How does a Galileo thermometer work?

Based on a thermoscope invented by Galileo Galilei in the early 1600s, the thermometer on your co-worker's desk is called a Galileo thermometer. A simple, fairly accurate thermometer, today it is mostly used as decoration. The Galileo thermometer consists of a sealed glass tube that is filled with water and several floating bubbles. The bubbles are glass spheres filled with a colored liquid mixture. This liquid mixture may contain alcohol, or it might simply be water with food coloring.

Attached to each bubble is a little metal tag that indicates a temperature. A number and degree symbol are engraved in the tag. These metal tags are actually calibrated counterweights. The weight of each tag is slightly different from the others. Since the bubbles are all hand-blown glass, they aren't exactly the same size and shape. The bubbles are calibrated by adding a certain amount of fluid to them so that they have the exact same density. So, after the weighted tags are attached to the bubbles, each differs very slightly in density (the ratio of mass to volume) from the other bubbles, and the density of all of them is very close to the density of the surrounding water.


If you have read this question, then you know that an object immersed in a fluid experiences two major forces: the downward pull of gravity and the upward push of buoyancy. It is the downward force of gravity that makes this thermometer work.

The basic idea is that as the temperature of the air outside the thermometer changes, so does the temperature of the water surrounding the bubbles. As the temperature of the water changes, it either expands or contracts, thereby changing its density. So, at any given density, some of the bubbles will float and others will sink. The bubble that sinks the most indicates the approximate current temperature.

Consider this example:

Let's say there are five bubbles in the thermometer:

  • A blue bubble that represents 60 degrees
  • A yellow bubble that represents 65 degrees
  • A green bubble that represents 70 degrees
  • A purple bubble that represents 75 degrees
  • A red bubble that represents 80 degrees

The blue bubble (60 degrees) is the heaviest (densest) bubble, and each bubble thereafter is slightly lighter, with the red bubble being the lightest. Now, let's say the temperature in the room is 70 degrees. Since the surrounding air is 70 degrees, we know the water inside the thermometer is also about 70 degrees. The blue and yellow bubbles (60 and 65 degrees, respectively) are calibrated so that they have higher densities than the water at this temperature, so they sink. The purple and red bubbles each have a density that is lower than the surrounding water, so they float at the very top of the thermometer. Since the green bubble is calibrated to represent 70 degrees, the same temperature as the water, it sinks slightly so that it is floating just below the purple and red bubbles -- thereby indicating the room's temperature!

Originally Published: Jun 21, 2001


Galileo Thermometer FAQ

What is a Galileo thermometer and how does it work?
The Galileo thermometer, invented by astronomer Galileo Galilei, works mainly on the buoyancy principle. It is used to determine the capability of an object to either sink or float. It consists of glass spheres that move up and down as the temperature changes. A rising glass sphere indicates a fall in temperature, whereas a rise in temperature is indicated by a falling glass sphere.
What is the clear liquid in a Galileo thermometer?
The clear liquid in a Galileo thermometer is ethanol, a colorless volatile liquid with very little odor. Although its density is less than water, it varies depending on the temperature, even more so than water.
How do you read a Galileo glass thermometer?
You can read the ambient temperature by looking at the spheres that float in the liquid. Now, read the tag that is attached to the middle bulb to get the temperature. If you don’t see a sphere in the middle, look at the lowest one to read the temperature.
How accurate is a Galileo thermometer?
This thermometer is better for approximations. It closely measures ambient temperatures, but because it uses buoyant forces its calculations may not be as accurate as modern digital thermometers.
Can a Galileo thermometer freeze?
Galileo thermometers do not freeze. However, you might want to keep it away from absorbing direct sunlight. This can fade the color of the spheres inside the tube, affecting its temperature measuring accuracy.