美国科学60秒 SSS 2013-01-15（在线收听）

 　　How do you to find 1 kilogram? Easy. It's the exact mass of medal cylinder called the international prototype kilogram. IPK, for short. Kept in controlled conditions in France. But the standard kilogram has gained weight since its creation in 1875. To trim and end its many replicas down to size, we need to clean them. The report is in the journal M*. When scientists analyzed the surface of an IPK replica, they found it carries tens of additional macro-grams. These changes are due to contaminates built up in the kilograms despite their careful storage of the glass. The additional mass as equal to just a grain of sand, but the kilogram is such an essential unit that tiny changes  to different replicas are important. To remove the contaminates, we need to standard and reproducible cleaning method. Researchers exposed one kilogram measure to ozone and ultraviolet light which would remove the cover base contamination without damage the medal. But to permanently standardize the kilogram we will need to find terms of fundamental coincidence, not a lump of medal in a jar.