4A10.20 Galileo's Thermometer


The temperature dependence of fluid density as a thermometric property


Galileo's thermometer is a sealed liquid-filled glass cylinder containing sealed glass floats (in this case labeled 18, 19, 20, 21, 22, 24 and 26 degrees C). The floats are adjusted in density so they sink at different temperatures. The current temperature is indicated by the the lowest marked temperature that is floating. Note: the temperature tags are small. A video camera is required for this demonstration.



  • [1] Galileo's thermometer
  • [1] 4 liter beaker (approximately 24 cm high)
  • [1] Roll of paper towels
  • [1] Bucket of ice
  • [1] Ice scoop

Important Notes

  • The thermometer is fragile, handle with care
  • The glass floats respond quickly when placed in cold water, but they respond slowly when removed from the water


  1. Read the temperature of the room on the thermometer.
  2. Pour the water (but not the ice) into the beaker.
  3. Place the thermometer in a beaker of water.
  4. Read the temperature of the cold water.


Additional Resources


  • PIRA 4A10.20
  • George Nickas, "A thermometer based on Archimedes' principle", AJP 57, 845-6, (1989)
  • Ronald Geballe, "Note on 'A thermometer based on Archimedes' principle' by George Nickas [Am J Phys 57, 845-846 (1989)]", AJP 59(1), 90, (1991)


  • Don't attempt this at home!

Last revised

  • 2018


  • The history of this device is discussed by Ronald Geballe in an article titled "Note on 'A thermometer based on Archimedes' principle' by George Nickas [Am J Phys 57, 845-846 (1989)]". Geballe quotes Taylor "In the whole of Galileo's voluminous writings the only allusion to the thermometer is an undated fragment in which he mentions the instrument as proving that the north wind is sometimes colder than ice or snow" and Geballe writes 'As to the sealed spirit in glass thermometer, both the familiar kind and the "lazy" one, W.E.K. Middleton writes that these were invented in Florence by "none less than the Grand Duke of Tuscany, Ferdinand II, one of the great family of the Medici" and later adduces evidence dating it to in or before 1641.'

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.