"[Scientists] should return to the plainness... of Observations on material and obvious things."
Robert Hooke, Micrographia (1664)
In a mercury thermometer, mercury in a small glass bulb expands into an evacuated, linear, uniform cross- section glass tube; the amount of expansion is used to measure the temperature of the bulb. Dante Gabriel Fahrenheit (1686-1736) left Gdansk, Poland, and eventually became a glassblower and scientific instrument maker in the Netherlands. His first glass thermometer (1709) used alcohol as the expanding fluid, but this has a limited temperature difference between its freezing and boiling points. In 1714 Fahrenheit turned to mercury, a liquid metal that expands uniformly over normal temperature ranges.
Fahrenheit insisted that thermometer results should be universally reproducible, and similar temperatures should be represented by the same number. To this end he introduced, in 1724, three "fixed" points and eight graduations on his thermometer tube. Zero degrees was the lowest temperature that he could obtain in the laboratory, the temperature of a mixture of water, water ice, and ammonium chloride. Thirty-two was the temperature of an ice, pure water mixture, and ninety-six degrees was the normal temperature of a human body. From 1717 Fahrenheit was selling thermometers from a base in Amsterdam, and these, and his temperature scale, became widely used throughout Britain, the Netherlands, and Germany.
More recently the Fahrenheit scale has been defined using the freezing and boiling points of pure water, at normal atmospheric pressure, as 32 and 212 degrees (0 and 100°C). Here our typical body temperature becomes 98,6°F (37°C).
