Temperature

The temperature scale is based on a fixed temperature, that of the triple point of water at which it's solid, liquid and gaseous. The freezing point of water was designated as 273.15 K, equalling exactly 0° on the Celsius tempera­ture scale. The Celsius scale, which is identical to the centigrade scale, is named after the 18th-century Swed­ish astronomer Anders Celsius, who first proposed the use of a scale in which the interval between the freezing and boiling points of water is divided into 100 degrees. By international agreement, the term Celsius has offi­cially replaced centigrade.

One feature of SI is that some units are too large for ordinary use and others too small. To compensate, the prefixes developed for the metric system have been bor­rowed and expanded. These prefixes are used with all three types of units: base, supplementary, and derived. Examples are millimetre (mm), kilometre/hour (km/h), megawatt (MW), and picofarad (pF). Because double pre­fixes are not used, and because the base unit name kilo­gram already contains a prefix, prefixes are used not with kilogram but with gram. The prefixes hecto, deka, deci, and centi are used only rarely, and then usually with metre to express areas and volumes. In accordance with established usage, the centimetre is retained for body measurements and clothing.

In some cases certain other units are allowed for a limited time, subject to future review. These include the nau­tical mile, knot, angstrom, standard atmosphere, hec­tare, and bar.