|Daniel Gabriel Fahrenheit thermometry
In 1701, Isaac Newton described a temperature scale in which zero was the freezing point of water and twelve was the human body temperature. Daniel Gabriel Fahrenheit was a glassblower who made barometers, altimeters, and thermometers. He invented the mercury-in-glass thermometer in 1714. He met and took ideas from Ole Rømer for his own scale. Fahrenheit originally established three calibration points for his scale. He set ninety six at the human body temperature; he set thirty two at the freezing point of water; and he set zero at the temperature of a mixture of ice, water, and ammonium chloride.
A mixture of equal amounts of ice, water, and ammonium chloride is a frigorific mixture. Together, these ingredients reach a stable temperature regardless of their initial temperatures. But something was wrong with Fahrenheit’s mixture; maybe the salts were impure or poorly dissolved; it didn’t work quite right.
Fahrenheit chose ninety six for body temperature to improve the simplicity of his scale. He started with Ole Rømer’s scale, but he didn’t like that water freezes at 7.5 °Rø or that body temperature is 22.5 °Rø. He multiplied Rømer’s degree by four, separating the freezing and boiling points by sixty degrees, but he wanted a difference of sixty four to make it easy to mark out the degrees using a compass to bisect the distances, so freezing became thirty two and body temperature became ninety six. Today the Fahrenheit scale is defined in reference to the Celsius scale, which is calibrated by any two of sixteen reference points—the vapor point of helium, the melting point of gallium, the triple or freezing points of various elements, and the triple point of Vienna Standard Mean Ocean Water. Water freezes at 0 °C or 32 °F, and boils at 100 °C or 212 °F. Today the United States is the only English-speaking country for whom the majority of citizens a balmy temperature of 25 °C doesn’t signify.