A weighty matter

. Not only might it lose a few atoms by evaporation or abrasion, or acquire a few by contamination, but it could be damaged or even stolen. But efforts to replace it by some measurement based on unvarying physical phenomena are proving difficult. At NPL at Teddington and at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, they are balancing electromagnetic and gravitational forces using pieces of apparatuses called watt balances. NPL says that recent improvements in the design of theirs have yielded an uncertainty of only 70 parts per billion, and there is a plan in place to reach 20 parts per billion. NIST on the other hand, with a different design claims 36 parts per billion uncertainty, and also intends to reach 20 parts per billion. The only problem is that the results from the two sets of apparatus disagree by 300 parts per billion. The other scheme, the International Avogadro Project, aims to define the kilogram in terms of a specified number of atoms of pure silicon. At present, the uncertainty of this experiment is 300 parts per billions, but the results disagree with the watt balance measurements by 1100 parts per billion. Research continues. For more information, see