In the process of development and perfection of measurement technologies it became clear that all standards are not ideal. At measurement standards of the basic units it is necessary to take not man-made objects but much more perfect samples already created by the nature. The National Institute of Standards and technologies of the USA together with the metrological organizations of other countries are taking the first steps toward revision of the international system of SI, which forms the basis of all modern measurements. In the new system all seven base units will be based on constants of the nature (currently only Kelvin, second and meter are based fully on constants of the nature). This will bring measurements to new levels of accuracy. In the possible revision of the SI system, the most significant change would be in the kilogramm. It is currently the only SI unit that is still defined by an artifact.

Within the existing SI, the mass unit is defined in terms of the International Prototype Kilogram (a 122 year old platinum-iridium cylinder). This definition of the kilogram creates certain problems, because the mass of this prototype changes slightly in time. A possible solution to this problem was proposed by two American scientists from the Institute of technology of Georgia in 2007. They suggested to consider as the standard of the weight a carbon cube from with strictly quantity of atoms. As the weight of each atom is constant,  the weight of this cube is also constant. Researchers have calculated that the cube with the weight 1 kilogram will consists 50184513538686668007780750 atoms, and the length of its side will be 8,11 centimeters. The first ideas of this research were published in 2007 in American Scientist. For three more years the scientists worked on details and have now presented a new article.

The American physicists have attended to a problem of the standard of kg and have chosen as “a reference” element carbon with an ulterior motive – before they were engaged in specification of number of Avogadro – one of the fundamental constants, defining, how many particles are contained in one mole of any substance. Though this number also is one of the most important things in chemistry, its exact value is not known. Its value is known with an uncertainty: N_a = (6.022145 ± 0.000016) × 10²³ mol⁻¹ (k = 2). The number of Avogadro was picked so that weight specified in grams equaled to weight of a molecule (atom) in nuclear mass units. The atom of carbon ¹²C has weight of 12 nuclear mass units so, one mole of ¹²C has a mass of 12 grams.

Having now respecified the number of Avogadro and having accepted its value equal 84446886³ (602214098282748740154456), researchers could calculate the necessary number of atoms of carbon in the standard.

However the scientists from Georgia have competitors. For example, at the US National Institute of Standards and Technology work is in progress for development of the concept of electronic kilogram using the so-called Watt balance. This way kilogram would be defined by current and voltage which are necessary for creation of the magnetic field, capable to counterbalance a weight of one kg. This way is in principle very good, as it allows achieving high precision (it is based on use of one more fundamental constant – the Planck constant). However the experiment is extremely complex.

One more variant for definition of the new kilogram standard is via a silicon sphere, which parameters are calculated in such a manner that it will contain a strictly known number of atoms. Such sphere has actually been created, but there were immediately the complexities similar to the complexities of the present standard – the sphere loses a part of the Si atoms with time and, besides, a film of silicon dioxide is formed on its surface.

At the moment it is not clear, whether there will be the carbon standard, silicon standard, electronic kilogram or scientists will think up a fourth, more convenient, way. But it seems rather probable that the cylinder stored in France within 120 years will soon retire. (Image: Wikipedia)