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The Cartesian Diver

As shown to the left of the title, first make the diver so that it just barely floats, and then cover the cylinder (or bottle) with an air-tight flexible closure such as a rubber membrane or even a stopper.

Then, as shown to the right of the title, if you press your finger on the membrane (or push the rubber stopper in a little further), the diver descends to the bottom.

Releasing your finger (or pulling up on the stopper a little) causes the diver to return to the surface as if by magic since you cannot see anything touching anything inside of the cylinder.

However, close observation of the diver shows that upon pushing down on the membrane and compressing the air above the water, that pressure is transmitted into the diver compressing its gas to a similar degree. The picture shows the two levels in the diver. Since the gas in the diver displaces less water upon compression, its bouyancy is diminished. The whole process of sinking is augmented by the fact that as the diver goes deeper, the hydrostatic pressure also increases squeezing the bubble even more.

For class use, it is good to make the diver more easily seen than merely an inverted test tube. Of course, if you want to be really tricky, make the diver out of an opaque material so that the bubble inside cannot be seen!

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