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| Keywords: MassStandards 005.jpg The platinum-iridium cylinder right was the primary standard kilogram for all metric measurements in the U S Known as Kilogram No 20 it was a copy of the International Prototype Kilogram which was preserved at the International Bureau of Weights and Measures at Sevres France Kilogram No 4 left under double bell jar a duplicate of No 20 was used as a secondary standard Noth cylinders were 39 mm in diameter and 39 mm high; they were made of 90 percent platinum and 10 percent iridium Re-comparison of Kilogram No 20 with the international standard in 1937 showed that the United States standard had changed by only one part in 50 million during approximately 50 years National Institute of Standards and Technology National Institute of Standards and Technology PD-USGov-NIST PD-USGov National Bureau of Standards National Institute of Standards and Technology Weights Metrology Kilogram Units of mass Units of measure | ||||
