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Demonstration of a cartesian diver

A Cartesian diver or Cartesian devil is a classic science experiment, named for René Descartes, which demonstrates the principle of buoyancy (Archimedes’ principle) and the ideal gas law.

Experiment description

The Cartesian diver experiment is set up by placing a "diver"—a small, rigid tube, open at one end, such as an hieyedropper—in a much larger container with some flexible component; for example, a two liter soft drink bottle. The larger container is filled with water, and must be airtight when closed. The "diver" is partially filled with a small amount of water, but contains enough air so that it is nearly neutrally buoyant, but still buoyant enough that it floats at the top while being almost completely submerged.

The "diving" occurs when the flexible part of the larger container is pressed inward, causing the "diver" to sink to the bottom until the pressure is released, when it floats again.

Explanation of the principles at work

Air in the diver makes it neutrally buoyant and therefore float at the water's surface. As a result of Pascal's Principle, when the pressure increases by squeezing the container, the least dense material, the air in the diver, is affected. Thus, the pressure on the water increases the pressure on the air bubble in the diver. The air compresses and reduces in volume, permitting more water to enter the diver. The diver now displaces a lesser weight of water than its own weight and becomes negatively buoyant (i.e. sinks), according to Archimedes’ principle. When the pressure on the container is released, the air expands again, increasing the weight of water displaced and the diver again becomes neutrally or positively buoyant and floats. If however, the buoyant force is equal to the compressed air force inside the diver, it will either float straight to the top or it will float in the middle of the container.

Other uses

In addition, the principle is used to make small toys often called "water dancers" or "water devils". The principle is the same, but the eyedropper is instead replaced with a decorative object with the same properties: a tube of near-neutral buoyancy. For example, a blown-glass bubble. 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 here.

The device also has a practical use for measuring the pressure of a liquid.

External links

Java applet showing how a Cartesian diver works

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 ==Experiment description==
-The Cartesian diver experiment is set up by placing a "diver"—a small, rigid tube, open at one end, such as an [[Pipette|eyedropper]]—in a much larger container with some flexible component; for example, a two [[liter]] [[soft drink]] [[bottle]]. The larger container is filled with water, and must be airtight when closed. The "diver" is partially filled with a small amount of water, but contains enough air so that it is nearly [[neutral buoyancy|neutrally buoyant]], but still buoyant enough that it floats at the top while being almost completely submerged.
+The Cartesian diver experiment is set up by placing a "diver"—a small, rigid tube, open at one end, such as an hi[[Pipette|eyedropper]]—in a much larger container with some flexible component; for example, a two [[liter]] [[soft drink]] [[bottle]]. The larger container is filled with water, and must be airtight when closed. The "diver" is partially filled with a small amount of water, but contains enough air so that it is nearly [[neutral buoyancy|neutrally buoyant]], but still buoyant enough that it floats at the top while being almost completely submerged.
 The "diving" occurs when the flexible part of the larger container is pressed inward, causing the "diver" to sink to the bottom until the pressure is released, when it floats again.