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As far as temperature measurements go, the most beautiful way to measure it is with a Galileo Thermometer. The attractive glass tubes with floating balls that change over time and temperature ever so slowly and gracefully.


But what is a Galileo Thermometer? How does it work? And perhaps more importantly how do you read one? Surprisingly this is a question we get asked a lot, but then again ‘Galileo Thermometer Reading’ is not a feature of most science classroom’s curriculum. There is also the minor detail that Galileo thermometers are not exactly precision instruments. A small one can have 6 degrees (F)  of  difference between the balls, which make for quite a measurements range of error. Even larger models have at least a couple of temperature difference between the diver globes. They also tend to lack a range of temperatures – most cover the temperatures from 68-84F degrees which makes them suitable only for heated/air conditioned indoor locations.


Galileo thermometers were not invented by Galileo himself, but Galileo did discover the principle that causes the Galileo Thermometers to work – the principle that liquids change density with the temperature. Thermometers that actually operate on this principle were actually developed in the later 17th century but not considered worthy of development as they were slow to show temperature changes. What we now know as a Galileo Thermometer was developed in the late 90’s as an attractive and functional gift item.

How do they work?

The Galileo Thermometer tube is a glass tube filled with some sort of liquid. Sometimes this liquid is just distilled water but at other times an alcohol or mineral spirits are used. These liquids are more sensitive to temperature changes that water (i.e they change faster). If you should break your Galileo thermometer you should clean up the liquid immediately and not ingest any of  the fluid. It may stink, but the fumes will only be a problem if you break it in an area with no circulation. So try not to break your Galileo thermometer in a small, locked closet.

The balls inside the thermometer each have a diferent fixed density measured  and adjusted using the weight of the glass ball, the temperature tag, and the liquid/air mix. The density of the liquid in the balls is not very affected by temperature. Now whenever a liquid is cooled or heated it will contract or expand respectively, this applies to the sensitive liquid in the tube. Each of the balls in the tube is adjusted to be slightly less dense than the density of the tube liquid at a set temperature. So the ball marked at 78 degrees will be just slightly less dense than tube liquid at that temperature, causing it to float. While the ball underneath it, marked and adjusted for 76 degrees will be more dense than the tube liquid and will sink.

How do you read a Galileo Thermometer?

It’s actually quite easy: Each diver has a temperature tag hanging from it. To read the temperature just look at the lowest ball that is floating. Sometimes the CloseupGalileodivers may not be floating or sinking but are floating between the floating and sinking divers then the temperature is somewhere between that diver and the top diver that has sunk. As we said: these are not precision instruments.

Now mind you, the tags on Galileo Thermometers tend to be numbers carved into brass. This makes them look elegant but does not make the numbers very readable. We said it was easy to read a Galileo thermometer, not that it was convenient.

Galileo Thermometers are a great gift item and look beautiful on a desktop or mantle. They may not be accurate, but there is no more attractive way to tell the temperature!

Want to buy Galileo Thermometers?


Comments on: "Galileo Thermometers – How they work and how to read them." (7)

  1. The explanation of how they work is incorrect. The density of the divers remains constant as neither their mass nor their volume changes (density = mass/volume). What changes is the density of the liquid in which they float – there is always an air space that the liquid can expand into, as you can see in the pictures.

    • You are correct, and it almost looks like I was setting things up to go that way in the first paragraph of the description and then did the explanation wrong. Huh. Anyway I will edit this.

  2. Elizabeth said:

    I have one of these on my office desk. A co-worker held it with both hands around the tube in order to force all the divers to sink. Since then, it hasn’t worked correctly (I have a normal thermometer in my office as well). How is this possible if everything is sealed?

  3. Velda Swinford said:

    all the bulbs went to the bottom does that mean it’s broken i got it for Christmas the grandkids and i love it i look every day to see if it’s working again

  4. I have two and I love the look of them

  5. mine broke and fell onto carpet so what do i use to clean it off the carpet please

  6. mine broke and fell onto carpet so what do i use to clean it off the carpet please, what do i use

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