Smoke detectors have evolved from a tool used to detect explosive gasses in mines. In 1939, Swiss Physicist Ermst Meili invented the ionization chamber detecting device used in smoke detectors today. Early smoke detectors of the 1950's were expensive and used only in commercial and public buildings. In the early 1970's, few smoke detectors were sold for installation in residential buildings. Improvements to batteries and electronics reduced the size and costs of smoke detectors in the mid 1970's. By 1977 more than 12 million smoke detectors were sold.
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