Everyday science

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.

Everyday science

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Everyday science

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Everyday science

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Everyday science

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Everyday science

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Everyday science

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Everyday science

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Everyday science

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Everyday science

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Everyday science

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Everyday science

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