# Kelvin Temperature Scale Definition

## Definition of Kelvin Temperature Scale

The Kelvin temperature scale is the most commonly used absolute temperature scale in the world. Here is the definition of the scale and a look at its history and uses.

### Key Takeaways: Kelvin Temperature Scale

• The Kelvin temperature scale is an absolute temperature scale that is defined using the third law of thermodynamics.
• Because it is an absolute scale, temperatures recorded in Kelvin do not have degrees.
• The zero point of the Kelvin scale is absolute zero, which is when particles have minimum kinetic energy and cannot get colder.
• Each unit (a degree, in other scales) is 1 part in 273.16 parts of the difference between absolute zero and the triple point of water. This is the same size unit as the Celsius degree.

## Kelvin Temperature Scale Definition

The Kelvin temperature scale is an absolute temperature scale with zero at absolute zero. Because it is an absolute scale, measurements made using the Kelvin scale do not have degrees. The kelvin (note the lowercase letter) is the base unit of temperature in the International System of Units (SI).

## Changes in Definition

Until recently, the units of the Kelvin scale were based on the definition that the volume of a gas at constant (low) pressure is directly proportional to temperature and that 100 degrees separates the freezing and boiling points of water.

Now, the Kelvin unit is defined using the distance between absolute zero and the triple point of water. Using this definition, one kelvin is the same magnitude as one degree on the Celsius scale, making it easy to convert between Kelvin and Celsius measurements.

On November 16, 2018, a new definition was adopted. This definition sets the size of the kelvin unit based on the Boltzmann constant. As of May 20, 2019, the kelvin, mole, ampere, and kilogram will be defined using thermodynamic constants.

## Usage

Kelvin temperatures are written with a capital letter "K" and without the degree symbol, such as 1 K, 1120 K. Note that 0 K is "absolute zero" and there are (ordinarily) no negative Kelvin temperatures.

## History

William Thomson, later named Lord Kelvin, wrote the paper On an Absolute Thermometric Scale in 1848. He described the need for a temperature scale with a null point at absolute zero, which he calculated to be equivalent to −273 °C. The Celsius scale at the time was defined using the freezing point of water.

In 1954, the 10th General Conference on Weights and Measures (CGPM) formally defined the Kelvin scale with a null point of absolute zero and the second defining point at the triple point of water, which was defined to be exactly 273.16 kelvins. At this time, the Kelvin scale was measured using degrees.

The 13th CGPM changed the unit of the scale from "degree Kelvin" or °K to kelvin and symbol K. The 13th CGPM also defined the unit as 1/273.16 of the temperature of the triple point of water.

In 2005, a subcommittee of the CGPM, the Comité International des Poids et Mesures (CIPM), specified the triple point of water refer to the triple point of water with an isotopic composition called the Vienna Standard Mean Ocean Water.

In 2018, the 26th CGPM redefined the Kelvin in terms of the Boltzmann constant value of 1.380649×10−23 J/K.

Although the unit has been redefined over time, the practical changes in the unit as so small that they do not appreciably affect most people working with the unit. However, it's always a good idea to pay attention to significant figures after the decimal point when converting between degrees Celsius and kelvin.

## Sources

• Bureau International des Poids et Mesures (2006). "The International System of Units (SI) Brochure." 8th Edition. International Committee for Weights and Measures.
• Lord Kelvin, William (October 1848). "On an Absolute Thermometric Scale." Philosophical Magazine.
• Newell, D B; Cabiati, F; Fischer, J; Fujii, K; Karshenboim, S G; Margolis, H S; de Mirandés, E; Mohr, P J; Nez, F; Pachucki, K; Quinn, T J; Taylor, B N; Wang, M; Wood, B M; Zhang, Z; et al. (Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants) (2018). "The CODATA 2017 values of h, e, k, and NA for the revision of the SI". Metrologia. 55 (1). doi:10.1088/1681-7575/aa950a
• Rankine, W. J. M. (1859). "A manual of the steam engine and other prime movers." Richard Griffin and Co. London. p. 306–307.
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