted their volume linearly (Charles's law) by about 1/273 parts per degree Celsius of temperature's change up or down, between 0 °C and 100 °C. Extrapolation of this law suggested that a gas cooled to about −273 °C would occupy zero volume.
Lord Kelvin
First absolute scale In 1848, William Thomson, who was later ennobled as Lord Kelvin, published a paper On an Absolute Thermometric Scale. The scale proposed in the paper turned out to be unsatisfactory, but the principles and formulas upon which the scale was based were correct. For example, in a footnote, Thomson derived the value of −273 °C for absolute zero by calculating the negative reciprocal of 0.00366—the coefficient of thermal expansion of an ideal gas per degree Celsius relative to the ice point. This derived value agrees with the currently accepted value of −273.15 °C, allowing for the precision and uncertainty involved in the calculation. The scale was designed on the principle that "a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T − 1)°, would give out the same mechanical effect, whatever be the number T." Specifically, Thomson expressed the amount of work necessary to produce a unit of heat (the thermal efficiency) as μ ( t ) ( 1 + E t ) / E {\displaystyle \mu (t)(1+Et)/E} , where t {\displaystyle t} is the temperature in Celsius, E {\displaystyle E} is the coefficient of thermal expansion, and μ ( t ) {\displaystyle \mu (t)} was "Carnot's function", a substance-independent quantity depending on temperature, motivated by an obsolete version of Carnot's theorem. The scale is derived by finding a change of variables T 1848 = f ( T ) {\displaystyle T_{1848}=f(T)} of temperature T {\displaystyle T} such that d T 1848 / d T {\displaystyle dT_{1848}/dT} is proportional to μ {\displaystyle \mu } .
When Thomson published his paper in 1848, he only considered Regnault's experimental measurements of μ ( t ) {\displaystyle \mu (t)} . That same year, James Prescott Joule suggested to Thomson that the true formula for Carnot's function was
μ ( t ) = J E 1 + E t , {\displaystyle \mu (t)=J{\frac {E}{1+Et}},}
… excerpt ends here. Continue reading the full article.
