Cartesian diver
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A Cartesian diver or Cartesian devil is a classic science experiment, named for René Descartes (Latin name was Cartesius), in which the "diver" - a small glass tube, open at one end such as an eye dropper - is placed in a much larger container with flexible walls, such as a 2-liter soft drink bottle, and adjusted so it barely floats at the top of the water in the larger container.
When the larger container is squeezed, the air inside the diver is compressed, reducing the overall displacement or buoyancy of the diver, which then sinks. One reason for using an eye dropper is that air cannot readily escape the end of the dropper, due to surface tension, however a pen cap can also be used. Ketchup or soy sauce packets from restaurants will also work.
Note that this experiment can also be done with a rigid bottle and a flexible cap (made of cork or some other elastic or flexible material).
The device also has a practical use for measuring the pressure of a liquid.
In addition, the principle is used to make small toys often called water dancers or water devils. If the tail of the glass bubble is given a twist, the flow of the water into and out of the glass bubble creates spin. This causes the toy to spin as it sinks and rises. An example of such a toy is the red "devil" shown to the left. It is topically identical to the eye dropper.
[edit] How the Cartesian diver works
The diver is an object suspended in a chamber of water, which is buoyant enough to barely float at the top of the chamber. When the chamber is squeezed, the pressure on the air captured in the diver increases, which eventually makes the diver neutrally or negatively buoyant. As the air in the diver becomes pressurized its volume is decreased, reducing the diver's buoyancy.
