Redefining the kilogram
1 Mar 2012 by Evoluted New Media
British researchers are leading the way when it comes to redefining the kilogram.
The kilogram is the only unit in the international system of units (SI) still defined by a physical artefact – a lump of platinum-iridium. The others – metre, second, Kelvin, ampere, mole and candela – are based on fundamental constants of nature, but researchers at the National Physical Laboratory (NPL) hope to change that.
The General Conference on Weights and Measures (CGPM) agreed in October 2011 that the kilogram should be defined in terms of Planck’s constant (h) – which relates the frequency of a photon to its energy. Its final decision was deferred until there was sufficient, consistent and accurate data to agree on a value for h.
The NPL paper – published in Metrologica – provides a measured value of h usinga piece of kit called the watt balance, which related electrical power to mechanical power. This allows it to make very accurate measurements of Planck’s constant in terms of mass, length and time. If h becomes fixed, the watt balance could provide a method of measuring mass.
“This is an example of how British science leading the world,” said project leader, Dr Ian Robinson. “NPL invented the watt balance and has produced an apparatus and measurements which will contribute to the redefinition.”
Measurements using watt balances have given uncertainties approaching the two parts in one hundred million required to base the kilogram on Planck’s constant. NPL’s equipment is being used at the National Research Council in Canada, who look set to provide considerable greater accuracy.
Further data is coming from NIST – the USA’s measurement institute. The equipment here is showing slightly different results, but these are being investigated further. If the results are found to be consistent, it would be the beginning of the end for the physical kilogram.
“This research will underpin the world's measurement system and ensure the long term stability of the very top level of mass measurement,” said Robinson. “Although the man on the street won't see much difference – you'll still get the same 1kg bag of potatoes – these standards will ultimately be used to calibrate the world's weighing systems, from accurate scientific instruments, right down the chain to domestic scales."
