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Standard kilogram celebrated 125 years
The international prototype without a protective case i>
In September 2014 marks 125 years since the birth of международного prototype of the kilogram . The decision to create the standard was adopted by the General Conference of Weights and Measures 7-9 September 1889 in Paris.
He kept at the International Bureau of Weights and Measures near Paris and is a cylinder with a diameter and a height of 39, 17 mm platinum-iridium alloy (90% platinum and 10% iridium). Such a composition is selected from the high density of platinum, so that reference can be made relatively small size: less matchbox adjustment.
The National prototype kilogram UK in a protective housing, the 18th copy of the international prototype i>
The mass of the international prototype of roughly corresponds to 1 liter of water at 4 ° C, and its weight depends on the height above sea level and gravity.
When produced international prototype, with 40 copies of them made of the same platinum-iridium alloy. They were sent to the National Bureau of Weights and Measures in different countries, scientists do not have to refer to the standard main each time for measurements.
National prototypes verify with basic prototype of every 40 years. Last check took place in 1989, and then the maximum difference in weight was 50 micrograms. These deviations are concerned scientists. They understand that the mass of a particular sample varies over time due to physical damage and the appearance of other artifacts.
The National prototype stored in the vault of the National Physical Laboratory i>
Unfortunately, the international prototype of the current anniversary, is likely to be the last. Now coming to the end of two experiments to establish a more precise mass standards. Their goal - to determine the mass through the natural constant, rather than through the reference sample.
One experiment involves the definition of the kilogram by Planck's constant. For this measure the current flowing through [wired] coil in the magnetic field against the force of gravity acting on a kilogram, объясняют Specialists UK National Physical Laboratory, where in honor of the 125th anniversary of the kilogram opened festive section on the site. It was in the UK in 1975, began to experiment with the watt balance, which is currently still in Canada.
Another method offered by German experts: within проекта Avogadro create silicon sphere the size of a grapefruit, which contains about 50 silicon atoms septillionov-28.
silicon spheres Avogadro i>
Since the known mass of silicon and density of the substance, the reference value of the kilogram can be linked to the volume of a sphere and, consequently, to the Avogadro constant.
The measurement of the mass of the sphere of the Avogadro i>
Kilogram was last SI unit, which is expressed through the physical standard. This indicates that 125 years ago physics are very well chosen material for the prototype. And even if it will soon withdraw from the use of, he rendered good service over the years.
Source: habrahabr.ru/post/238273/
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