Today, let's look at Fahrenheit's thermometers. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Daniel Fahrenheit, the man who put thermometry on a solid footing, was born in the Polish city of Gdansk in 1686. When he was 15 his parents both died from eating poisonous mushrooms. The city council put the four younger Fahrenheit children in foster homes. But they apprenticed Daniel to a merchant, who taught him bookkeeping and took him off to Amsterdam.

And there he found out about thermometers. The Florentine thermometer, invented in Italy some 60 years before, caught his fancy. So he skipped out on his apprenticeship and borrowed against his inheritance to take up thermometer making. When the city fathers of Gdansk found out, they arranged to have the 20-year-old Fahrenheit arrested and shipped off to the East India Company. He dodged the Dutch police until he became a legal adult at the age of 24. At first Fahrenheit had gone on the run; but then he continued traveling -- through Denmark, Germany, Holland, Sweden, and Poland -- to learn and study.

Ulrich Grigull, who tells Fahrenheit's story, notes that Florentine thermometer scales were quite arbitrary. No two were alike. Makers set the low point on the scale during the coldest day in Florence that year and the high point during the hottest day.

Fahrenheit wanted thermometers to be reproducible. He realized that the trick was not to use the coldness or hotness of a particular day or place, but to find materials that changed at certain temperatures. Isaac Newton had had the same idea a few years earlier, but he wasn't a professional thermometer maker, and he was ignored.

Between 1707 and 1714 Fahrenheit worked out an alcohol thermometer scale based on three points: Zero was freezing point of a salt-water mixture, 32 degrees was the freezing point of water, and body temperature was called 96 degrees. Body temperature was a little off in this scale, but it was close. In 1714 he startled the world with a pair of thermometers that both gave the same readings. No one had ever managed to do that. Later he made mercury thermometers that let him use the boiling point of water instead of the human body temperature.

The turning points of inventive genius are subtle. Fahrenheit made sense of temperature by seeing temperature scales in abstract terms. He realized, independently of Newton, that the scales must be set by universal material properties. But he also did what Newton failed to do. He manufactured fine thermometers that carried his thinking into the world.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Grigull, U., Fahrenheit, a Pioneer of Exact Thermometry. Heat Transfer 1966, the Proceedings of the 8th International Heat Transfer Conference, San Francisco, 1966, Vol. 1, pp. 9-18.

Grigull, U., Newton's Temperature Scale and the Law of Cooling. Wärme- und Stoffübertragung, 1984, Vol. 18, pp. 195-199.

This episode has been substantially revised as Episode 1300.