A technique used to produce positive ions by bombarding atoms and molecules with high-energy electrons. It was devised by the Canadian-American physicist Sir Arthur Dempster (1886–1950) in 1918 and used in some of the earliest work on mass spectroscopy. The technique has been used extensively ever since. A positive ion results because the bombarding electron knocks out an electron from the target atom or molecule. The ratio of the mass to charge of the resulting positive ion is then determined.