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Measuring the Planck constant with an aluminium cube

Richard Davis of the BIPM has developed a simple experiment to determine the value of the Planck constant to better than 1 % by measuring the mass of a 20 g aluminium cube. The details have been published in the Journal of Chemical Education and the article is currently listed among the 20 most downloaded articles of the past 12 months.

The article is entitled "What is a kilogram in the revised International System of Units (SI)?" (J. Chem. Educ., 2015, 92(10), 1604-1609 published by the American Chemical Society). It takes the student through a simple experiment that begins by measuring the mass and dimensions of an aluminium cube and ends with a measurement of the Planck constant, h, to well within 1 % accuracy. Along the way, one determines the atomic mass of aluminium, the atomic mass of carbon-12 and the atomic mass constant (or dalton) all to the same relative uncertainty as h. Although not needed to determine h, the student also sees that the results now in hand yield the value of the Avogadro constant, again to the same relative uncertainty as the constants already determined. Essentially the student determines the mass of a single aluminium atom from the measured mass of the cube, its volume and properties of an aluminium crystal. The Planck constant is finally calculated with the help of additional reference data that are independent of the kilogram. It is then shown that accurate measurement of the Planck constant in the present SI is a pre-requisite to redefining the kilogram through a fixed value of h.

The article follows on from Resolution 1 of the 25th meeting of the CGPM which noted that further work of the BIPM should alert user communities to the proposed revision of the SI, which will probably occur in the autumn of 2018.

The article is open access thanks to its having been selected by the ACS Editors' Choice initiative. It currently ranks among the 20 most downloaded articles of the past 12 months. (One may note on the Journal of Chemical Education website that only six of the 20 articles were published in this century.) A short slide show narrated by the author can also be viewed.

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