New method for measuring a kilogramme developed

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 6.022x1023 - 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.

When silicon crystalises, it forms cubic cells of eight atoms each. Thus, it is possible to calculate the number of atoms in such a sphere by examining the ratio between the total crystal volume and the volume occupied by each silicon atom, which can in turn be calculated by measuring the cubic cell.

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.

But in a day and age when science is a truly global endeavour, having just one physical standard against which all others must be calibrated is an impediment to progress, researchers said.

The standard itself is subject to subtle fluctuations in mass over time due to surface reactions, they said.

That's why the international metrology community is working to redefine the kilogramme in terms of a constant of physics instead of a physical object.

After years of discussion and research, the kilogramme will be officially redefined in terms of Planck's constant in 2018.

The research was published in the Journal of Physical and Chemical Reference Data.