New method developed for measuring a kilogramme

London : The humble kilogramme may be given a new and more precise definition that for the first time will not require reference to a physical object.

An international effort to redefine the kilogramme by 2018 has been helped by recent efforts from a team of researchers from Italy, Japan and Germany to correlate two of the most precise measurements of Avogadro’s number and obtain one averaged value that can be used for future calculations.

Avogadro’s number is approximately 6.022×1023 — an almost unfathomably large quantity, greater than the number of grains of sand on Earth or even the number of stars in the universe.

But the number, which represents the number of discrete particles like atoms or molecules in a “mole” of a substance, is a useful way to wrangle these tiny particles into more meaningful quantities.

A mole of water molecules, for instance, is only a few teaspoons of liquid. Because Avogadro’s number is linked to a number of other physical constants, its value can be used to express other units, such as the kilogramme.

The team has calculated Avogadro’s number several times in the past. Each time, they obtained a value for Avogadro’s number by counting the number of atoms in a one kilogramme sphere of highly pure Silicon.

Currently, the kilogramme weight standard is a platinum-iridium cylinder about the size of a golf ball, housed in the International Bureau of Weights and Measures in Sevres, France.

Source | Free Press | 16 July 2015

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