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The Cartesian Diver

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Author: Marc Rosner

When the density (mass divided by volume: D = m/V) of an object is greater than the density of the surrounding fluid or gas, the object sinks. When the density of the object is less, it rises. This is why ice floats in water and helium balloons in air, and why lead sinks in water and in air. Submarines control their own density by pumping seawater in and out of chambers. Divers wear weights to keep their density close to that of water; otherwise they would have to fight their tendency to float. A Cartesian diver is a simple device with a variable density that floats and sinks at your command.

Materials

  • large widemouthed plastic jar with tight-fitting lid (A mayonnaise jar works well.)
  • water
  • eyedropper or pen cap
  • clay or wire
  • flat-sided plastic bottle
  • soy sauce packet

Procedure

  1. Fill a jar to the rim with water.
  2. Put an eyedropper in the water and adjust it so that it barely floats. You can make these adjustments:
    1. If necessary, put some clay or wire around the tip to weight it so that it points downward.
    2. Adjust the amount of water inside the dropper. An air bubble is necessary in the top of the dropper.
    3. If the dropper sinks, make it lighter by reducing the water inside (increasing the air) or by removing some clay or wire from the tip. When the eyedropper is adjusted properly, if you push down on it, it should travel down for a bit, slowly come to a stop, and return to the surface.
  3. Seal the lid on the jar. Now squeeze the sides of the jar. The diver should sink to the bottom until you release your grip.
  4. As you squeeze the jar, you increase the pressure on the water and on the air bubble inside the dropper. This makes the bubble smaller. The reduced volume of the bubble makes the density of the diver greater than that of water, so it sinks. The density of water is 1 g/cc. This value is the basis of the metric system.

    The Cartesian Diver

  5. Try these variations on the Cartesian diver:
    1. If you do not have a dropper available, you can use a pen cap, which is open at one end.
    2. With skill, you can get your diver to sink in a flat-sided glass bottle. Even glass bends under pressure.
    3. Make a reverse Cartesian diver. Adjust the diver so it barely sinks to the bottom. Squeeze the jar a bit before sealing it. When you squeeze the jar gently at the bulge, the diver will rise from the reduced pressure of this configuration. This is because you are pushing the compressed sides outward, reducing the force they exert on the water inside. The air bubble in the Cartesian diver expands, and the loss of density makes the diver buoyant in the surrounding liquid.
    4. The Cartesian Diver

      Buoyancy is an upward force exerted on an object by its surrounding medium (liquid or gas).
    5. Make a "gourmet" diver. Use a sealed soy sauce packet as a diver. It usually has the right amount of air and liquid inside.

References

Diving into Darkness: A Submersible Explores the Sea by Rebecca L. Johnson (Minneapolis: Lerner, 1989).

Robert Fulton: From Submarine to Steamboat by Steven Kroll, Katherine Kirkpatrick, and Steve Kroll-Smith:(New York: Holiday House, 1999).

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