Leó Szilárd (February 11, 1898 – May 30, 1964) was a Hungarian physicist and inventor who conceived the nuclear chain reaction in 1933, patented the idea of a nuclear reactor with Enrico Fermi, and in late 1939 wrote the letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb. He also conceived the electron microscope, the linear accelerator (1928, not knowing Gustav Isings 1924 journal article and Rolf Widerøe's operational device) and the cyclotron. Szilárd himself did not build all of these devices, or publish these ideas in scientific journals, and so their credit often went to others. As a result, Szilárd never received the Nobel Prize, but two of his inventions did.
When Chicago was evacuated ahead of an advancing Race army, Szilard and the rest of the physicists working on the project traveled across much of the US to Denver, where they resumed work on the project. It was here that Szilard and his fellow scientists completed work on the atomic pile, and the first U.S.-built atomic bomb, the Fat Lady.