Galileo's Thermometer

How Does a Galileo Thermometer Tell the Temperature?

Mar 10, 2007

Description

A Galileo thermometer consists of a sealed vertical glass tube containing a clear liquid, usually water. Inside this tube are several brightly colored calibrated glass bubbles that float at various levels in the tube. There will typically be a few floating at the top and a few sitting on the bottom. The bubble near top sinking a little, or the bubble near the bottom starting to float, or perhaps the average of those two, indicates the temperature. The bright colors of the floating bubbles are decorative and not related to the functioning of the thermometer. The glass bubbles usually contain a liquid that is typically some combination of water, alcohol, and coloring.

How it works

As the room temperature changes, the temperature of the water in the vertical tube also changes. This temperature change causes the water to expand or contract. As the volume of the water changes, its density (defined as the mass divided by the volume) changes. The ancient Greek scientist and philosopher, Archimedes, discovered that the buoyant force on an object immersed in a fluid equals the weight of the fluid displaced by that object - Archimedes' Principle. As a result of this principle, an object will float in a fluid if it is less dense than the fluid and sink if it is more dense.

The floating bubbles in the Galileo thermometer have carefully calibrated densities. This calibration can be accomplished by adjusting the amount and composition of the liquid in the bubble and by adjusting the size and weight of the little tag indicating the temperature that is fastened to the bottom of the bubble. The bubbles less dense than the liquid in the glass tube float to the top and the more dense bubbles sink to the bottom. As the temperature changes, the density of the liquid in the vertical tube changes. Hence more bubbles will float or sink accordingly.

History

There is some controversy about the historical details, but the basic story is something like this. Ancient Greeks may have used simple temperature sensing devices, but Galileo is usually given credit for the invention of the first modern thermometer. The device that Galileo invented was however not technically a thermometer. It did not have a numerical temperature scale, so it is more properly called a thermoscope than a thermometer. Galileo made his first rudimentary thermoscope in the 1590s, most likely in 1593, but it was not based on the same principle as the modern Galileo thermometer that bears his name. It consisted of a long slender tube that sucked up water to a level that depended on the temperature of the water. He also discovered that the density of a liquid changes with its temperature - the principle that led to the Galileo thermometer. The first thermometer with a numerical temperature scale was made by Santorio Santorio who was a medical doctor and used it for medical purposes.

The copyright of the article Galileo's Thermometer in Physics History is owned by Paul A. Heckert. Permission to republish Galileo's Thermometer in print or online must be granted by the author in writing.

Jun 12, 2008 9:17 AM

Guest :

I appreciated the detailed description of the Galileo thermometer and the easy access to sources. We broke ours and really wanted to replace it.
Martin Gingold

Nov 23, 2008 3:01 PM

Guest :

I have a galileo thermometer that boke recently. The bottom of the tube broke but none of the floating bubbles broke. We made a stand for the tube, (now inverted) put water in it and placed the floating bubbles in order back in the tube. We have been watching it for several weeks and the bubbles are at the top and will not drop. I have a smaller model and the bubbles rise and drop. Why won't my broken one do that? Is there something that needs to be in the water?

Nov 23, 2008 4:59 PM

Paul A. Heckert :

It is possible that the liquid in your Galileo thermometer was not water. The density of the floating bubbles and the liquid in the tube must be very accurately calibrated relative to each other so that the bubbles will rise and fall as the temperature changes. Without knowing the density of the liquid in your thermometer originally, it will be difficult to get it to work.

Nov 25, 2008 6:25 PM

Guest :

what does it look like

Dec 1, 2008 1:49 PM

Guest :

i think this product is very special

Dec 10, 2008 12:15 PM

Guest :

Jan 2, 2009 10:26 PM

Guest :

thanx this info helped me a lot on my history project:D
where can i buy one?
byeeeeee

Jan 8, 2009 10:38 AM

Guest :

I just recently purchased a Galileo thermometer as a Christmas gift..When it was received (via UPS) and was opened at Christmas, all of the balls except for one sank to the bottom. They will not float up even though the temperature around has varied significantly. Will it ever work????

Jan 8, 2009 4:41 PM

Paul A. Heckert :

If it didn't work as soon as you opened it, it sounds like a manufacturing defect. Perhaps it has the wrong density liquid. Does it have a warranty?

Jan 13, 2009 7:43 AM

Guest :

I recently broke one by accident. I'd had it several years. The clear liquid is not water. I don't know what it is, but it had a distinctly motor oil smell and a slightly slippery feeling. It was hard to clean up--especially off the network cabling--and required detergent. None of the hand-blown bubbles broke, so I was dealing with just the clear liquid.

Jan 19, 2009 2:20 PM

Guest :

I still have no idea how to read this thing

Jan 20, 2009 4:42 AM

Guest :

thnx it realy did help in my science project

Mar 5, 2009 7:54 PM

Guest :

This helped a lot when I had to write a paper on thermometers.
Thx!!!!

Mar 8, 2009 6:08 AM

Guest :

Thankyou for your explanation, it would be nice to know how to make one. How to calibrate the bubbles and what about the weight of the tags?

Mar 31, 2009 5:05 PM

Guest :

thanks for the information that helped me with my SS Project :)

Apr 15, 2009 9:43 AM

Guest :

why was it invented?

Apr 20, 2009 4:57 PM

Guest :

Guest: Mine is NOT broken, but all this past week all the "bubbles" have been clustered at the bottom. The ambient temperature here in Florida has been in the low to mid 80's every day. Otherwise, it has been fairly active, with a variety of "positions"--we have never learned how to read it properly. Is humidity a factor??