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What is a Galileo Thermometer?

A Galileo thermometer is a sealed glass cylinder containing a clear liquid, usually ethanol, and several glass vessels. Ethanol shows a significant change in density according to changes in temperature, making the thermometer more accurate. The glass spheres inside the cylinder are filled with a coloured liquid and have a calibrated medallion hanging from the bottom with a specific temperature engraved on each medallion. The metal tags are calibrated to a unique weight, and their density allows the sphere to float or sink in the ethanol at specific temperatures. The liquid inside the glass spheres is for decorative purposes only.   

How does a Galileo Thermometer work?

Galileo discovered that the density of a liquid is a function of its temperature: the density of most liquids decreases as the temperature increases.

Ethanol obtains its heat through the ambient air on the outside of the glass cylinder. It is quite sensitive to temperature changes and will expand and contract accordingly. As the air temperature cools, the liquid contracts and the density increases. At higher air temperatures, the liquid expands and its density decreases. The spheres with a lower density than the liquid will float to the top, while those with a higher density will sink.

How do you read a Galileo Thermometer?

As the temperature changes, spheres will rise and fall within the glass tube. The lowest floating bulb is the one nearest to the ambient temperature. If all of the spheres float to the top, then the air temperature is colder than the temperature reading on the lowest medallion. If the bulbs all sink, then the air temperature is warmer than the reading on the highest tag.

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