National Standard of Mass, Kilogram No. 20

- Image ID: HRF8A1
Science History Images / Alamy Stock Photo
Image ID: HRF8A1
The National standard of mass, known as Prototype Kilogram No. 20, is a platinum-iridium cylinder 39mm in diameter and 39mm high. It is an accurate copy of the international standard kept at the International Bureau of Weights and Measures at Sevres, France. Prototype No. 20 was furnished to the United States by the International Bureau in accordance with the metric treaty of 1875. Recomparison with the international standard in 1948 showed that the United States standard has remained constant within one part in 50 million over approximately 60 years.