The international prototype (standard) of the kilogram is stored at the International Bureau of Weights and Measures (located in Sèvres near Paris) and is a cylinder 39.17 mm in diameter and height made of a platinum-iridium alloy (90% platinum, 10% iridium). Originally, a kilogram was defined as the mass of one cubic decimeter (liter) of pure water at 4 ° C and standard atmospheric pressure at sea level.
So why did it get heavier?
Using the unique Theta-sensing technology, Prof. Peter Cumpson and Dr. Naoko Sano from the University of Newcastle analyzed the surface of the standard and determined that the International Prototype (Standard) Kilogram quantified hydrocarbon contamination on its surface. Their research shows that the International Prototype (Standard) of the Kilogram has probably gained tens of micrograms in mass since the standard was introduced in 1875. But since this is a kilogram that all kilograms in the world must correspond to, in the theoretical sense at least all kilograms will be technically heavier too.
While an increase in the kilogram reference by tens of micrograms may seem insignificant, Peter Cumpson argues that any discrepancy between the International Prototype (Reference) for the kilogram and its replicants can be problematic. "There are cases of international trade with high value materials - where every last microgram must be accounted for." Accumulated hydrocarbons can be removed by exposure to a mixture of ultraviolet rays and ozone.
The situation is complicated by the fact that in the world there are 39 most accurate copies of the reference kilogram stored in Paris, which at one time were sent to various countries of the world to maintain a unified system of measures and weights. Now, according to a number of scientists, they are beginning to differ more and more from each other in weight due to climatic and other conditions that exist in different parts of the planet.
"The industrialization that has taken place in the world and the nature of the functioning of modern society have led to the fact that the surface of the reference kilogram acquired a plaque in the form of additional particles," says a research team from the University of Newcastle. - The world standard weight system is now under the threat of creating chaos in it. We were horrified when we saw that mercury particles had accumulated on the surface of one of the 40 reference kilograms stored in the UK,”said project manager Professor Peter Campson.
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According to the information received by the publication, the question of how to carry out a special "dust cleaning" of the surfaces of all 40 reference kilograms in order to return them to a single weight is now being discussed.
At a conference of the Royal Scientific Society in London on January 24-25, Richard Davis, former head of the mass division of the International Bureau of Weights and Measures (based in Sèvres, near Paris), proposed softening the definition. As a temporary measure before redefining "kilogram mass", he proposed averaging the disputed masses. So, if the results of two experiments of different types in determining the mass do not quite agree, they should simply be averaged, and the resulting value should be declared a new standard, quotes a proposal from the scientist Nature News.
However, this plan has many opponents. “The decision to simply average two inconsistent results would be perfectly mathematically correct, but this is not a scientific approach,” said Michael Hart, a physicist at the University of Manchester.
Averaging advocates see it as a way forward. “Ideally, we want to achieve perfect measurement convergence. But if that doesn't work, you need to use a mathematical approach,”said Terry Quinn, former director of the International Bureau of Weights and Measures. He emphasizes that the differences in question are too small to create real problems for the practical application of the measure.
This difference is a serious problem in the long run, because it cannot be judged whether the copies are getting heavier or, conversely, the reference is lighter.
Therefore, scientists have long been planning to replace the "material" standard of the kilogram, expressing it in terms of invariable constants of physics, which are determined with high accuracy and do not change. If this succeeds, the mass of the kilogram will become as unchanged as the laws of the universe.
Scientists use two approaches to find the exact expression for the kilogram: in the first, the mass of the kilogram is determined using electric and magnetic fields, and in the second, they try to express it in terms of Planck's constant, one of the basic quantities of quantum mechanics.
The second method involves direct recalculation of the number of atoms in a crystalline silicon sphere. This result allows the kilogram to be redefined in terms of Avogadro's constant, which is the number of molecules per mole of matter and, in fact, relates the atomic mass of an element to the mass of a macroscopic body.
In recent years, measurements by both methods have been carried out with an accuracy of 30 ppb - this is the limit of the most accurate measurements of the mass of a platinum-iridium cylinder. However, the results of the two types of experiments diverge by 175 billion shares - this is much more reasonable from a methodological point of view.
This does not allow officially redefining the measure of the kilogram, since the reasons for the discrepancy are unclear and cannot yet be eliminated. That is why a proposal was put forward about the elementary averaging of two calculations and such a mathematical redefinition of the kilogram.
The task of redefining the standard of mass becomes more and more acute over time. “The longer we wait and confer, the greater the mass drift between the original standard and its replicas. Then we will not be able to agree at all about what mass should be considered a reference. The situation looks almost crazy,”admits Quinn.
So far, scientists have agreed that silicon atoms are ideal for the project due to their stability. Under standard conditions, they are almost never destroyed, and all damage is easily calculated. “We want to redefine a kilogram based on the mass of an atom, we want to count the atoms in one kilogram of a certain crystal,” said Professor Peter Becker “We measure the volume of a sphere and the volume of a silicon atom, and when you divide the silicon sphere by the volume of an atom, you get the number of atoms - everything is very simple". Scientists from many countries, including Russia, take part in the manufacture of the standard. 2 million euros were spent on the manufacture of a silicon sphere, and it was created for almost 5 years. According to the project manager, physicists have already calculated how many silicon atoms should be in one kg and have begun to "assemble"although this standard will not be perfectly accurate, because so far scientists cannot "assemble" a standard in the literal sense of the atoms.
The production association "Electrochemical Plant" in Zelenogorsk, Krasnoyarsk Territory, will supply raw materials to create a new international standard for the kilogram - an ideal ball made of a single crystal of silicon-28 isotope, the press service of the enterprise told Interfax.
The standard will be made in the form of an ideal ball from a single crystal of silicon-28 isotope.
The crystal will be grown at the Institute of Crystal Growth Germany from polycrystalline silicon obtained at the Institute of Chemistry of High-Purity Substances in Nizhny Novgorod. The initial raw material for the standard - silicon tetrafluoride - was supplied by Zelenogorsk ECP.