A European Informational Website
learn more
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.
The kilogram is the only SI base unit that employs a prefix,[1] and the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property.
The pound, being defined as exactly 453.59237 grams, results in the kilogram being approximately equivalent to 2.205 avoirdupois pounds[2] in the Imperial system and the customary system of weights and measures used in the United States.
The kilogram was originally defined as one thousand times "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a meter, and at the temperature of melting ice",[3] and later as the mass of one litre of pure water at standard atmospheric pressure and at the temperature at which water has its maximum density (277.13 K, 3.98 °C). 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 90% platinum and 10% iridium (by weight) and machined into a right-circular cylinder (height = diameter) of 39 mm. The international prototype is kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) in Sèvres on the outskirts of 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 40 years. The international prototype is one of three cylinders made in 1879. In 1883, it was found to be indistinguishable from the mass of the kilogram standard at the time, and formally ratified as the kilogram by the 1st CGPM in 1889.[4]
By definition, the error in the repeatability of the current definition is exactly zero; however, any changes in the standard over time can be found by comparing the official standard to its official copies. Because the official copies and the official standard are made of roughly the same materials and kept under the same conditions, comparing the relative masses between standards over time estimates the stability of the standard. 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.[5] The observed variation in the prototype has intensified the search for a new definition of the kilogram.
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 of France 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 margrave 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.
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:
In a similar manner that the metre was redefined to fix the speed of light to an exact value of 299,792,458 m/s, there are proposals to redefine the kilogram in such a way to fix other physical constants of nature to exact values.
CIPM RECOMMENDATION 1 (CI-2005):[6] 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),
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 9.8 newtons (the SI unit of force). A value of 9.80665 m/s² (980.665 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.)
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.
(K)ong-kin ()іляграм