The Galileo Thermometer
A Galileo thermometer is a thermometer made of a sealed glass cylinder containing a clear liquid and several glass vessels of varying densities. As temperature changes, the individual floats rise or fall proportion to their respective density.
It is named after Galileo Galilei because he discovered the principle on which this thermometer is based—that the density of a liquid changes in proportion to its temperature—and invented a thermoscope based on this principle.
Floating in the liquid inside the cylinder are a number of sealed glass bulbs containing coloured liquid. Attached to each bulb is a metal disc whose weight is adjusted to give the bulb the correct buoyancy. As the liquid in the cylinder changes temperature, its density changes and the bulbs are free to move – rising or falling to reach a position where their density is either equal to that of the surrounding liquid or where they are brought to a halt by other bulbs. The bulbs differ in buoyancy by a very small amount and are ordered such that the least dense is at the top and densest at the bottom so that they form a temperature scale.
The temperature is typically read from the metal disc hanging from each bulb. Usually a gap separates the top bulbs from the bottom bulbs and then the temperature is between the tag readings on either side of the gap. If a bulb is free-floating in the gap, then its tag reading is closest to the ambient temperature.
To achieve satisfactory accuracy, the weights must be manufactured to a tolerance of less than 1/1000 of one gram (1 mg)
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