Cartesian Diver

A classical science experiment

Category: Science Time frame: half an hour
Age: Where:
Level: easy What you need: easy to get items
Keywords: Cartesian Diver, diver, Descartes, air, water, pressure, buoyance
Article written by Andreas


According to Wikipedia, this experiment got named for Rene Descartes. Check out the video to see the experiment: by pressing on the flexible cap, the diver (a small glass in our case) begins to sink.

There's some air in the floating glass, just enough so that it doesn't sink. The large glass is completely filled with water. When you press the flexible cap of the large glass, the air inside the floating glass is compressed (because water cannot be compressed). This reduces the buoyancy of the small glass and it begins to sink.

 To do this on your own you need:


The small glass has to fit into the large one. The experiment works best, if the small glass is large enough so it cannot turn upside down. Plastic doesn't work as it doesn't sink.


 The tricky part is to get the right amount of air into the small glass. To do this, take the straw and put the small glass onto the tip of it (as shown below). When dipping it into the large glass (filled with water) the air can escape. Then gently blow into the straw until there's enough air to lift the glass. Pinch the straw to make sure the air cannot escape again.


Then completely fill the large glass (no air should remain at its top). Cap it with the rubber glove and fix it with the rubber band. Test it. If the small glass stays on top, there's too much air in it.