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The Committee of Weights and Measures will decide the fate of the kilogram by the end of the week.

A copy of the standard kilogram stored in the chamber of measures and weights
Image: Andrew Brookes, National Physical Laboratory / SPL

On October 15-16, the International Committee on Weights and Measures will hold a meeting, which will determine the further sequence of actions to determine the kilogram standard. Earlier, in 2011, the experts of the commission decided to abandon the material standard and determine the kilogram value in terms of fundamental constants – the speed of light and the Planck constant. The basis of the “new kilogram” will form the well-known ratio E = mc 2 . This is reported by the journal Nature .

The main recommendation of the committee for the transition to the new mass standard was the measurement with sufficient accuracy of the Planck constant. This requires three independent measurements, two of which must be fundamentally different. In order to meet this requirement, as well as to achieve the required accuracy, researchers took about three years of work. The study involved three organizations: the National Institute of Standards and Technology (NIST, USA), the National Metrology Institute (PTB, Germany) and the National Research Council (NRC, Canada).

NRC and NIST used in their measurements the method of watt weights – in them the mass equal to the reference kilogram is commensurate with the electromagnetic forces of attraction between the two conductors. Due to the ability to directly measure in the experiment some fundamental constants, the device allows you to directly obtain the value of the Planck constant.

PTB, in turn, used an alternative approach – a direct calculation of Avogadro’s number based on the number of atoms in a ball made of a 28 Si single crystal . The Avogadro project’s experiment is to measure the volume of a one-kilogram ball with laser interferometry as accurately as possible, and also to measure the packing density of atoms in it — X-ray diffraction is used for this. It is interesting to note that enriched silicon was produced in Russia using centrifugation (uranium enrichment is similarly done)

By simply dividing the volume of a ball by the density of atoms, you can get the exact number of atoms in the ball, which is used in determining the Avogadro number. It, through other fundamental constants (the relative mass of the electron, the fine structure constant, and a number of other quantities), is related to the Planck constant.

One of the problems to be solved by the commission is the use of the new standard. In particular, the rules for accessing scientists to Watt scales is still in development.

The platinum-iridium ingot, which is now used as a reference, can change its mass with time due to the processes of evaporation and diffusion. Like any material object, it can be destroyed, which cannot be done with physical constants.

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