California University Cyclotron, or the Calutron, is a mass spectrometer which was created during the Manhattan Project by Earnest O. Lawrence for the isotope separation of 235U from 238U. When nuclear fission was realized by European physicists in the late 1930's, the race to create a nuclear bomb began. The Americans and the Allied Forces started the Manhattan Project, a top secret project to develop nuclear weapons before the Germans. After the discovery of fission, it was also realized that the only naturally occurring fissile material was 235U, which makes up approximately 0.711% of uranium in the Earths crust. Therefore, in order to create an effective nuclear weapon, the percentage of 235U would have to be increased to around 90%. This meant that techniques for the enrichment of uranium were necessary. In response to this, the Y-12 facility was built in Oak Ridge, Tennessee in order to house the 1152 Calutrons during World War two (WW2). The facility was an industrial-scale uranium enrichment plant which provided most of the fuel for the "Little Boy" nuclear weapon, which was dropped on Hiroshima in 1945. 
In a mass spectrometer, a vaporized quantity of a sample (natural uranium containing 0.711% 235U and 99.289% 238U) is bombarded with high energy electrons which causes them to become positively charged ions. These positively charged ions are then accelerated through a magnetic field. When passing between the poles of a magnet, a beam of naturally occurring uranium ions which has a very narrow range of energy levels (or monoenergetic), will split into several streams according to their momentum. The momentum of the uranium isotopes 235 and 238 will be slightly different due to the fact that 238 has a larger mass than 235 (see isotope separation for a detailed explanation of isotopes and their respective masses). For this reason, there will be a stream of 235U ions and a stream of 238U ions as depicted in figure 2 on the right. These streams, due to their differing masses and momentum's, will have different radius curvatures. Each of the two isotopes are collected in cups at the ends of the semicircular path.