“À tous les temps, à tous les peuples!” Towards a Biography of the Pavia University Platinum kg (1800-2020)

In the Museum for the History of Pavia University shines a precious, small cylinder of platinum (weight 1 kg; ca 3,9 cm (d) x 3,9 cm (h)), with its original box signed by Jean Nicolas Fortin (1750-1831) and inscribed: KILOGRAMME Conforme à la Loi du 18 Germinal an 3 présenté le 4 Messi an 7. Fortin. The reference is to April the 7th 1795 and June the 22nd 1799. In 2019 the cylinder underwent a careful X-ray fluorescence analysis confirming that it is composed of platinum (only very small traces of iridium) and thus its 18th (or very early 19th) century origin. How did the platinum kilogram get to Pavia? In 1876 Giovanni Schiaparelli (1835-1910) director of the Brera Observatory in Milan and professor at Pavia, transferred it officially from Brera to the Pavia Physics Institute directed by Giovanni Cantoni (1818-1897). This followed the establishment in 1875 of the Bureau International des Poids et Mesures (BIPM) that led to 1889’s new prototype made of 90% platinum and 10% iridium. The transfer from Paris to the Brera observatory is more obscure. It might involve three scientists who had personal links with Napoleon: Lorenzo Mascheroni (1750-1800), Alessandro Volta (1745-1827), and Barnaba Oriani (1752-1832). At the beginning of the 19th century three official kilograms were in Milano, one of them in Brera. In 2019 also the “new” Paris prototype had to retire. Pavia’s platinum kilogram is now a grandfather and stands in all its beauty as a witness to the usefulness of the ever more precise but never definitive search for a ‘stable’ unit of measurement.