Book: # 1

The SI unit of mass, the kilogram, is defined as the mass of a specific platinum iridium alloy cylinder kept at the International Bureau of Weights and Measures at Sevres, France. 

This mass standard was established in 1901, and there has been no change since that time because platinum iridium is an unusually stable alloy. The Sevres cylinder is 3.9 centimeters in diameter and 3.9 centimeters in height. A duplicate is kept at the National Bureau of Standards in Gaithersburg, Md.