Femtometre
The femtometre (American spelling femtometer) symbol fm derived from the Danish and Norwegian word 'fifteen', ) is an SI unit of length equal to 10 metres, which means a quadrillionth of one metre. This distance used to be called a fermi and was so named in honour of Italian-American physicist Enrico Fermi, as it is a typical length-scale of nuclear physics. Definition and equivalents. 1000000 zeptometres = 1 femtometre = 1 fermi = 0.000001 nanometre = For example, the charge radius of a proton is approximately 0.831 femtometres while the radius of a gold nucleus is approximately 8.45  femtometres. 1 barn = 100 fm History. The femtometre was adopted by the 11th "Conférence Générale des Poids et Mesures", and added to the SI in 1964, using the Danish word for "15" and the similarity in spelling with "fermi". The fermi is named after the Italian physicist Enrico Fermi (1901–1954), one of the founders of nuclear physics. The term was coined by Robert Hofstadter in a 1956 paper published in "Reviews of Modern Physics" entitled "Electron Scattering and Nuclear Structure". The term is widely used by nuclear and particle physicists. When Hofstadter was awarded the 1961 Nobel Prize in  Physics, it subsequently appeared in the text of his 1961 Nobel Lecture, "The electron-scattering method and its application to the structure of nuclei and nucleons" (December 11, 1961).