The Designated Diver

( A Cartesian Diver for Super-teachers)


This is an extension of the usual Cartesian diver demo.


-density and buoyancy
-effect of pressure on the volume of a gas
-incompressibility of liquids and hydraulic force


- a flat-sided glass bottle with a solid rubber stopper
[an old 12 oz. liquor bottle works very well]

- a glass dropper with a rubber bulb


Fill the bottle with water and lower a glass dropper half-filled with water into the bottle. The dropper should float with very little of the rubber bulb out of the water. (It may take several tries to get the optimum flotation level.)

Add enough water to fill the bottle completely, leaving only a small bubble of air trapped when the stopper is inserted.

By rotating the stopper, you can adjust the pressure on the air bubble inside the dropper to make it sink or float. Adjust it so the dropper barely floats to the top.

Gripping the bottle with two fingers and pressing the flat sides together can be enough to sink the Diver. Readjust the stopper to get the Diver to respond to whatever pressure you wish.

Students (and even adults) will be amazed that you can bend the glass sides of the bottle with your fingers. If you wish, surreptitiously rotate the stopper back out a bit before a strong student tries it. The Diver won't sink!

Reverse the change before a student with weaker hands tries to bend the glass.


Not enough pressure is involved to result in breaking the bottle, but as you adjust the rubber stopper, be careful that your fingers do not come in contact with any chips in the glass rim of the bottle.


The pressure exerted on the air bubble at the top of the bottle is also transmitted through the incompressible liquid to the air trapped inside the dropper bulb. As the bottle is squeezed, the level of water rises in the stem of the dropper showing that the air is being compressed. Since the compressed air displaces less water, the buoyant force on the Diver is reduced and the Diver sinks. When the pressure on the glass bottle is released, it resumes its normal shape. This reduces the pressure on the air inside the bulb, which expands again, displacing more mass of water thus increasing the buoyant force.

When the buoyant force becomes greater than the force of gravity on the Diver, the Diver floats to the surface.



As with most of the demos that I can't recall the origin of I believe that I first saw Irwin Talesnick show this one. It's been the source of lots of fun over the years.


Related Materials available from Flinn Scientific:

Catalog No. Description

AP9082 Cartesian Diver Construction Kit

AP8721 Squidy-Cartesian Diver

AP4548 Hook Cartesian Diver