A Modern Take on Galileo’s Thermometer and Goethe’s Barometer

By Michael Parks, P.E., for Mouser Electronics

Most of us associate the name Galileo with astronomy. However, like many of his of Renaissance contemporaries, Galileo Galilei had research interests that cut across a wide spectrum of scientific arenas. As part of his interest in physics, he discovered a principle of liquids that demonstrated density changes in proportion with the liquids’ temperatures. This fundamental principle would be applied in a practical manner by Galileo’s pupils in 1666 (two years after Galileo’s death).

The so-called Galileo thermometer, called the termometro lento (slow thermometer) in Italian, is a wonderfully simple invention that arguably blends science and art in an elegantly visual way. The singular glass tube is filled with a concentrated solution of ethanol and water. The density of the ethanol varies with temperature changes more than water and as such causes a half-dozen or so colorful glass balls to float or sink. Interesting fact: It is not the liquid within the colorful glass balls that affects their buoyancy. Rather, it is the metal tags dangling underneath, which indicate to the user the ambient temperature, that adjust to alter the effective density of each ball. As the ambient temperature decreases more balls float. Conversely, as the temperature increases more balls sink.

Almost 70 years later, a German statesman by the name of Johann Wolfgang von Goethe would leverage his interest in meteorology and work by the Italian physicist Evangelista Torricelli to invent the glass globe barometer. (Fun fact: Torricelli was one of Galileo’s pupils who invented the Galilean thermometer.) The glass globe or Goethe barometer visually indicates the ambient atmospheric pressure by providing a mechanism with an open-ended spout that is attached to an otherwise sealed glass vessel in which water can rise or fall. As the atmospheric pressure rises, the water can push down the spout. As the pressure drops, the water can flow higher up the spout, and typically, lower pressure indicates the approaching of nastier weather such as a thunderstorm.

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