The 118-year-old reference kilo is kept in a vault at the International Bureau of Weights and Measures (BIPM) at Sèvres, southwest of Paris. It now appears to have lost weight when compared with its copies, like the above.

Although 50 micrograms is equivalent to the weight of a fingerprint, for scientists the inconstant weight is complicating calculations.

"They depend on a mass measurement, and it's inconvenient for them to have a definition of the kilogram which is based on some artifact," physicist Richard Davis of BIPM told the Associated Press.

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