It was Galileo Galilei who came up with the concept of buoyancy. His concepts are used in this thermometer therefore it is named after him. A Galileo thermometer is a thermometer made of a sealed glass cylinder containing a clear liquid and several glass vessels of varying density. The glass floats rise or fall in proportion to their respective density and the density of the surrounding liquid as the temperature changes. In other words, the individual rises or falls in proportion to the change in the density of the surrounding liquid.

Each vessel is filled with colored liquids that have a specific density. By comparing the positions of the individual vessels, the temperature can be estimated. The change in the density of the clear outer liquid i.e ethanol or kerosene(such liquids which are more prone to change in temperatures)  results in a change in the apparent height of the bulbs above/below the outer liquid. The thermometer reads out this apparent height by observing the position of the metal tag attached to the bulb under the clear outer liquid. The lower the apparent height, the higher the apparent temperature. When the thermometer is immersed in water, the thermometer reading corresponds to the temperature of the water. 

Woking Principle of Galileo Thermometer:

Its working principle is based on the concept of buoyancy. With the increasing temperature of the atmosphere the temperature of the liquid inside the thermometer increases.Increasing temperature results in the expansion of the liquid, which results in decreasing density. Hence the heavier vessel( with more density) will sink with increasing temperature and the lightweight vessel floats. If the temperature is decreased the heavy glass will float.

Principle of buoyancy: The principle of buoyancy states that a floating object will rise in water until it reaches a point where the weight of the object and the water are equal. The force of gravity acts downward on all objects. If a floating object has more weight than the amount of water it displaces, it will float upwards. The buoyant force is the upward force exerted by water. It is proportional to the density of the object and inversely proportional to the volume of the object.

F buoyant​=F up​−F down​

Are Galileo thermometers accurate?

These thermometers are mostly used for decoration purposes. These give approximate values and are based on the principle of buoyancy. There are not accurate up to 100% in results. There might be slight variations in its results. The accuracy range also depends upon the position and other external factors. If the thermometer is placed in direct sunshine it will show a higher temperature than normal room temperature.

How do you read the temperature on a Galileo thermometer?

Galileo thermometer has a reading hanging to the glasses that shows the temperature. The glass which lies in the middle of the liquid i.e partly submerged will indicate the current temperature. There are also rare cases in which no glass lies in the middle, then the average readings of vessels on the top and bottom are taken. This can happen due to huge differences in temperatures in between them.

Is Galileo thermometer Still Useful:

The answer to this question is conditional. As there are the latest digital thermometers that give 100% accurate results so Galileo thermometer is not comparable to modern thermometers in function. However, Galileo thermometer has historical importance. It can be used in schools and organizations.It can help a lot to kids to understand the concept of buoyancy because it is the practical implication of that concept. This thermometer is easily available in the online market and scientific stores. It is easy to use and understandable.

Galileo chain thermometer is also a type of thermometer which is based on the same principle as described above. This thermometer has a single sphere that floats over a chain. Readings are marked on the outer glass. the bottom is marked as the maximum and the top is marked as the minimum temperature. Galileo chain thermometer is different from an ordinary Galileo thermometer as the former has a single sphere and has a chain to support it.

As described above this is a very simple device that can be built easily with dew items that are easily available. To construct this thermometer at home a glass tube, a few small spheres with colored water, metal tags, and ethanol. There is a need to properly calibrate the spheres to mark the tags with accurate readings. It is fun and easy to do so.