Optical engineer Katie Green examines a silicon sphere at the CSIRO's Australian Centre for Precision Optics. Counting the atoms in perfect silicon spheres could replace the definition of the kilogram. Credit:Bryan OBrien For the first time all seven of the metric units for measurement will be based on universal physical constants rather than physical artefacts. Meeting last week in Paris, the 104th meeting of the International Committee for Weights and Measurements (CIPM) was satisfied that after years of experiment it has found a new method to define the kilogram. The president of the CIPM is Barry Inglis. He is Australia's former chief metrologist, the founding chief executive of Australia's National Measurement Institute and the first Australian to chair the prestigious international committee. Speaking from Paris, Dr Inglis told Fairfax Media: "We are now in a position to decide on a definition of a kilogram that is better and more reliable than the uncertainty of the existing mass."

Barry Inglis, president of the International Bureau for Weights and Measures, addresses the general congress in 2014. Credit:BIPM The current definition of the kilogram is based on the "International Prototype", or Le Grand K. It is a cylindrical lump of platinum-iridium alloy sitting in a vault in the Pavillon de Breteuil, Sevres, near Paris. Six other near-identical alloy masses were made with it in 1889 to establish an international standard for the kilogram. Illustration: Cathy Wilcox The problem is that after 126 years the seven kilogram masses no longer weigh the same: they are out by about 60 micrograms. And because the International Prototype is deemed the standard, it is unclear whether it has become lighter or the others heavier. Most likely it is Le Grand K losing mass through minute amounts of gas diffusing from the artefact or atoms leaving the artefact from handling.

The weights of the seven kilogram masses have diverged since 1889. Credit:BIPM Since the establishment of the metric system during the French Revolution, metrologists - as those wizards who perform feats of measurement are known - have looked to place measurement on the most precise of bases. A copy of the International Prototype of the kilogram inside three glass bells in Sevre, France. Credit:BIPM The metric system now encompasses seven base units: mass (kilogram), distance (metre), time (seconds), electric current (ampere), temperature (Kelvin), substance (mol) and luminosity (candela). Of those seven, six are based on universal constants of nature. For example, a metre is defined as the length of the path travelled by light in a vacuum during a time interval of 299,792,458ths of a second.

The kilogram has been the hold-out. To shift from Le Grand K, two methods have been developed. One is to manufacture near-perfect spheres of silicon crystal and literally to count the atoms using X-ray diffraction. The other is to develop an insanely accurate electromagnetic scale (a watt balance) and from the electric charge measured reverse ferret via Planck's constant to Avogadro's number. Avogadro's number is defined as the number of atoms in 12 grams of the carbon isotope C-12. It works out to be an almost unfathomable number - about six followed by 23 zeroes, or 600 billion trillion - more than the number of stars in the universe or grains of sand on all Earth's beaches. Planck's constant is a mind-bogglingly tiny number - it is about 6.6 billion trillionths of a trillionth of a trillionth; or rather zero point 33 zeros followed by 66. It links the energy of a photon (light particle) to the frequency of its wavelength. Until now the uncertainty between the two methods has been greater than the uncertainty in the loss of mass from the physical artefact, the International Prototype of the kilogram.

Dr Inglis told Fairfax Media that the uncertainty has now been eclipsed. "The agreement between the two methods is down to less than 20 parts per billion," he said. There will be some more fine tuning as more experimental results come in, Dr Inglis said. A final recommendation on the value of Planck's constant and Avogadro's number - and from there a definition of the kilogram - will be made in two years. It will then be endorsed by the general assembly of the International Bureau of Weights and Measures in 2018. When the switch from physical artefact to physical constant is taken, of course, it will be based on the current mass of the International Prototype. So if that has indeed diminished by some micrograms, the new kilogram will be based on the slimmed down model. So next time you weigh yourself, be kind and deduct 60 micrograms for every kilogram.