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The kilogram (symbol: kg) is the SI base unit of mass. A gram (grain in now obsolete Portugese spelling, in Latin granum) is defined as one thousandth of a kilogram. See 1 E 0 kg for comparisons with other masses.

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Multiples SI prefixes are used to name multiples and subdivisions of the kilogram. The most commonly used ones are:

tonne = 1,000 kilograms (strictly speaking, this should be named megagram, but the name is never used)
gram = 1/1000 kilogram
milligram = 1 thousandth of a gram
microgram = 1 millionth of a gram (ie. 10-6 gram)


The kilogram is the only one of the SI units which is defined in relation to an artifact rather than to physical properties.

The kilogram was originally defined as the mass of one liter of pure water at a temperature of 4 degrees Celsius and standard atmospheric pressure. This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram.

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 kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype 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.

It seems, that the original kilogram has lost about 50 micrograms in the last 100 years. The reason is still unknown (reported in Der Spiegel in 26/2003. Therefore the search for alternatives is intesified:

Proposed future definitions

There is an ongoing effort to introduce a definition by way of fundamental or atomic constants. The proposals being worked on are:

  • The Watt balance uses the current balance[?] that formerly was used to define the ampere to relate the kilogram to a value for Planck's constant, based on the definitions of the volt and the ohm.
  • The Avogadro approach attempts at defining the kilogram by a fixed count of silicon atoms. As a practical realization, a sphere will be used where the size is measured by interferometry.
  • The ion accumulation approach involves accumulation of gold atoms and measuring the electrical current required to neutralise them.
  • The levitated superconductor approach relates the kilogram to electrical quantities by levitating a superconducting body in a magnetic field generated by a superconducting coil, and measuring the electrical current required in the coil.

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