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International System of Units (view source)
Revision as of 11:33, April 27, 2007
, 11:33, April 27, 2007→Lead Agency
* Since the SI symbols are symbols they are not altered to fit in with the surrounding text. The case of the symbol does not change - kelvin is always written as 'K' and kilograms is always written as 'kg'. Furthermore, they are never written in italics. Thus <i>The woman lost 25 kg in weight</i> is incorrect and should be <i>The woman lost </i>25 kg<i> in weight</i>
* Since the SI symbols are symbols they are not altered to fit in with the surrounding text. The case of the symbol does not change - kelvin is always written as 'K' and kilograms is always written as 'kg'. Furthermore, they are never written in italics. Thus <i>The woman lost 25 kg in weight</i> is incorrect and should be <i>The woman lost </i>25 kg<i> in weight</i>
==Units of volume in the metric system==
The liter was a unit of volume in the original 1795 definitiion of the metric system.It was defined as a cubic decimeter, i.e. a volume of 1/1000th of a cubic meter. However, the 1901 redefinition of the liter as the volume of 1 kg of water at the temperature when it takes its maximum density (3.98 °C) yields a a volume greater than a cubic decimeter by 28 parts per million. For everyday purposes this was not a problem, but for highly accurate scientific purposes it then became important to specify whether one actually meant a liter or cubic decimeter. The liter was thus not recommended for scientific purposes and technically became no longer part of the metric system. Officially, volume is measured in cubic meters and its multiples and submultiples, such as the cubic centimeter. The 1964 definition of the liter changed it back to exactly a cubic decimeter but technically it is a now a unit approved for use alongside the metric system but not part of it, alongside the minute, hour, year and others.
In the English system of weights and measures used in the USA, different units of volume are used for dry goods than for liquids. The Queen Anne gallon of 231 cubic inches is used for liquids and the Winchester gallon is the basis of the Winchester bushel of 2150.42 cubic inches. However in the metric system no distinction is made between units of volume used for dry goods and units of volume used for liquids. The cubic meter and its multiples and submultiples are used for all volumetric purposes. This is part of a general difference of approach between the two systems. The English system tends to use different measures for the same quantity depending on the context whereas the metric system uses the same measure. For example, in the English system pressure is measured in inches of mercury for weather, pounds per square inch for tire pressure and pounds per square foot for floor pressure. In the metric system these would be all measured in pascals or kilopascals.
==Lead Agency==
==Lead Agency==
As scientific and engineering measurements become more accurate over time, the measurement standards they are compared against need to be continually refined. The National Institute for Science and Technology (NIST), based in Gaithersburg, Maryland, carries out world-class research in metrological science. NIST represents the United States in international committees on metrolgy.
As scientific and engineering measurements become more accurate over time, the measurement standards they are compared against need to be continually refined. The National Institute for Science and Technology (NIST), based in Gaithersburg, Maryland, carries out world-class research in metrological science. NIST represents the United States in international committees on metrolgy.