Galileo thermometer

A Galileo Thermometer is a device made of a sealed glass cylinder, containing a clear liquid (water,  for example), and several floating glass bubbles, each filled with a different color liquid, and a small metal tag hanging from it.


Each bubble represents a certain temperature (which is indicated on the metal tag).

All the bubbles are produced, and filled with liquid, so to have the same density (mass-to-volume ratio).

The metal tags, being of very similar volume to each other, are slightly different by weight, so that the bubbles+tags, are different very slightly from each other in their "average density", yet, being very close to that of the surrounding water.

As the temperature changes, some of the bubbles are floating at the top part of the cylinder, while others are sinking at the lower part of it.
The lower bubble of the floating ones (the one closest to the middle section of the tube) indicates the room temperature
 Why ???
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When the surrounding air temperature changes,the water temperature inside the cylinder changes as well, causing it to expand or to contract, thereof changing its density.

At any given density, some of the bubbles will float,while others will sink.The bubble floats at the most lower part of the floating bubbles, has the most similar density to that of the water, at that certain temperature.

When the surrounding air temperature changes, the water temperature inside the cylinder changes as well, changing the specific weight of the liquid inside the bubbles.

As the specific weight decreases, the bubble sinks, while in case it increases, the bubble floats.

The glass bubbles are expanding and contracting, as a result of the changes in the surrounding water temperature.

The change in their volume results in floating or sinking of them, in accordance to the temperature.

When the surrounding air temperature changes, the volume of the air inside the bubbles changes simultaneously.

When the air volume decreases, the bubble will sink, and when it increases, it will float.

1.  medina anat (2013-12-20 15:00:00)
Address the question
Lovely selected topic especially in light of the use of scientific knowledge for use in design.
I think distractors are written in a way that can lead to mistakes, if we consider the level of knowledge required of junior high school. Should perhaps refine the wording.
In addition, I think you want the link contains information that could help resolve the question.
2.  Elazar Michael (2014-12-20 15:00:00)

This question is very interesting and well formulated. I wonder though whether it’s not too difficult for children; they might find it hard to identify the correct answer.

The geographical point is relevant to the question, but the author might add there more information pertaining to the question. The remark contained in the point mentions Archimedes, while the questions deals not only with density but also with temperature (a subject which has nothing to do with Archimedes), so the author might consider discussing this aspect too.

A correction: Syracuse is not in Greece, it’s in Sicily; the confusion might arise from the fact that the region of Sicily and southern Italy was once called “Magna Graecia”.

3.  Galkin Amit (2023-01-20 16:00:00)
Thanks, Anat

I followed your advice, and edited both question and answers, hope they are more clear, now.

A demonstration of the Galileo thermometer in operation was definitely planned to be part of the question, however I've got the actual thermometer only recently. It will be added soon.

Thank you for your comments and map point addition.

4.  Galkin Amit (2023-01-20 16:00:00)
Thanks, Michael

Thank you for lighting up the Greece/Italy mistake.

As for Archimedes, following your comment, I have added a brief explanation to clarify the connection between the terms used her: mass - volume - density - weight - buoyancy - temperature.

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