Don’t let its looks deceive you. It is a fairly accurate thermometer.
While Galileo thermometers are mostly used as desk decorations nowadays, if they are manufactured correctly, they can be fairly accurate thermometers. The Galileo thermometer is made up of a sealed glass tube filled with liquid and some glass spheres. Each sphere has a little tag denoting a temperature measurement attached to it. These spheres float or sink, depending on the temperature of the room.
To read the temperature off a Galileo thermometer, simply read the temperature of the lowest floating glass sphere. In the case of the picture above, it would be the red one. If you can find a sphere that floats in the middle of the tube, that’s even better.
The way a Galileo thermometer works is by taking advantage of the fact that a liquid’s density changes with respect to the temperature it is. Of course, the density changes are very minute, but with a little care, you can actually take advantage of these changes, allowing to measure the temperature of the liquid.
With that in mind, a Galileo thermometer is fairly easy to understand. As you might guess, each of the spheres in the thermometer are weighted slightly differently (by filling them with different amounts of liquid). This allows you to “tune” the density of each of the spheres individually so that they match the density of the liquid at that temperature.
The liquid that is used in the Galileo thermometer is somewhat difficult to determine. I did a little bit of digging around and couldn’t find a conclusive answer. However, it is very unlikely that the liquid is water. It is more likely to be a hydrocarbon, or a mix of water and a hydrocarbon, such as kerosene, paraffin or naphtha. Why isn’t water used? Well, it can be used, but it takes a lot more effort to make a good thermometer, because water’s density changes very little over the temperature range you’d want to measure (usually between 20°C and 30°C). The spheres are probably filled with a water/alcohol mixture, while the tube is filled with water.
The Galileo thermometer is a simple invention that shows you how you can measure something in a very creative way. There are many around in stores and are somewhat fun to watch.