Today, meteorologists use a wide range of instruments to measure ambient air temperature. Centuries ago, much simpler and, at times, “interesting” devices such as the Galileo thermometer were used.

Although Galileo Galilei did not invent the instrument now known as the Galileo thermometer, it bears his name because its operation is based on the relationship between a liquid’s density and temperature. The device itself was developed by members of the Accademia del Cimento in Florence during the seventeenth century.

Although it bears Galileo’s name, the instrument is not based on the thermoscope associated with him. Galileo’s thermoscope relied on the expansion and contraction of air, whereas the Galileo thermometer operates through changes in a liquid’s density.

In this article, we take a closer look at this historical meteorological device and how it functioned to indicate atmospheric conditions.

Before delving into its inner workings, though, we first need to define what precisely a Galileo Thermometer is:

Galileo thermometer Definition​​​​​​​​​

A Galileo thermometer is a meteorological instrument consisting of a sealed glass tube filled with a clear liquid containing small glass bulbs of varying densities. Ambient temperature changes also alter the liquid’s density, causing different bulbs to rise or fall, which indicates the temperature.

Each bulb is partially filled with a differently coloured liquid. Small metal tags of different weights hang below the bulbs to adjust their overall density, while the number on each tag indicates a specific temperature.

Any changes in air temperature change the density of the liquid as well. This causes the bubbles inside the liquid to rise and fall in response to changes in the fluid’s density.

By observing the different heights at which the glass bulbs are floating, the temperature can be determined. This is usually done by identifying the number on the tag below the lowest bulb floating near the top of the tube. If one bulb is floating on its own between the upper and lower groups, the number on its tag indicates the temperature.

If this sounds confusing to you, you are not alone. If I only described to you what a Galileo thermometer looks like and how it responds to temperature changes, it would be difficult to understand what is really happening and why.

One needs to understand the principles and forces at work that make all the parts in this thermometer behave the way they do and how they all work together to help determine the atmospheric temperature.

The first priority, therefore, is to make sure every principle is fully explained and understood. And that will be the focus of the next section.

(If you are familiar with these principles or want to skip all the technical jargon, you can jump over the next section and move directly to the section explaining how a Galileo Thermometer works.)

In order to better understand how a Galileo Thermometer works, one first needs to clarify three principles to make sure you understand how they influence the individual components that form part of the thermometer:

1. 1. Buoyancy: Many explanations of the Galileo Thermometer start by emphasizing the fact that the instrument works on the principle of buoyancy.
   2. Density: The Galileo Thermometer is named after the scientist mainly because it operates according to the principle that the density of a liquid changes as its temperature changes.
   3. Gravity: Gravity plays a major role in the downward pull of all objects. This is especially important for the Galileo Thermometer to operate correctly.

So which one of these three principles is the Galileo Thermometer based on? Actually, all three principles play an equally important part in making it work.

The best way to understand the relevance and importance of all three principles is to see how each one works and its role in making the Galileo Thermometer work.

Buoyancy is the upward force or thrust of a liquid on an object submerged in it. It is the very principle that all vessels designed to float on top of the water are based upon.

It is very closely related to the principle of density. Density is probably the most significant factor that determines an object’s buoyancy.

A simple example will illustrate how this works in practice. Take a tennis ball or football, and try to push it underwater in a bucket or bathtub. What happens?

It immediately starts resisting your action and shoots to the surface as soon as you release it. This is because the overall density of the ball is so much less than that of the water.

Now try to do the same with a heavy piece of metal like lead. You will immediately notice that you do not experience any “pushback” from the water while you submerge it.

If you release it, the piece of lead drops to the bottom. The metal’s density is so much greater than that of the water that the “upward force” of the liquid is not strong enough to prevent its downward movement.

As you just observed, the density of an object compared to that of the liquid it is submerged in determines its buoyancy (ability to float).

Although most of us understand what density is, it is a bit harder to define in words. ScienceDaily puts it best by describing density as mass per unit of volume.

This means density is closely related to an object’s mass. But mass has to be put in context by comparing it to the object’s volume (physical size).

Technically, density can be calculated by dividing mass by its volume.

This is best understood by seeing it in practice. Let’s use the example of a piece of iron of a certain size. Now compare that to a piece of foam of exactly the same size.

Clearly, the piece of iron is much heavier than the foam. This is because it has a much higher density than the similarly sized foam.

We can easily prove this by using the equation to calculate density: Density = mass/volume.

Let’s say the piece of iron weighs 157 grams (0.35 pounds) and the foam weighs 2 grams (0.004 pounds). We already know they have the same volume (size), for example, 20 cubic centimetres (1.22 cubic inches).

Do the calculation, and you will see that the iron has a much higher density of 7.85 g/cm³ compared to the foam’s density of 0.1 g/cm³.

Simply put, if two solid objects of the same volume have different weights, the heavier object has a higher density.

Gravity can be defined as the Earth’s force that pulls all objects towards its centre. Although all objects experience the same gravitational acceleration, the strength of the gravitational force acting on an object depends on its mass and is described as its weight.

Gravity impacts every single object on Earth. Every one of us is aware of and experiences gravity every single day (sometimes in a very unpleasant way, especially when you lose your balance and fall from a considerable height).

It should come as no surprise then that gravity plays a vital role in the functioning of a Galileo thermometer. It is the balance between the downward force of gravity and the upward buoyant force of the liquid that determines whether the glass bulbs rise or sink inside the tube.

Now that all the principles and forces at work have been explained and are out of the way, it is time to see how they all work together to make a Galileo Thermometer work.

From the description earlier in this article, we already know what a Galileo Thermometer looks like and what it consists of.

What one may not realize is that each of the coloured glass bulbs in the liquid has approximately the same density. It is the numbered metal tags hanging below each bulb that adjust the overall density of each one.

The individual tags each have a different weight that corresponds to the number on them, which shows the specific temperature each glass bubble represents.

From the previous section’s explanations, we will also know that it is the density of objects in a liquid that determines their ability to float (their buoyancy). And it is this very principle that makes a Galileo thermometer work.

Each glass bulb in the thermometer has a slightly different overall density. At a specific atmospheric temperature, one bulb may have approximately the same density as the surrounding liquid. The other bulbs will be either slightly less or slightly more dense than the liquid, causing them to rise or sink.

It is important to remember that air temperature directly influences the density of the liquid. If the temperature increases, it warms up the liquid, making it less dense. If it decreases, the air cools down the liquid, making it more dense.

The glass bulbs inside the liquid respond to this change in the liquid’s density. Some bulbs that are now less dense than the liquid will start to rise to the top. At the same time, the bulbs with a density higher than that of the liquid will begin to sink to the bottom of the tube.

The glass bulb, which now has approximately the same density as the surrounding liquid, will be floating around halfway between the ones at the bottom and the ones at the top. This glass bulb in the middle shows the actual air temperature at the time, as indicated by the number on its tag.

If no bulb is floating in the middle, the temperature can be determined by identifying the number on the tag below the lowest bulb floating near the top of the tube.

And that is how the Galileo thermometer works. It is a simple-looking process, but as you have learned, with a lot of different forces at work behind the scenes.

As the air temperature keeps changing, the glass bulbs continue to rise or sink in response to changes in the liquid’s density.

Although not used for meteorological purposes anymore, it is easy to see why Galileo Thermometers are still so popular. They are not just beautiful to use as ornaments, but the constant movement of the glass globes inside the liquid makes them fascinating to watch.

It also helps that, despite its fairly rudimentary and dated mechanism, it can still provide a reasonably accurate indication of the ambient temperature.

Until next time, keep your eye on the weather!