Some of the measurements we take for granted were not known in the ancient world. Among them is temperature. Certainly, ancient people knew that it was cold sometimes and hot sometimes, but no one had a way of putting a number to it. It was Galileo who made the first steps toward a quantitative means of measuring temperature. He noted that gases expand when heated. Using this principle, he developed a thermoscope, an inaccurate gas thermometer. Although Galileo did not know it, the gas he used –– air –– changed in volume according to outside air pressure as well as heating.

A really good thermometer [the word comes from Greek thermos=hot and metron=measure] was not made until 1714, when Gabriel Fahrenheit made the first mercury thermometer. Instead of air Fahrenheit used changes in the expansion of mercury to measure temperature. He set 0° as the lowest temperature he could reach by freezing salted water, hoping to avoid negative temperatures.

Anders Celsius did not create a new type of thermometer, but he did create the scale that is most commonly used around the world today. In one of the great mistakes of science, Celsius first set his scale with 0° as the temperature of boiling water and 100° as the temperature of freezing water. Wiser heads prevailed, however, and the scale was reversed. Until 1948 the scale Celsius devised was known in the United States as centigrade, but that year scientists agreed to rename it after its inventor. Gradually the world has gotten used to the idea that 37°C means 37 degrees Celsius, not 37 degrees centigrade.

More accurate thermometers have been developed over the years. Instead of using the simple expansion of materials upon heating, other thermometers use differences in the coefficient of expansion of two metals, the amount of energy emitted at different temperatures by certain substances, or the changes in electrical resistance at different temperatures. Fahrenheit’s idea of a temperature scale with no negative numbers was achieved with the introduction of the Kelvin scale (sometimes called the absolute scale). Lord Kelvin observed in 1848 that Charles’s law, which states that gases lose 1/273 of their volume for every degree below 0°C, implies that at –273° the volume becomes zero. He proposed that –273° must be the lowest temperature obtainable, since nothing has a negative volume.

This concept has since been verified. Today absolute zero is set more accurately at –273.15°C (–459.7°F). On the Kelvin scale, then, the freezing point of water is 273.15 K (no degree sign is used) and the boiling point is 373.15 K.