Galileo Thermometers

Galileo at age 38. Image Courtesy NASA’s Johnson Space Center

Thus the mass of any material contained within a unitary volume of space tends to decrease as the temperature increases. Therefore density is inversely proportional to temperature; as temperature increases, the density of materials decreases. Each different material exhibits its 'energy behaviour' in its own unique way. This can be used to correlate the density of a material with its temperature. A Galileo thermometer is constructed using small glass spheres to make a series of floating environments within a larger tube that is usually filled with water. Each sphere contains a specific amount of water and air or another liquid and air, and is tagged with a precisely calibratedcounterweight.

The counterweightis marked with a specific temperature. Each sphere thus has a specific density at a specific temperature. The spheres float within the primary liquid at a level determined by the difference in their densities. Because each of the spheres changes density with temperature at a different rate, the difference between the densities of the two materials decreases in a predictable manner. The level at which any particular sphere floats within the primary liquid changes accordingly. In this way, the temperature is clearly indicated by the sphere floating at the lowest level within the primary liquid.