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U.S. National Prototype Kilogram, which currently serves as the primary standard for measuring mass in the U.S. It was assigned to the United States in 1889 and is periodically recertified and traceable to the primary international standard, "The Kilogram", held at the ''Bureau International des Poids et Mesures'' (BIPM) near Paris. The http://physics.nist.gov/cuu/Units/kilogram2.html international prototype, made of platinum-iridium, is kept at the BIPM under conditions specified by the 1st CGPM in 1889. The kilogram or kilogramme, (symbol: kg) is the SI base unit of mass. It is defined as being equal to the mass of the international prototype of the kilogram. It is the only SI base unit that employs a prefix , and the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property. A kilogram is approximately equivalent to 2.205 avoirdupois pounds in the Imperial system and the customary system of weights and measures used in the United States. History The kilogram was originally defined as the mass of one litre of pure water at standard atmospheric pressure and at the temperature at which water has its maximum density (3.98 degrees Celsius). This definition was hard to realize accurately, partially because the density of water depends slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition. To avoid these problems, the kilogram was redefined as precisely the mass of a particular standard mass created to approximate the original definition. Since 1889, the SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium of 39 mm height and diameter and is kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures), near Paris. Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype ("Le Grand Kilo") roughly every 10 years. The international prototype kilogram was made in the 1880s. By definition, the error in the repeatability of the current definition is exactly zero; however, in the usual sense of the word, it can be regarded as of the order of 2 micrograms. This is found by comparing the official standard with its official copies, which are made of roughly the same materials and kept under the same conditions. There is no reason to believe that the official standard is any more or less stable than its official copies, thus giving a way to estimate its stability. This procedure is performed roughly once every forty years. The international prototype of the kilogram seems to have lost about 50 micrograms in the last 100 years and the reason for the loss is still unknown (reported in Der Spiegel, 2003 The gram The gram or gramme is the term to which SI prefixes are applied. The reason the base unit of mass has a prefix is historic. Originally, the decimal system of units was commissioned by Louis XVI and in the original plans, the kilogram was supposed to be called the grave. A gramme was simply an alternative name for a thousandth of a grave, properly named milligrave, and a tonne was an alternative name for 1000 graves, properly named kilograve. However, the metric system didn't come in effect until after the French Revolution. At that time, the name "grave" had become politically incorrect, since it is an alternative word for the title "count" (cognate with the British "markgrave" and the German "Graf"), and nobility titles were not considered compatible with the notion of égalité. The gram was also the base unit of the older CGS system of measurement, a system which is no longer widely used. Proposed future definitions There is an ongoing effort to introduce a new definition for the kilogram by way of fundamental or atomic constants. The proposals being worked on are: Atom-counting approaches Fundamental-constant approaches In a similar manner that the metre was redefined to fix the speed of light to an exact value of 299792458 m/s, there are proposals to redefine the kilogram in such a way to fix other physical constants of nature to exact values. This would have the effect of defining Planck's constant to be h = 6.6260693 J s. This is consistent with the current 2002 CODATA value for Planck's constant which is 6.6260693 ± 0.0000011 J s. This would have the effect of defining Avogadro's number to be NA = 6.0221415 elementary entities per mole and, consequently, a simpler and concise definition for the mole. This is consistent with the current 2002 CODATA value for the Avogadro constant which is 6.0221415 ± 0.0000010 mol-1. This would have the effect of defining the electron mass to be me = 9.1093826 kg. This is consistent with the current 2002 CODATA value for the electron mass which is 9.1093826 ± 0.0000016 kg. This redefinition of the kilogram has the effect of fixing the elementary charge to be e = 1.60217653 C and would result in a functionally equivalent definition for the coulomb as being the sum of exactly 6 241 509 479 607 717 888 elementary charges and the ampere as being the electrical current of exactly 6 241 509 479 607 717 888 elementary charges per second. This is consistent the current 2002 CODATA value for the elementary charge which is 1.60217653 ± 0.00000014 C. International Committee for Weights and Measures|CIPM RECOMMENDATION 1 (CI-2005) CIPM RECOMMENDATION 1 (CI-2005) Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants The International Committee for Weights and Measures (CIPM), Link with weight When the weight of an object is given in kilograms, the property intended is almost always mass. Occasionally the gravitational force on an object is given in "kilograms", but the unit used is not a true kilogram: it is the deprecated kilogram-force (kgf), also known as the kilopond (kp). An object of mass 1 kg at the surface of the Earth will be subjected to a gravitational force of approximately 9.80665 newtons (the SI unit of force). Note that the factor of 980.765 cm/s² (as the CGPM defined it, when cgs systems were the primary systems used) is only an agreed-upon conventional value (3rd CGPM (1901), CR 70) whose purpose is to define grams force. The local gravitational acceleration g varies with latitude and altitude and location on the Earth, so before this conventional value was agreed upon, the gram-force was only an ill-defined unit. (See also g, a standard measure of gravitational acceleration.) Examples 1.6726 yg SI multiples When the Greek small letter mu ('µ') in the symbol of microgram is technically unavailable it should be replaced by Latin small letter 'u', but other informal abbreviations like 'mcg' (confusingly also used to designate the obsolete term "millicentigram", equal to 10 µg) can also be encountered in practice. In the pharmaceutical industry, 'mcg' is used in the place of 'µg' to designate "microgram." The decagram is alternatively spelled 'dekagram'. The megagram (1000 kg) is also more commonly known as the (metric) tonne (t), also spelled ton (the long ton is a measure of 2240 lb, whereas the short ton is 2000 lb). The unit tonne is accepted to be used with the SI and may take the same prefixes, see also metre-tonne-second system of units. See also | |||||||
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