Galileo's thermometer


The device is constituted by glass cylinder containing a liquid with a density that considerably increases on decreasing of the temperature. Within the cylinder there are some glass cruets with a colored liquid inside. These cruets have different average densities and bring name-plates where one can read the temperature.


When the device has reached the thermodynamic equilibrium with the external surroundings, you can read the temperature by observing the number on the lowest cruet floating. If the external surroundings is at a very low temperature, the liquid in the cylinder becomes denser of every cruet, thus all of then float. On the contrary at high temperature they sink.

Termometro di Galileo
A particular of the thermometer
At middle temperatures only the cruets denser than the liquid fall down to the bottom: the lowest one floating is just less dense than the liquid thus it approximately marks the temperature.

We can wonder why the cruets don't change their density, in fact temperature changes also for them. The answer is very simple: the glass their shell is made of expands and shrinks so little than you can disregard it for these variations of temperature (the thermometer works at temperatures between 10 and 30 Celsius degrees). The volume, and also the density, of the cruets can be considered constant.