The international radium standard and the curie

By 1910 radium was sold for cancer treatment at enormous prices, and its value depended entirely on measurement. At the Radiology Congress in Brussels in September 1910 an international committee, including Rutherford, Soddy, Boltwood and Stefan Meyer, agreed on a common standard. They named its unit the curie: the amount of radium emanation (radon) in equilibrium with one gram of radium. Marie Curie insisted the unit be tied to a full gram rather than a tiny practical fraction.

She was asked to prepare the primary standard. In August 1911 she weighed out 21.99 milligrams of the purest radium chloride and sealed it in a thin glass tube a few centimetres long. In March 1912 the committee compared it in Paris, by its gamma rays, with standards made in Vienna by Otto Hönigschmid. They agreed, and the Paris tube became the International Radium Standard, kept at the International Bureau of Weights and Measures at Sèvres.

Her laboratory then tested radium tubes for hospitals and industry. The curie remained the unit of radioactivity for decades; it has since been replaced in the SI by the becquerel.