thermometrum

Thermometer comes from Greek thermós (hot, warm) and métron (measure). The word appeared in the early 1600s. The instrument it names has a tangled origin: Galileo built a thermoscope (a device that showed temperature changes but could not measure them precisely) around 1593. Santorio Santorio, an Italian physician, added a graduated scale around 1612. The Florentine Accademia del Cimento built sealed liquid thermometers in the 1650s. No single inventor created the thermometer. It was assembled in stages.

The problem was scale. Gabriel Fahrenheit, a German-Dutch physicist, developed his mercury thermometer and temperature scale in 1724. He set 0° at the coldest temperature he could reliably produce (an ice-salt mixture), 32° at the freezing point of water, and 96° at human body temperature (later adjusted to 98.6°). Anders Celsius, a Swedish astronomer, proposed his scale in 1742 with 0° at boiling and 100° at freezing — the reverse of modern Celsius. Carl Linnaeus or one of his colleagues inverted it after Celsius's death.

The clinical thermometer — designed to measure body temperature — transformed medicine. Carl Wunderlich, a German physician, published a study in 1868 based on over a million temperature readings from 25,000 patients, establishing 37°C (98.6°F) as the normal human body temperature. Before Wunderlich, fever was a subjective diagnosis. After him, it was a number. The thermometer turned a sensation into a measurement.

Digital thermometers have replaced mercury in most applications. Infrared thermometers can measure temperature without contact — the forehead scanners used during the COVID-19 pandemic are a direct descendant of Torricelli's and Galileo's experiments. The word still means 'heat measure.' The heat is measured in milliseconds now, by electronics that the word's Greek coiners could not have imagined.