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Cartesian divers

A Cartesian diver can be made by blowing a small bulb on the end of a length of glass tubing. The excess tubing can then be cut off to give the shape shown in Figure 1. Allow the glass to cool down before cutting the end off and certainly before going on to put any water in it.

Next you should put a little water into the bulb until it is just under half full.
Now put you diver into a tall plastic bottle with the bulb upright. Screw the lid of the bottle on and your diver should float towards the top of the water. (Figure 2). You may need to adjust the amount of water in the diver to make sure that it floats.

When you squeeze the bottle (you may have to squeeze quite hard) your diver should start to sink down towards the bottom of the bottle (Figure 3). When you let go the diver will rise towards the top of the bottle again.



The diver rises or sink depending on its density. When its overall density (glass surround and air) is less than that of water it will float and when its overall density (glass and air) is greater than that of water it will sink.

At the start the volume of the air bubble in the diver is large and so the overall density of the diver is low. As long as the overall density of the diver is less than the density of water the diver will float. Also, because of the large volume of air in the diver a correspondingly large amount of water is displaced the diver floats (See Figure 4).

The sides of the bottle are now squeezed hard (Figure 3).

The water in the bottle is virtually incompressible, except under enormous forces, and so the only thing that can be compressed is the air in the diver (See Figure 5). The volume of this air is reduced and so the overall density of the diver increases. Its total mass has remained the same but because its volume has decreased its density will have increased (Density = Mass/Volume).

If the bottle is squeezed hard enough the density of the diver will increase until it is greater than the density of water the diver will therefore sink.

Releasing the pressure on the bottle will let the air in the diver expand again, decreasing its overall density to less than that of water and allowing it to float upwards.

You might like to wonder what it would be like if the liquid in which the diver was placed had a greater density than that of water. Do you think that it would make the experiment easier or more difficult to perform?

Another additional experiment is to add some salt to the water to see what affect this has.

Alternative forms of diver can be made with plastic thermometer tubes or straws closed at one end by a small blob of plasticene.

© Keith Gibbs 2020