The is a system of measurement used in most of the . It is also called the International System of Units, or .
British imperial Units
Imperial units were defined in the United Kingdom in 1825. Imperial units were used in countries that were part of the . While many of these countries, including the United Kingdom, have officially adopted SI, the older system of units is still used
US customary units are the official units used in the US. These are similar to the British imperial units and are also based on the units used in the United Kingdom from before . But some of the units are different to the British ones. For example, there are 20 imperial fluid ounces in an imperial pint, but 16 US fluid ounces in a US pint. Additionally, the US fluid ounce is slightly bigger than the imperial fluid ounce. The result is that US pints and gallons are smaller than imperial pints and gallons. In the United States, the metric system has been legal for trade since 1866 but other measurements such as the gallon, inch, and the pound are still widely used.
SI Units are the most widely used system of units. They are the most common system for everyday in the world, and are almost universally used in the realm of science. The name derives from the French phrase, Système International d'Unités, or in English International System of Units. The system consists of a set of seven base units together with a set of prefixes from which all other units are derived.
The emergence of an internationally recognized system of units at a time of increasing international cooperation and trade is highly significant. It has provided a necessary common base for the scientific, technical, and industrial exchange that has fostered a growing consciousness of the need to approach issues from a .
The was officially adopted in after the . During the history of the metric system, a number of variations have evolved and their use spread around the world replacing many traditional measurement systems.
By the end of a number of different systems of measurement were still in use throughout the world. Some of these systems were metric system variations while others were based on the Imperial and American systems. It was recognized that additional steps were needed to promote a worldwide measurement system. As a result, the 9th General Conference on Weights and Measures (CGPM) in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.
Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units recommended were the , , , ampere, (later renamed kelvin), and the candela. In 1960, the 11th CGPM named the system the International System of Units. The seventh base unit, the mole, was added in 1970 by the 14th CGPM.
SI units are still sometimes referred to as the , especially in the United States, whose population has not widely adopted it, and in the , where conversion is only partial. SI units are a specific canon of measurements derived and extended from the Metric system; however, not all metric units of measurement are accepted as SI units. This international system of units is now either obligatory or permissible throughout the world. It is administered by the standards organization: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures, BIPM).
Length
The most important unit is length: one was originally defined to be equal to 1/10,000,000th of the distance from the pole to the along the meridian through . (Prior discussions had often suggested the length of a seconds pendulum in some standard gravity, which would have been only slightly shorter, and perhaps easier to determine.) This is approximately 10 percent longer than one yard. Later on, a rod with a rigid, X-shaped cross section was produced to serve as the easy-to-check standard for one meter's length. Due to the difficulty of actually measuring the length of a meridian quadrant in the eighteenth century, the first platinum prototype was short by 0.2 millimeters. More recently, the meter was redefined as a certain multiple of a specific radiation wavelength, and currently it is defined as the distance traveled by light in a vacuum in a specific period of time. Attempts to relate an integer multiple of the meter to any meridian have been abandoned.
Mass
The original base unit of mass in the metric system was the gram, chosen to match the mass of one cubic centimeter of water. For practical reasons, the reference standard that was deposited at the Archives de la république on June 22, 1799, was a kilogram (a cylinder of ). One kilogram is about 2.2 pounds. In 1889, the first General Conference on Weights and Measures (CGPM) sanctioned a replacement prototype, a cylinder of a 90 percent platinum, 10 percent alloy; this has served as the standard ever since and is stored in a Paris vault. The kilogram became the base unit in 1901.
Also in 1901, a kilogram of distilled pure water at its densest (+3.98° ) under a standard atmosphere of pressure was used to define the liter, a more convenient unit than the very large cubic meter. Because this liter turned out to be different from the cubic decimeter by about 28 millionths, this definition was abandoned in 1964 in favor of the cubic decimeter.
The kilogram is the only base unit not to have been redefined in terms of an unchanging natural phenomenon. Such a definition, said to be in terms of an artefact (the cylinder in Paris), is particularly inconvenient, because, in principle, it can be used only by traveling to Paris and, with permission, comparing one's own candidate standard to the reference one. For this reason, as well as the effort required to protect the standard from absorption or dispersion of gases and vapors, at a meeting of the Royal Society in on February 15, 2005, scientists called for the mass of the standard kilogramme in Paris to be replaced by a standard based on "an invariable property of nature"; but no decision on redefinition can be taken before 2007.
Temperature
The metric unit of temperature originally was the centigrade or Celsius scale. This was