Who invented the Thermometer

Galileos thermoscope

Galileo’s Thermometer. How Does a Galileo Thermometer Tell the Temperature? Galileo thermometers are popular ornamental thermometers. How do they work?


There is some controversy about the historical details, but the basic story is something like this. Ancient Greeks may have used simple temperature sensing devices, but Galileo is usually given credit for the invention of the first modern thermometer.

The device that Galileo invented was however not technically a thermometer. It did not have a numerical temperature scale, so it is more properly called a thermoscope than a thermometer. Galileo made his first rudimentary thermoscope in the 1590s, most likely in 1593, but it was not based on the same principle as the modern Galileo thermometer that bears his name. It consisted of a long slender tube that sucked up water to a level that depended on the temperature of the water. He also discovered that the density of a liquid changes with its temperature – the principle that led to the Galileo thermometer. The first thermometer with a numerical temperature scale was made by Santorio Santorio who was a medical doctor and used it for medical purposes.


A Galileo thermometer consists of a sealed vertical glass tube containing a clear liquid, usually water. Inside this tube are several brightly colored calibrated glass bubbles that float at various levels in the tube. There will typically be a few floating at the top and a few sitting on the bottom. The bubble near top sinking a little, or the bubble near the bottom starting to float, or perhaps the average of those two, indicates the temperature. The bright colors of the floating bubbles are decorative and not related to the functioning of the thermometer. The glass bubbles usually contain a liquid that is typically some combination of water, alcohol, and coloring.

How it works

As the room temperature changes, the temperature of the water in the vertical tube also changes. This temperature change causes the water to expand or contract. As the volume of the water changes, its density (defined as the mass divided by the volume) changes. The ancient Greek scientist and philosopher, Archimedes, discovered that the buoyant force on an object immersed in a fluid equals the weight of the fluid displaced by that object – Archimedes’ Principle. As a result of this principle, an object will float in a fluid if it is less dense than the fluid and sink if it is more dense.

The floating bubbles in the Galileo thermometer have carefully calibrated densities. This calibration can be accomplished by adjusting the amount and composition of the liquid in the bubble and by adjusting the size and weight of the little tag indicating the temperature that is fastened to the bottom of the bubble. The bubbles less dense than the liquid in the glass tube float to the top and the more dense bubbles sink to the bottom. As the temperature changes, the density of the liquid in the vertical tube changes. Hence more bubbles will float or sink accordingly.

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