Scientists play weighting game as Kg redefined

Scientists at the National Physical Laboratory have released new research results that could affect how we measure a kilogram – the last SI unit based on a manufactured object.

International Prototype kilogram (IPK), which is made from an alloy of 90% platinum and 10% iridium

Currently the kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of 1L of water.

This month the General Conference on Weights and Measures – the body who define The International System of Units (SI) - will consider a range of issues related to the maintenance of the SI, including a proposal to prepare for the redefinition of the Kg.

This may mean redefining mass to rely on a physical constant - a physical quantity that is universal in nature and constant in time - rather than the less accurate current means. However, to do so they first need find a suitable physical constant.

For 30 years scientists across the world have been looking to measure the value of a physical constant as accurately as possible using the existing man-made unit for a Kg. Once they have achieved this the fixed value of one of these constants can be incorporated it into a new definition of the kilogram.

The most likely constant could be one that uses Planck’s constant. Research at the National Physical Laboratory employs an indirect comparison of mechanical and electrical power to measure the Kg using length, time and quantum mechanical effects. However, the results suggest that more work is needed.

When attempting to measure Planck’s constant using its Mark 2 watt balance, the NPL showed a significant discrepancy with results produced at The National Institute of Standards and Technology (NIST) in the US. As both watt balances, though different in design, are using the same principle the results should be closer together.

Seton Bennett, Deputy director at the National Physical Laboratory said: “Given the discrepancy in these results, further results are required before the CGPM could consider adopting a new definition for the kilogram. A decision to replace the International Prototype with a definition based on Planck’s constant requires substantially more research by laboratories across the world, building on the strong foundations already laid by the National Physical Laboratory and its international partners.”

Work on the NPL Mark II watt balance will go on to March 2008 to reduce the uncertainties of the apparatus and either confirm the NPL result or reveal the source of this discrepancy.