German physicist (1686–1736)
Possibly owing to a business failure, Fahrenheit emigrated to Amsterdam from his native Danzig (now Gdańsk in Poland) to become a glass blower and instrument maker. He specialized in the making of meteorological instruments, and proceeded to develop a reliable and accurate thermometer. Galileo had invented the thermometer in about 1600, using changes in air volume as an indicator. Since the volume of air also varied considerably with changes in atmospheric pressure liquids of various kinds were quickly substituted. Fahrenheit was the first to use mercury in 1714. He fixed his zero point by using the freezing point of a mixture of ice and salt as this gave him the lowest temperature he could reach. His other fixed point was taken from the temperature of the human body, which he put at 96°. Given these two fixed points the freezing and boiling points of water then work out at the familiar 32° and 212°. One advantage of the system is that, for most ordinary purposes, negative degrees are rarely needed.
Using his thermometer, Fahrenheit measured the boiling point of various liquids and found that each had a characteristic boiling point, which changed with changes in atmospheric pressure.