How does a Galileo thermometer function?

The thermometer on your colleague's desk is known as a Galileo thermometer, based on a thermoscope invented by Galileo Galilei in the early 1600s. It is a simple yet fairly accurate device, now primarily used as a decorative item. The Galileo thermometer consists of a sealed glass tube filled with water and several floating glass spheres. These spheres are filled with a colored liquid mixture, which might contain alcohol or simply be water mixed with food coloring.

Each bubble has a small metal tag attached, displaying a number and degree symbol that indicates the temperature. These metal tags serve as calibrated counterweights. The weight of each tag differs slightly, and since the bubbles are all hand-blown glass, their size and shape are not identical. The bubbles are calibrated by adding a precise amount of fluid so that they have the same density. Once the metal tags are attached, each bubble has a slightly different density (the ratio of mass to volume), but all bubbles have a density very close to that of the surrounding water.

If you've come across this question, you're aware that any object submerged in a fluid experiences two key forces: gravity pulling it downward and buoyancy pushing it upward. It's the force of gravity that allows this thermometer to function.

The fundamental concept here is that as the surrounding air temperature shifts, so does the temperature of the water around the bubbles. This temperature change causes the water to either expand or contract, altering its density. At any given density, some bubbles will float, while others will sink. The bubble that sinks the furthest points to the current temperature.

Let's look at an example:

Imagine there are five bubbles in the thermometer:

The blue bubble, representing 60 degrees, is the densest, and each subsequent bubble is progressively lighter, with the red bubble being the lightest. Now, imagine the room temperature is 70 degrees. Since the surrounding air is at 70 degrees, the water inside the thermometer is also approximately 70 degrees. The blue and yellow bubbles (60 and 65 degrees, respectively) are calibrated to have densities greater than the water at this temperature, causing them to sink. Meanwhile, the purple and red bubbles have lower densities than the surrounding water, which makes them float at the top of the thermometer. Since the green bubble is calibrated for 70 degrees, the same as the water, it sinks just a little, floating slightly below the purple and red bubbles, thus indicating the room's temperature!