large scale production of low boron content graphite. The resulting product was designated AGOT graphite ("Acheson Graphite Ordinary Temperature") by National Carbon. With a neutron absorption cross section of 4.97 mbarns, the AGOT graphite is considered as the first true nuclear-grade graphite. By November 1942 National Carbon had shipped of AGOT graphite to the University of Chicago, where it became the primary source of graphite to be used in the construction of Chicago Pile-1. Government support Szilard drafted a confidential letter to the President, Franklin D. Roosevelt, warning of a German nuclear weapon project, explaining the possibility of nuclear weapons, and encouraging the development of a program that could result in their creation. With the help of Eugene Wigner and Edward Teller, he approached his old friend and collaborator Albert Einstein in August 1939, and convinced him to sign the letter, lending his prestige to the proposal. The Einstein–Szilard letter resulted in the establishment of research into nuclear fission by the U.S. government. An Advisory Committee on Uranium was formed under Lyman J. Briggs, a scientist and the director of the National Bureau of Standards. Its first meeting on 21 October 1939 was attended by Szilard, Teller, and Wigner. The scientists persuaded the Army and Navy to provide $6,000 for Szilard to purchase supplies for experiments—in particular, more graphite. In April 1941, the National Defense Research Committee (NDRC) created a special project headed by Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program. Compton's report, submitted in May 1941, foresaw the prospects of developing radiological weapons, nuclear propulsion for ships, and nuclear weapons using uranium-235 or the recently discovered plutonium. In October he wrote another report on the practicality of an atomic bomb. For this report, he worked with Fermi on calculations of the critical mass of uranium-235. He also discussed the prospects for uranium enrichment with Harold Urey. Niels Bohr and John Wheeler had theorized that heavy isotopes with odd atomic mass numbers were fissile. If so, then plutonium-239 was likely to be. In May 1941, Emilio Segrè and Glenn Seaborg produced 28 μg of plutonium-239 in the cyclotron at the University of California, and found that it had 1.7 times the thermal neutron capture cross section of uranium-235. At the time only such minute quantities of plutonium-239 had been produced, in cyclotrons, and it was not possible to produce a