A pretty picture and a bit of physics

This will be the last of the images from our weekend visit to the Cape; a pretty picture of a Galilean Thermometer. I think they’re beautiful and this one doesn’t disappoint. Being the sort of person who is naturally curious about such things, I was surprised to discover that neither the volume of the spheres nor the composition of the colored fluid they contain has anything to do with the way in which this intriguing instrument works. The key to understanding its operation is to know that each of the little tags is a precisely calibrated counterweight which imparts a slightly different density to each of the spheres. So how does this seventeenth century gizmo work? See if this makes sense. As the ambient temperature changes, so does the density of the fluid in the tube (which is, by the way, a mix of alcohol and water). You will recall that there is an inverse relationship between temperature and density such that the fluid in the tube will become less dense as temperatures rise and more dense as they fall. As the density of the fluid changes so too does the relationship between the buoyant force (pushing up) and the gravitational force (pulling down). The little spheres maintain their relative order in the column at all times. As temperature increases, the fluid in the tube becomes less dense and the gravitational forces pulls more strongly against the buoyant force and the balls fall (in relative order of density). As temperature falls, the fluid in the tube becomes more dense and the buoyant force becomes stronger, pushing more forcefully against the gravitational force, and the balls rise in the column. The tag on the sphere which rests comfortably in the middle of the column indicates the ambient temperature. I really do appreciate this instrument for its beauty and its mechanical elegance.


21 thoughts on “A pretty picture and a bit of physics

  1. Lovely image. It’s useful to take items like that to a class full of younger folk and ask them how it works. It’s fun encouraging them to refine and test their ideas whether as individuals or in groups.

    • Many folks have taught courses with (unimaginative) titles such as ‘How Things Work.’ Perhaps there’s a course-development project in there for you. Something a bit more rigorous than the gee-whiz stuff that I write about.

  2. Is this my old thermometer? Always loved having it in my room. Can’t believe it made it so many years intact without falling prey to one of my wayward elbows! Lovely image and explanation.

  3. This IS a pretty contemporary image considering how old the thing must be! Where exactly is the “dial ” that tells you what the temperature is. You explanation was fascinating! I am amazed at how clean and clear the glass balls are. How is this possible?? : ) PS. Never took physics at BHS. Perhaps if you were my teacher I might not have been petrified to take it!

    • Actually, this is a new addition to the place and is only a few years old. Also, the temperature of the room is indicated by the little brass tag of the sphere that is floating in the middle of the cylinder (others will be above and below it). On the afternoon I took this photograph the heaviest sphere in this group (which indicated a temperature of 66 degrees) was floating, along with all the others, at the top of the tube, indicating that it was colder than 66 in the room! Next time you’re down there you should take a look at this thermometer to determine the temperature for yourself! D

    • Thanks much Seonaid! I was telling Joanna about your new Gravatar and told her of your use of the term ‘crinkly bits,’ and she had a very, very, good (knowing) laugh!

    • If anyone would appreciate this conversation-piece, you would. They aren’t all that expensive … so there’s a Holiday gift to yourself for this coming season.

  4. We were given one of these thermometers as a present long ago. For a change, I actually did understand the workings of something and your description explains it perfectly. Ours does not contain any colored water though, just the nicely engraved little tags. I agree with elkement … the outer variable density liquid should be the colorful one … maybe even like a lava lamp. 🙂

    • I took this photo when we were down at the Cape last weekend. If I had shown a somewhat larger view you would have seen that all the spheres were crammed at the top of the cylinder … and the lowest had a temp tag of 66F … so, the ambient was something quite a bit cooler than that. There is no heat in this single-season summer house … it was rather cool over night! We brought our winter comforter from the Farm and were AOK though. We had a bright and sunny day here … hope you had the same and put that Canon through its paces. D

      • I did. One more day until back to the day job. Forecast sounds good. It is too bad you weren’t there this week. Sunny for a few days and the weather will be in the 70s. But you are probably going to have some decent weather there too.

  5. I remember buying my Dad a small one of these which he was fascinated with. I just loved the look of it. If he were here today he’d be so interested in your explanation.

  6. I have no idea what you said, but the picture is smashing. It is contemporary, colorful and very interesting. Loved the composition, but you left me in the dust as far as the explanation. My mind stopped deciphering technical lingo before I was born.

  7. A pretty image and device indeed! I find that such devices might actually make it harder to understand the underlying effects as the colored liquids – and their densities – are irrelevant … so beauty may be distracting 😉 Perhaps just the “outer” liquid – whose density’s change with temperature is important – should be the colored one!

    • But then the thing wouldn’t be nearly as pretty! And, I don’t think that many folks really care how it works … the part about the mechanics was meant for geek-types only. D

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