This website is reader-supported. When you buy through links on our site, we may earn a small affiliate commission.

What is a Galileo thermometer?

Last Updated: 29.11.21


Science most definitely has a beautiful side, too, and the Galileo thermometer is one of the instruments that translate the exactitude of scientific observations and principles to useful objects that adorn any interior space.

Before taking a look at what a Galileo thermometer is, let’s take a moment and talk about the well-known scientist whose name it is carrying.

Who was Galileo Galilei

Born in Italy in 1564, Galileo Galilei was an astronomer, engineer, physicist, philosopher, and mathematician whose work greatly influenced many of the principles known at the time and altered the course of scientific research and knowledge towards modernity.

Although some of his findings were controversial during his lifetime, they were, of course, correct, thus setting a new path for science. His most notable work that was regarded as highly controversial was on heliocentrism and Copernicanism, which placed the sun at the center of the Solar System.

During his lifetime he was tried by the Inquisition, found suspect of heresy, and forced to recant. Galileo Galilei spent the rest of his days under house arrest. However, during this period he wrote some of his best-known works on kinematics and strength of materials.

He was also the one who discovered that the density of liquid changes as a result of increasing and decreasing temperatures, which, evidently, leads us to our topic, the Galileo thermometer.


The true inventors

Although you would expect that this thermometer was invented by the famous scientist, funnily enough, that is not actually the case. Galileo Galilei did invent an air thermometer, better known as a thermoscope, around 1603, but it was not this one.

What we call today the Galileo thermometer was, in fact, invented by a group of academicians and technicians known as the Accademia del Cimento of Florence. It’s true that this group included Galileo’s pupil, Torricelli and Torricelli’s pupil Viviani, but that is pretty much the only connection between the instrument and Galilei himself.

The writings of the time described the device as slow and lazy, and this is accurate, since it uses the changing density of liquids and the principle of buoyancy to measure the outside temperature. Buoyancy determines whether an object floats or sinks in a liquid, this way explaining how ships made of steel float in the water.

It’s actually pretty hard to find another more beautiful way to measure the temperature than this device, although it’s true that the readings are fairly accurate and not as good as those provided by a regular thermometer.

How it’s made

Manufactured since the end of the 17th century, the Galilean thermometer is made up of a sealed glass cylinder, and inside there’s a transparent liquid and a series of bulbs that also carry an attached weight.

Each one of the weights attached to a bulb is also tagged with an engraved number and a degree symbol. When it comes to the weights, they are actually calibrated counterweights and each one is different from the others.

Another particularity that makes this measuring instrument very appealing is the different coloring of each bulb. This is added so that all the liquids involved have the same density, but it’s also the element that adds an overall attraction of what is most definitely a very special thermometer.


How it works

You might remember from your science class that an object immersed in a fluid experiences two different forces: gravity which is pulling it downward and buoyancy which is pushing it upward. In the case of this thermometer, the force of gravity is the one that makes it work.

Besides being a beautiful object by itself, the Galileo thermometer uses the bulbs to read temperatures, as they rise and fall depending on these principles. The basic idea is that, as the temperature of the air changes, it transfers to the temperature of the liquid surrounding the bulbs which, as a result, changes its density.

Each bulb has the same volume and the same density, therefore the same magnitude of gravitational and buoyant forces act upon it at any given moment. However, each of these bulbs is also defined by the mass suspended from its base which increases the relative weight of the bulb.

This means that the effect of gravity is also altered compared to the other ones. As the density of the surrounding fluid changes, the force of gravity manages to overcome the buoyant force, thus making the bulbs sink or float. The principle is that as density decreases, the buoyant force does the same.

The temperature of each bulb remains constant so that as the temperature of the surrounding fluid increases, its density decreases. Since the buoyant force follows in the same way, the gravity continues to pull downwards and makes the bulb sink.

How to read a Galileo thermometer

At any given point, some of the bubbles float while others sink, and the one that sinks the most actually indicates the approximate temperature of the surroundings.

A small Galileo thermometer can have 6 degrees Fahrenheit of difference between the bulbs, which means that there’s enough room for errors. Most models cover temperatures between 68 and 84 degrees Fahrenheit which means they are suitable only for indoor spaces that are heated or air conditioned.

In order to read a Galileo thermometer all you have to do is to look at the lowest bulb that is floating and ignore the ones that are touching the bottom of the container. The bulbs that are neutrally buoyant that show the ambient temperature.

If there’s no bulb floating in the gap formed by the sunken and the rising bulbs, simply use the lowest bulb from the floating cluster to read the temperature. This thermometer is certainly not the most precise alternative, but it’s a very clever way to showcase some basic physics principles and have a beautiful object around the house.

If you would like to read about more precise measuring alternatives, you can check out our other articles on topics such as finding an effective infrared thermometer or finding out how an instant thermometer can help you out in your everyday life.



Ioana Moldovan

Ioana’s professional experience in the optics field has helped her understand the value of passing her knowledge forward. Her curious personality helps her gather useful information for her readers and her goal is to make technical information fun and accessible to everyone.

Notify of
Inline Feedbacks
View all comments Protection Status