Dempster, Arthur Jeffrey (1886-1950), was an American physicist. In 1935, Dempster discovered uranium 235 (U-235), a rare isotope (form) of the element uranium. This uranium isotope is a key substance in the atomic bomb.
Dempster was one of the developers of the mass spectrograph, an instrument for sorting out atoms by their mass. Using this instrument, he discovered many isotopes and obtained highly accurate measurements of their masses. Such information is essential for measuring transformations of mass and energy. Dempster also made important contributions to the development of the atomic bomb during World War II (1939-1945).
Dempster was born in Toronto, Ontario, on Aug. 14, 1886. He graduated from the University of Toronto. He moved to the United States in 1914, and he became a United States citizen in 1918. In 1916, Dempster took his doctor of philosophy degree at the University of Chicago. He taught physics there from 1919 until his death on March 11, 1950.
