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Density - Galilean Thermometers (Mark Pichaj)


"Kinetic-Molecular Theory Meets Archimedes' Principle"


“Using Density to Find Temperature with Galilean Thermometers”


Ubiquitous; adapted by Mark Pichaj

Principles Illustrated

My goodness—where do we start? 


•  The transfer of heat energy and its effect on temperature and molecular motion.

•  Molecular motion and expansion and contraction.

•  The effect of volume change on density.

•  Why density effects buoyancy, as explained by Archimedes and the Principle of Flotation.

•  To summarize, the relationships among temperature and density, and density and buoyancy.

Kinetic-Molecular Theory meets Archimedes Principle and the Principle of Flotation in a steel cage match!


Grade Eight, Focus on Physical Science:

    8.a.—d. (Density & Buoyancy) 

Grades Nine-Twelve:


        1.b. & 2.f.  (Unbalanced forces cause a change in motion)


        7.a.  (Kinetic-molecular theory)


(1)  INTRODUCE THERMOMETERS:  Have students observe two so-called "Galilean Thermometers" and ask them to read the temperature.  (With a separate thermometer of known accuracy, measure the classroom temperature as  a reference, and let students know what it is.)

(2)  PREPARE WATER BATHS:  Half-fill a large beakers (1-2 L) with tap water, and heat it to over 50°C.  Half-fill another large beaker with ice and water, so that the temperature of the water is under 10°C.  (Perform this step ahead of time, if possible.)

(3)  PREDICT RESULTS:  Ask students to predict the relationship of the bobs rising or falling with a change in the temperature.  Have them write down their prediction on their Demonstration Log Sheet, and then explain to the student next to them why they think that is going to happen.

(4)  PERFORM EXPERIMENT:  Stand one Galilean Thermometer in the hot water and the other in the cold.  (It may take some time for the bobs to begin to respond because of the slow heat transfer from the hot and cold water baths to the glass, and from the glass to the buoyant fluid.)

(5)  WAIT FOR RESULTS:  While waiting for the bobs to rise or fall, regale your students with stories of Galileo's studies of expanding and contracting fluids (he used a thin, sometimes coiled, water-filled tube with air bubbles in it to demonstrate expansion and contraction) or a discussion of kinetic-molecular theory, or of Archimedes and how density effects buoyancy.  (If there is no response after 10-15 minutes, remove the Galilean Thermometers and carefully invert them once to mix the buoyant fluid before replacing them in their respective water baths.)

(6)  OBSERVE & DISCUSS RESULTS:  When the bobs finally do respond, take the temperature of each water bath, and discuss your students' predictions.  Unmercifully mock those who failed correctly to predict the response of the bobs to a change in temperature, and make those who did correctly predict the bobs' response explain why they behaved in this manner.  Your goal is an explanation that references the kinetic-molecular theory of matter and includes a causal connection among heat energy transferred and temperature change, molecular motion and volume change, density and buoyancy.  Good luck.   

Questioning Script

Prior knowledge & experience:

  Thermometers measure temperature, usually by an increase in temperature corresponding to an expanding liquid being forced by its increase in volume to rise in a confined tube.

Root question:

  What is the relationship between temperature and volume change, volume change and density change, density and buoyancy?  Or, more simply, students will predict whether the balls inside a Galilean thermometer will move up or down with a known change in temperature.

Target response:

  After viewing the repsonse of the Galilean Thermometer in cold and warm water, the students will understand the relationships between temperature and volume change (as explained by the Kinetic-Molecular Theory, the inversely-proportional relationship between expansion and density, and the relationship of density of a fluid to its buoyant force.

Common Misconceptions:

  As temperature “goes up” (becomes greater), the movement of a thermometer must “go up” (an increase in vertical displacement), as well.

Photographs and Movies

A so-called "Galilean Thermometer"

(The temperature is found by reading the temperature stamped on the tag

attached to the neutrally-buoyant bob floating in the middle—in this case, the orange one.

Galileo never actually made a thermometer like this one, but it still operates on the same principle:

a change in temperature causes a change in volume which causes a change in density,

which we can observe when the change in density causes a change in buoyancy)

 A close-up view of two floating bobs...

(Since the bobs are made of blown glass, which is a solid,

they expand and contract only minimally with a change in temperature,

and so remain at a nominally constant volume no matter how hot or cold the buoyant fluid gets.

Since their mass obviously doesn't change either, they also remain at a constant density.

Thus, any change in their buoyancy is solely a function of the change in density of the buoyant fluid,

which varies as the volume expands or contracts with a change in temperature.

        (Wikimedia Commons/Public Domain)

How to Read a "Galilean Thermometer"

(A neutrally-buoyant bob is the same density as the buoyant fluid, and gives the temperature directly;

if all bobs have negative and positive buoyancy, the temperature is found by averaging the floater & sinker in the middle.

Frankly, the accuracy of these temperature readings are poor, and vary widely from thermometer to thermometer.)

(Wikimedia Commons / from Greg Hewgill)

The Principle of Operation of a "Galilean Thermometer"

(The buoyant fluid expands and contracts with a change in temperature and so changes in density, too.

but the glass bobs don't expand or contract very much, so their density remains constant.

A denser fluid produces more buoyant force as predicted by Archimedes' Principle!)

cooler/contracted/denser           warmer/expanded/less dense

= more buoyant force                      =  less buoyant force

=  bob FLOATS                             =  bob SINKS

(Wikimedia Commons / from Greg Hewgill)

(1)  A YouTube video of a warming Galilean Thermometer by time-lapse photography:

"Galileo Thermometer Time-Lapse"

(2)  Galileo's contribution to thermometry on HowStuffWorks: “How does a Galileo thermometer work?”:

(3)  An eight-foot tall single-bob Galilean Thermometer:

(4)  Time lapse photography of the behavior of a Galilean Thermometer over changes in temperature:

(5)  Commercial advertisement showing a good view of a conventional Galilean Thermometer:

(6)  Commercial advertisement showing a good view of a new single-bob type of Galilean Thermometer:

(7)  Another image of a new, more modern-looking single-bob Galilean Thermometer from Think Geek:


(1)  Wlikipedia: Galileo Thermometer

(2)  Galilean Thermometer, by Greg Hewgill:

(3)  HowStuffWorks: “How does a Galileo thermometer work?”: