from the University of California. In July 1941, an Office of Scientific Research and Development (OSRD) contract was awarded to Columbia University to study gaseous diffusion. With the help of the mathematician Karl P. Cohen, they built a twelve-stage pilot gaseous diffusion plant at the Pupin Laboratories. Initial tests showed that the stages were not as efficient as the theory would suggest; they would need about 4,600 stages to enrich to 90 percent uranium-235. A secret contract was awarded to M. W. Kellogg for engineering studies in July 1941. This included the design and construction of a ten-stage pilot gaseous diffusion plant. On 14 December 1942, the Manhattan District, the US Army component of the Manhattan Project, as the effort to develop an atomic bomb became known, contracted Kellogg to design, build and operate a full-scale production plant. Unusually, the contract did not require any guarantees from Kellogg that it could actually accomplish this task. Because the scope of the project was not well defined, Kellogg and the Manhattan District agreed to defer any financial details to a later, cost-plus contract, which was executed in April 1944. Kellogg was then paid $2.5 million. For security reasons, the Army had Kellogg establish a wholly owned subsidiary, the Kellex Corporation, so the gaseous diffusion project could be kept separate from other company work. "Kell" stood for "Kellogg" and "X" for secret. Kellex operated as a self-contained and autonomous entity. Percival C. Keith, Kellogg's vice president of engineering, was placed in charge of Kellex. He drew extensively on Kellogg to staff the new company, but also had to recruit staff from outside as well. Eventually, Kellex would have over 3,700 employees. Dunning remained in charge at Columbia until 1May 1943, when the Manhattan District took over the contract from OSRD. By this time Slack's group had nearly 50 members. His was the largest group, and it was working on the most challenging problem: the design of a suitable barrier through which the gas could diffuse. Another 30 scientists and technicians were working in five other groups. Henry A. Boorse was responsible for the pumps; Booth for the cascade test units. Libby handled chemistry, Nier analytical work and Hugh C. Paxton, engineering support. The Army reorganized the research effort at Columbia, which became the Special Alloyed Materials (SAM) Laboratories. Urey was put in charge, Dunning becoming head of one of its divisions. It would