English: Sealed glass tube of radium chloride kept by the US Bureau of Standards that served as the primary standard of radioactivity for the United States in 1927, and several subsidiary working standards. . This was officially known as the "Secondary radium standard no. 6" which had in 1913 been calibrated against the International Radium Standard, a similar tube of radium chloride prepared by Marie Curie, kept by the Bureau Internationale des Poids et Measures (International Bureau of Weights and Measures) at Sevres, France. This was determined to contain 20.68 mg of radium chloride, corresponding to 15.44 mg of radium in 1913. Since radium decays at a rate of about 1% in 25 years, at the time of this picture it was estimated to contain 15.360 mg of radium