While they are commonly used for decorative purposes today, Galileo thermometers are actually carefully calibrated devices that can measure air temperature very accurately. These thermometers look very complicated but actually operate based on some relatively simple scientific principles. The materials used to make a Galileo thermometer vary slightly but are fairly simple and ordinary.
Galileo thermometers consist of a vertical glass tube filled with clear liquid. Inside the tube are glass bulbs also filled with liquid, with a tag hanging from the bottom. As the temperature changes, the bulbs float up and down. The lowest floating bulb has the tag that is the correct temperature. This type of thermometer is accurate but somewhat cumbersome. It is appealing because the bulbs inside the tube contain colourful liquids, which makes it aesthetically appealing as well as useful.
How Galileo Thermometers Work
Galileo thermometers operate on some very basic scientific principles. Objects in a liquid will float or sink according to their density; to put things simply, denser objects sink. Liquids are unique in that their density decreases slightly as the temperature increases. The densities of each bulb in the tube are different. This principle is what makes the thermometer work.
Materials for a Galileo Thermometer
The materials needed for a Galileo thermometer are fairly simple. Glass bulbs are attractive and traditional, but any small waterproof container could potentially be used. A sealed glass cylinder is required for the bulbs to float in, as well as water and tags for the temperatures. The bulbs can be coloured with food colouring, which makes them easier to see as well as more attractive. Some Galileo thermometers contain other liquids, but these are optional.
Although the materials needed to create the Galileo thermometer are somewhat simple, calibrating it can be quite challenging. The density of the bulbs themselves is actually the same. The tags hanging from the bulbs are counterweights, and all weigh different amounts. This makes the overall density of each bulb different, since weight is related to density. This means that the bulb tagged with the correct temperature will be the same density as the water that it is in, which causes it to float in the middle of the thermometer. The other bulbs will either sink or float to the top.
Calibrating the Thermometer
You can use a water density chart to calculate how much weight is needed at different temperatures to make the bulbs match the density that water would be at the temperatures each of them represents. This can be very difficult since it requires a lot of math and precise scales. A less precise but much easier method is to use a thermometer and to heat or cool the water column to the desired temperature, put a bulb in and add weight to it until it floats in the middle of the column, thus calibrating the bulb.