Scientists from the National Institute of Standards and Technology (NIST, USA) measured the Planck constant with great accuracy. This was done primarily to redefine the kilogram in terms of fundamental physical constants. The point is that the reference platinum-iridium bar stored in the Chamber of Measures and Scales is subject to diffusion and evaporation effects, due to which its mass is not constant. The corresponding article was published in the journal Metrologia .
In 2011, the General Conference on Measures and Weights adopted a resolution on the redefinition of all basic units so that their numerical values were determined exclusively using fundamental physical constants and the properties of atoms. At the same time, for the introduction of a new kilogram, the main recommendation was the measurement of Planck’s constant with sufficient accuracy with the further expression of the kilogram through the value found, and the mass of the platinum-iridium standard at the time of determination will be equal to the new kilogram with a certain error.
Three organizations were involved in measuring the numerical value of the Planck constant: the National Institute of Standards and Technology (NIST, USA), the National Metrology Institute (PTB, Germany) and the National Research Council (NRC, Canada). At the same time, NIST and NRC use the Kibble scale (the former name is the Watt scale) as an installation, while PTB directly measures the volume and mass of a 28 Si single crystal ball and also the packing density of atoms in it, measuring Avogadro number. It, in turn, is expressed in terms of fundamental constants, including the Planck constant.
Today, the fourth generation of Kibble weights is working at NIST. In principle, they are arranged as follows: a sample is installed on one side of the scale, the mass of which must be measured. Instead of the second bowl, a coil in a magnetic field is used, which balances the balance due to the Ampere force acting on the coil. Read more about installing NIST-4 here .
Higher accuracy of measurements compared to previous results of the installation is due to the following factors: firstly, more statistics are collected. Secondly, the scientists discovered that they had previously somewhat overestimated the influence of the magnetic field of the coil on the installation. Third, the researchers more accurately measured the effects associated with the movement of the coil in a magnetic field. All this made it possible to increase the measurement accuracy from 34 × 10 −9 standard deviations to 13 × 10 −9 . The value of Planck’s constant in the experiment turned out to be equal to h = 6.626 069 934 (89) × 10 −34 J · s.
The end of experiments in all three laboratories, as well as the redefinition of the kilogram itself, is scheduled for 2018. It is worth noting that at the same Conference on Measures and Weights it is planned to redefine other physical constants, including ampere, Kelvin and a mole.