Equipment: A Galileo's thermometer (a sealed liquid filled glass cylinder containing sealed glass floats labeled 18, 19, 20, 21, 22, 24 and 26 degrees C), a 4 liter beaker, ice water and paper towels.

What it does: The floats are adjusted in density so they sink at different temperatures. Each float carries a tag indicating the temperature at which it sinks. Temperature is indicated by the float with the lowest marked temperature which is floating.

Concepts Demonstrated: The temperature dependence of fluid density as a thermometric property.

Classroom assembly:

1. Read the temperature of the room on the thermometer.
2. Place the thermometer in a 4 liter beaker (height = approximately 24cm) of cold water.
3. Read the temperature of the cold water.

• The glass floats respond quickly when placed in cold water. They respond slowly when removed from the water.
• 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.'

Cautions: The thermometer is fragile. Handle with care.

Setup Time: Short.

Difficulty: Straightforward.

Visibility: The temperature tags are small. A video camera is required for this demonstration.

Related demonstrations:

References: 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)

Original Construction: Purchased. There seem to be several manufacturers of these things. A google search for "Galileo's thermometer" should turn up a retailer.