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New value of Avogadro constant determined by EMRP project meets target uncertainty for redefinition of the kilogram

Researchers from EMRP project kNOW (SIB03) have completed a watt-balance measurement of the Planck constant having a preliminary uncertainty of 30 x 10-8 and correlated two of the most precise measurements of the Avogadro constant to obtain a value whose relative uncertainty is less than 2 x 10-8. The new value, determined in collaboration with the National Measurement Institutes of Japan and USA, now falls within the targeted uncertainty necessary for the redefinition of the kilogram.

The kilogram is the last remaining SI base unit to be based on a physical object - a platinum-irridium cylinder kept at BIPM in France - rather than a fundamental constant of nature. A new definition in terms of the Planck constant would offer improved long-term stability and opportunities for dissemination. Before the kilogram can be brought into line with the other SI base units, there needs to be international agreement on the Planck constant. Two approaches have been adopted to obtain this value: one is to use a watt balance; the other is to derive the constant from the Avogadro constant, obtained by counting the number of silicon atoms in two one-kilogram silicon spheres.

After re-measuring the silicon spheres in March 2015, a team including the SIB03 researchers correlated the new value of the Avogadro constant with previously reported values, to give an averaged value with an uncertainty below the agreed threshold for redefinition. The watt balance and Avogadro techniques now give nearly identical values of the Planck constant, paving the way to the new kilogram, expected in 2018.

This is a major step towards fulfilling the aims of SIB03 'Realisation of the awaited definition of the kilogram - resolving the discrepancies', which set out to understand, and hopefully resolve, the inconsistency between the determinations of the Planck constant and the Avogadro constant in preparation for the upcoming redefinition.