It's how the mass of the International Prototype Kilogram and its copies have drifted over time: http://www.bipm.org/en/bipm/mass/ipk/ http://www.bipm.org/en/b

It's how the mass of the International Prototype Kilogram and its copies have drifted over time: http://www.bipm.org/en/bipm/mass/ipk/ http://www.bipm.org/en/bipm/mass/ipk/ (verifications tab)