Cartesian Divers

Posted on January 20, 2010


This post is for everyone who might want to play with Cartesian divers, but especially for the Fast Track kids from Arisia 2010.  For a hands-on science session this year, I showed some of the kids how to make Cartesian divers, but the materials we used made it difficult to do it with so many kids and so little time.  Not everyone got theirs to work, and they were very prone to leaking.  So in this post I’ll explain some easy ways to make Cartesian divers at home.

But first, the matter of the name.  There is some confusion out there on the internet about whether they are called “Cartesian divers” or “artesian divers”.  For a while, it had me confused too.  Cartesian divers are named after René Descartes, so the correct name is “Cartesian divers”, but the spread of the confusion is understandable because “artesian“, as it happens, also has a meaning related to fluid mechanics.  There used to be a province in France called Artois where a lot of the water under the ground was under pressure.  If you drill a well in such a place, water will flow up through the well without any pumping.  In Artois many such wells were drilled to take advantage of the effect.  Wells like that came to be known as “artesian”, named after the province.

Next, I’ll explain the way I learned to make a Cartesian Diver when I was little.  It involves two test tubes (one little, one big), a small piece of sheet rubber (e.g. cut from a rubber glove), and something to hold the rubber on the big test tube (recently I used a rubber band and duct tape).  You fill the big test tube with water and put it aside.  Then you filled the little test tube with water leaving just enough air so that it floats when you put it upside down in water.  You either have to flip it really fast, use a tiny test tube so the water stays in due to surface tension, or use a straw to fill it with the right amount of air once it’s in the water.  You put the little test tube upside down into the big test tube, with the air bubble still in the little one so it floats.  Then you attach the rubber sheet seal to off the top of the bigger test tube.

Now, when you press in the rubber with your thumb, the little test tube sinks to the bottom.  When you stop pressing, it floats back up!  How does that work?  By pressing on the rubber, you pressurize the water inside.  Water is an incompressible fluid, so it doesn’t “scrunch together” when you do that.  But air, on the other hand, is very compressible.  So … you’re press on the rubber, the rubber presses on the water, and the water presses on that little air bubble that’s keeping the little test tube afloat.  That compresses (“scrunches up”) the air bubble.  It’s the same amount of air, but in a smaller volume.  That means it gets more dense.  Things that are denser than water sink in water and things that are less dense float.  Remember how we made the air bubble just big enough that the little test tube would float?  Well we just made it a little more dense by compressing it.  So now it sinks!  Release the pressure and it all goes back to normal and it floats back up.

For the kids at Arisia, I tried using balloons instead of bits of rubber glove because I thought they’d be easier to squeeze.  They were easier to squeeze, but they leaked so much that you had to start over again every time you squeezed it!  So after Arisia I searched the internet for the quickest, easiest, and most durable way to make a Cartesian diver I could find.  The easiest method I found was so easy that I felt kind of sheepish for having bothered with test tubes at all!

  1. Get an empty, plastic drink bottle.
  2. Get a ketchup packet (maybe several different kinds – they don’t all work perfectly).
  3. Drop the ketchup packet into the bottle.
  4. Fill the bottle with water (all the way to the top).
  5. Put the cap tightly onto the bottle.
  6. Squeeze the bottle and the packet sinks, release and it floats!

NOTE: This is simple enough that a video may be overkill, but I want to start using videos in general, so I’ll let this be a test.  🙂

Why does this work?  They don’t fill ketchup packets all the way up with ketchup.  There’s a little bit of air in there.  So… squeezing the bottle pressurizes the water, the water presses on the packet which presses on the ketchup which presses on the air bubble trapped inside.  It turns out that most ketchup packets have just the right amount of that compressible air relative to their weight to make them barely float!  Sound familiar?  Squeezing makes the packet more dense, so it sinks.

It’s unclear who first discovered this method, but here are some websites that describe it:,,

Here are some other methods and explanations for how to make Cartesian divers:,,,

I know that I had a learning experience at Fast Track this year!  As overwhelming as it was to try to help so many kids at once, I had a great time.  I hope I’ll be able to try again next year with some new activities.

Thank you very much for reading!