Definitions

cartesian-devil

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 experiment is set up by placing a "diver"—a small, rigid tube, open at one end, such as an 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 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

The diver is an open ended object in a airtight container of water. Air in the diver makes it buoyant enough to barely float at the water's surface. When the container is squeezed, the pressure affects the least dense material in the container, which is the air inside the diver. This occurs because of Pascal's Principle, which states that pressure on a fluid is transmitted unchanged throughout the fluid. The pressure from the fluid being squeezed also presses on the air bubble in the dropper. 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.

Archimedes’ principle states that when an object—in this case, the diver—displaces a weight of water greater than its weight it floats and is said to be positively buoyant. Since the diver now displaces a lesser weight of water than its own weight, it is negatively buoyant, and sinks. When the compression of the container is released, the air expands again, increasing the weight of water displaced. The diver again becomes positively buoyant and re-floats.

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 in to 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.

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