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Kilogram became intangible


The participants of the 26th General Conference on Measures and Weights, which takes place in Paris, made a historic decision to redefine four of the seven basic units of the International System of Units (SI) – kilogram, ampere, Kelvin and pray. By this decision, the kilogram was “untied” from the material carrier standard, and is now determined through the Planck constant. From this point on, all SI units are tied to fundamental physical constants. The broadcast of the conference session, where this decision was unanimously adopted, was conducted on the  website of the International Bureau of Weights and Measures.

The current decision completes the reform, which lasted several decades: back in 1983, the meter was tied to the value of the speed of light in a vacuum (and the value of speed was fixed, all programs for measuring the speed of light in a vacuum were discontinued, because its value became precisely known by definition. In 2005, the researchers decided to choose three more constants for redefining other units. Planck’s constant was chosen as the basis for determining the unit of mass, kilogram, elementary electric charge (electron charge) unit current, and the Boltzmann constant – thermodynamic temperature.

In order to complete this transition, it took several years to measure the constants with high accuracy .

Until this day, the mass unit, a kilogram, was tied to a man-made artifact, a cylinder made of an alloy of platinum and iridium, which is stored in the French city of Sevres. The results of several international comparisons of national copies made from the same material showed that their masses vary relative to the main standard in the range of ± 50 micrograms over 100 years. How much the mass of the main standard has changed is unknown, since there is nothing to compare it with. For many types of measurements, such a deviation can lead to unreliable results.Now approved a new definition of kilogram, based on Planck’s constant. The installation, with which you can implement a new mass standard, is called a Kibble scale . In such scales, the standard is a load that balances the repulsive force between a permanent magnet and a current-carrying coil. Thus, the mass of the object can be found due to the equality of the electric and mechanical forces (for more information about the device of this device, you can read here). Constant Planck “hides” in the equations describing the operation of the electrical part of the installation, and without fixing it, it was impossible to redefine the kilogram. It is noteworthy that the same setup was previously used just to refine the Planck constant, but for this purpose a different mode of operation was used. Specifically, the Planck constant, in particular, is devoted to our text “Last Reference .  ”

Thanks to the new definition of the kilogram, each country will be able to reproduce the reference installation independently at any time, without resorting to reconciliation with the main reference.

The conference also approved a new definition of ampere. The previous definition, approved in 1948, was based on the measurement of the force acting on parallel conductors with current. Now the scientists decided to fix not only the numerical value of Planck’s constant for the kilogram, but also the numerical value of the electric charge – for the new definition of ampere.The unit of temperature, Kelvin, until today was defined as 1 / 273.16 of the thermodynamic temperature of the triple point of water . This definition created its inconvenience, since there are always impurities of heavy isotopes of hydrogen and oxygen in water, and they can significantly shift the triple point. Therefore, metrologists had to create a separate standard – the Vienna Standard of averaged ocean water. Her recipe is: 0.00015576 moles of deuterium per mole of ordinary hydrogen; 0.0003799 mol 17 O per mole 16 O, and 0.0020052 mol 18 O per mole 16A. In addition, in order to determine the exact values ​​in other ranges, scientists had to control the melting and boiling points of several other substances. The new definition of Kelvin is based on the Boltzmann constant, which will now be exactly equal to 1.380649 × 10 -23 J × K -1 (joules per kelvin).

Up to this time, the mole was tied to the number of atoms in 0.012 kilograms of stable carbon-12, that is, it was bound to mass. In the new version of the SI system, it will be determined through the fixed Avogadro constant, that is, it will be equal to 6.02214076 × 10 23  particles.

The draft reform was approved today, but it will come into force on World Metrology Day, May 20, 2019.

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