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The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and no heat energy remains in a substance—is defined as zero kelvin (0 K). The unit increment of the Kelvin scale is the kelvin (symbol: K), which is the SI unit of temperature and is one of the seven SI base units. The Kelvin scale is named after British physicist and engineer William Thomson, 1st Baron Kelvin (1824–1907). Definition of the kelvin The unit “kelvin” and its scale by international agreement, defined by two points: absolute zero, and the triple point of specially prepared (VSMOW) water. This definition also precisely relates the Kelvin scale to the Celsius scale. Absolute zero—the temperature at which nothing could be colder and no heat energy remains in a substance—is defined as being precisely 0 K and –273.15 °C. The triple point of water is defined as being precisely 273.16 K and 0.01 °C. This definition does three things: 1) it fixes the magnitude of the kelvin unit as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water; 2) it establishes that one kelvin has precisely the same magnitude as a one-degree increment on the Celsius scale; and 3) it establishes the difference between the two scales’ null points as being precisely 273.15 kelvins (0 K = –273.15 °C and 273.16 K = 0.01 °C). Temperatures in Kelvin can be converted to other units per the table at top right. Some key temperatures relating temperatures on the Kelvin and Celsius scales are shown in the below table. A For Vienna Standard Mean Ocean Water at one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature. Older definitions of the Celsius scale once defined the boiling point of water under one standard atmosphere as being precisely 100 °C. However, the current definition results in a boiling point that is actually 16.1 mK less. For more about the actual boiling point of water, see VSMOW water in temperature measurement. SI prefixed forms of kelvin SI prefixes are often employed to denote decimal multiples and submultiples of the kelvin. The most commonly used factors of kelvin are listed below. Uppercase/lowercase and plural form usage When reference is made to the unit kelvin (either a specific temperature or a temperature interval), kelvin is always spelled with a lowercase k unless it is the first word in a sentence. When reference is made to the Kelvin scale, use an uppercase K. Until the 13th General Conference on Weights and Measures (CGPM) in 1967-1968, this unit was called a degree just as other temperature degrees are, distinguished from other degrees with the adjective “Kelvin” in “degrees Kelvin,” or often as “degrees absolute” (which was more ambiguous, since it could also refer to degrees Rankine). When the units were degrees, the plural was formed by adding an “s” to degree and like other adjectives in English, the adjective identifying the scale was unchanged in the plural. After the name change, the plural of kelvin is “kelvins.” When reference is made to the “Kelvin scale,” the word “kelvin”—which is normally a noun—functions adjectivally to modify the noun “scale” (like “Georgia peach”) and is capitalized. Temperatures and intervals Because the kelvin is an individual unit of measure, it is particularly well-suited for expressing temperature intervals: differences between temperatures or their uncertainties (e.g. “Agar exhibited a melting point hysteresis of 25 kelvins,” and “The uncertainty was 10 millikelvins”). Of course, the kelvin is also used to express specific temperatures along its scale (e.g. “Gallium melts at 302.9146 kelvin”). Note that to avoid confusion, especially when temperatures and uncertainties both appear in the same document, the singular form of kelvin (no “s” at the end) is preferred when referring to temperatures; the plural form is preferred when referring to intervals. One disadvantage of the kelvin is that intervals and specific temperatures on the Kelvin scale both utilize the exact same symbol (e.g. “Agar exhibited a melting point hysteresis of 25 K,” and “The triple point of hydrogen is 13.8033 K”). Thus, wherever ambiguity might arise due to the dual use of the symbol K within a document, it is preferable to use the symbol for denoting temperatures and to express the intervals using the full unit name in its plural form, kelvins, (e.g. “The helium temperature was 650 mK… and our standard deviation in this set of experiments was 15 millikelvins.”) Formatting and typestyle for the K symbol The kelvin symbol is always a roman (non-italic) capital K since the lowercase version is the SI prefix for 1 × 103. The admonition against italicizing the symbol K applies to all SI unit symbols; only symbols for variables and constants (e.g. P = pressure, and c = 299,792,458 m/s) are italicized in scientific and engineering papers. As with most other SI unit symbols (angle symbols, e.g. 45° 3′ 4″, are the exception,) there is a space between the numeric value and the kelvin symbol (e.g. “99.987 K”). The special Unicode kelvin sign Unicode, which is an industry standard designed to allow text and symbols from all of the writing systems of the world to be consistently represented and manipulated by computers, includes a special “kelvin sign” at U+212A. One types & & When viewed on computers that properly support Unicode, the above line appears as follows (size may vary): Depending on the operating system, Web browser, and the default font, the “K” in the Unicode character may be narrower and slightly taller than a plain uppercase K; precisely the opposite may be true on other platforms. However, there will usually be a discernible difference between the two. If the computer being used to view a particular Web page doesn’t support the Unicode kelvin sign character ( &), it may be canonically decomposed by the browser into U+004B (uppercase K) and the two would appear identical. In still other computers, the kelvin symbol is mapped incorrectly and produces an odd character.
Accordingly, for Web use, it is better to use the simple uppercase K to represent the kelvin symbol so it can be properly viewed by the widest possible audience. Why technical articles use a mix of Kelvin and Celsius scales In science (especially) and in engineering, the Celsius and Kelvin scales are often used simultaneously in the same article (e.g. “…its measured value was 0.01023 °C with an uncertainty of 70 µK…”) This issue had been addressed by both Resolution 3 of the 13th CGPM (1967/68) which stated that This practice of using both “°C” and “K” is usually avoided in non-technical articles intended for the general public where both the kelvin and its symbol, K, are not well recognized and could be confusing. History of the Kelvin scale Below are some historic milestones in the development of the Kelvin scale and its unit increment, the kelvin. For more on the history of thermodynamic temperature, see Thermodynamic temperature: History of thermodynamic temperature. See also | |||||||
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