DOI 10.1393/ncc/i2016-16296-7 Communications: SIF Congress 2015 IL NUOVO CIMENTO 39 C (2016) 296 The Fermiac or Fermi’s Trolley F. Coccetti( * ) Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi” Via Panisperna 89a, Roma, Italy received 28 April 2016 Summary. — The Fermiac, known also as Fermi’s trolley or Monte Carlo trolley, is an analog computer used to determine the change in time of the neutron population in a nuclear device, via the Monte Carlo method. It was invented by Enrico Fermi and constructed by Percy King at Los Alamos in 1947, and used for about two years. A replica of the Fermiac was built at INFN mechanical workshops of Bologna in 2015, on behalf of the Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, thanks to the original drawings made available by Los Alamos National Laboratory (LANL). This reproduction of the Fermiac was put in use, and a simulation was developed. 1. – Description The Fermiac [1] is a little brass trolley invented by Enrico Fermi and built by Percy King in 1947. It is a 30 cm long hand-operated computer conceived to study the change in time of the neutron population in a nuclear device. The neutron population would either increase or decrease or remain constant in time, representing a supercritical, sub- critical or critical system respectively. The Fermiac was used at the Los Alamos National Laboratory (LANL) for about two years by the Bengt Carlson’s T-division group, and by scientists such as Ulam, King, Carlson and Metropolis. It was put aside in 1949 because of the general availability of new electronic computers. The Fermiac makes use of the Monte Carlo method to trace the histories of neutron movements in a nuclear device. This device originally had no official name, and it became also known as Fermi’s trolley or Monte Carlo trolley. The original Fermiac, shown in fig. 1, is on display in the Bradbury Science Museum in Los Alamos. ( * ) E-mail: fabrizio.coccetti@centrofermi.it Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 1