Kilogram
the kilogram is those SI - fundamental unit of the mass. The unit symbol of the kilogram is kg.
It is in several respects a special unit. It is the only SI fundamental unit, only by a comparison article (prototype), the Urkilogramm,is fixed. The name of units kilograms begins Kilo “( v. with the SI resolution „. griech.: chilioi „thousands “); therefore may not be formed decimal parts and multiple kilograms outgoing from the kilogram with resolutions or prefixes, instead one leads them of Gram off, which - after SI rules and standardization - not when milli kilogram may be designated.
To table of contents |
old persons definition
the kilogram was originally definedas the mass of a Kubikdezimeters (DM ³) water with maximum density (thus with 3,98 °C) and given pressure. However the unit kilogram was somewhat largely dimensioned at that time too. Actually water has a maximum density of only 0.999975 kg/dm ³.
Current definition
since 1889 forms the Urkilogramm ( the international kilogram prototype) into of Paris the reference value for the unit kilogram. Its mass amounts to by Definitionem 1 kg. It becomes in a safe deposit of the international office forWeights and mass (BIPM) in Sèvres with Paris keeps.
This concerns a cylinder of 39 millimeters of height and diameter, which consist of an alloy of 90% platinum and 10% iridium. Countries, those the metric system joinedare, thus the Meterkonvention signed, this Urkilogramms is in the possession of copies. The international commission for weights and mass (CIPM) decides when these copies are compared with the Urkilogramm. So far there were such comparisons around 1950 andlast around 1990. Here one stated that copies of the Urkilogramms in the course of the years had become apparently heavier than the original. Possibly, so an explanation, was simply too frequently deseamed the Urkilogramm. A member country of the Meterkonvention can however at any timeits copy to the BIPM to bring leave, in order to let it compare with the work copies of the BIPM. The Federal Standards Laboratory (PTB), which the German dupe of the Urkilogramms possesses, this so far about every ten years did.
planned redefinition
At present world-wide is worked to redefine the kilogram so that it can be derived from a fundamental constant of physics. This project got a special urgency, since it was stated that those approximately 40 copies opposite the Urkilogramm in the meantime outexhibit unsettled reasons on the average a deficiency in weight of approximately 50 micro gram. Since these copies were always made of the same material and after the same procedure, it is more probable that a Urkilogramm became easier its40 copies became heavier.
Here momentarily different beginnings are pursued:
- Provide a ball of highly pure silicon with a certain (and counted) number of silicon atoms. A such ball was already manufactured and measured on nanometer accuracy. The measured mass yielded however around onetiny amount from the calculated mass. This was attributed partially to the fact that the isotope composition silicon be measured exactly could not sufficiently. Natural silicon consists of three isotopes with the atomic masses 28, 29 and 30. One strivesnow to make a ball of isotope-pure Silizium-28 which is however technically and financially enormously aufwändig.
- Determines the weight of an mass-afflicted body by a Watt balance, whereby mechanical is compared with electrical achievement. Here become in two steps: 1) the riverin a coil, with a Kraft magnetic in a magnetic field one produces and: 2) the tension induced by movement of the coil in this magnetic field measured (river × tension = electrical achievement with the unit Watt). In addition must the speedthe moved coil and the acceleration due to gravity, which produces the Gewichtskraft of the piece of weight, to be measured.
- Determine the mass of an atom with the help of an ion beam (electrically charged atoms) and gathering the ions to a balancable mass. By measurement of the electric current of theIon beam and the time can then the mass of an atom in the unit kilogram be computed.
These procedures are still in the development and it are not certain, which will replace the Urkilogramm from them.
On the earth's surface experiencesa mass of a kilogram a Gewichtskraft on approximately 9.81 Newton (dependent on local acceleration due to gravity).
see also
Web on the left of
| Wiktionary: Kilogram - word origin,Synonymous one and translations |
- http://www.ptb.de/de/wegweiser/einheiten/_index.html “the PTB as Hüterin of the units”
- Internet portal of the international office for weights and mass (in English and French)
