Kilogram 18, UK copy of SI unit of mass. The UK's National Standard Kilogram, copy number 18 of the International Prototype, was allocated in 1889 when the standard was established. It is kept at the National Physical Laboratory (NPL) in a specially-designed glass and stainless steel enclosure. This is airtight and the temperature is maintained at 19 degrees Celsius. The International Prototype Kilogram, a cylinder of platinum-iridium alloy, is held at the Bureau International des Poids et Mesures (BIPM), at Sevres, France. For Kilogram 18 alone, see image C016/2049. Photographed at the NPL, Teddington, UK.