Arthur Jeffrey Dempster (14.08.1886—11.03.1950) was a Canadian physicist and chemist. He was born in Toronto (Canada). He graduated from the University of Toronto (1909). In 1911—14 he continued his studies in Göttingen, Munich and Würzburg. From 1914 he lived in the USA. From 1919 he worked at the University of Chicago (professor from 1927).
His research was in the field of mass spectrometry, the chemistry of isotopes, electrical discharges in gases and the physics of accelerators. He built the first mass spectrometer (1918) and the first double-focusing mass spectrograph, discovered a number of isotopes (of potassium, lithium, magnesium, calcium and zinc) and in 1935 uranium-235. He constructed the curve of the packing fractions for the heavy elements. He proposed a version of the direct method of accelerating ions.