Theodore Alvin Hall (October 20, 1925 – November 1, 1999) was an American physicist and an atomic spy for the Soviet Union, who, during his work on United States efforts to develop the first and second atomic bombs during World War II (the Manhattan Project), gave a detailed description of the "Fat Man" plutonium bomb, and of several processes for purifying plutonium, to Soviet intelligence.

The Great Depression hurt Barnett's business significantly; when it was no longer able to support the household, the family moved to Washington Heights in Upper Manhattan.

[1] Even at a young age, Theodore showed an impressive aptitude for mathematics and science, mostly being tutored by his elder brother Edward, who was 11 years his senior.

[4][5] In the fall of 1936, despite the protests of their parents, Edward, his brother, legally changed both his and Theodore's last name to Hall in an effort to avoid antisemitic hiring practices he was experiencing that were prevalent at the time.

[1][5] At the age of 18, on the recommendation of Prof. John Van Vleck, Hall was hired as the youngest physicist to be recruited to work on the Manhattan Project at Los Alamos.

[4][6] At Los Alamos, after first helping to determine the critical mass of uranium used for "Little Boy",[1] Hall was assigned to conduct experiments on and tests of the implosion system ("Fat Man").

It was widely known inside the confines of Los Alamos, that Lieutenant General Leslie Groves, director of the Manhattan Project, had revealed to a group of top physicists there at a dinner that the real target of the US atom bomb was the Soviet Union,[8] a shocking statement that led one top physicist, Josef Rotblat, to resign from the Project, and others like Niels Bohr and Leo Szilard to vainly petition first Roosevelt, and later Truman to halt it, not use it on people in Japan, or to inform the Soviets about it.

On the pretext of returning to his home in New York City for his 19th birthday in October 1944, Ted Hall visited the headquarters of Amtorg, the Soviet Union trading company located in a loft building on 24th Street in Midtown Manhattan.

The two eventually met with Anatoly Yatskov, the New York station chief operating under the cover of being a Consular clerk, who two weeks later transmitted the information about both young men to NKVD headquarters in Moscow using a one-time pad cipher.

[citation needed] Kurnakov reported in November 1944: Rather tall, slender, brown-haired, pale and a bit pimply-faced, dressed carelessly, boots appear not cleaned for a long time, slipped socks.

[1] In June 1946, Hall's security clearance was revoked by the US Army, not over suspicions of being a Soviet asset but because of discovery of a letter he had received from his British sister-in-law, his brother Edward's wife Edith, who inquired jokingly of him: "I hear you're working on something that goes up with a big bang!

Furloughed out of the military with an honorable discharge and a citation from President Truman for the achievements of the Army's Special Engineering Detachment at Los Alamos, Hall headed that fall for the University of Chicago, where he finished out his master's and doctoral degrees in physics, met his wife, and started a family.

[13] At Cambridge he created the Hall Method of continuum normalization, developed for the specific purpose of analyzing thin sections of biological tissue.

The FBI and Justice Department claimed this was because their only evidence was the Venona document and that the US did not want to let the Soviets know they had broken their elaborate and supposedly "unbreakable" code.

Alan H. Belmont, the number-three man in the FBI, claims he decided at that time that information coming out of the Venona project would be inadmissible in court as hearsay evidence, so its value in the case was not worth compromising the program.