[Physics]

[Cartesian Divers]

This project idea comes to you from Venture Engineering and Science Camp in Hamilton, Ontario.

 [Venture Engineering and Science Camp]
[Purpose:]
To investigate the concepts of buoyancy, density, and transmitted pressures.

[Theory:]
Objects float or sink as a result of their density. Density can be described as the amount of weight in a specific volume. An object is buoyant (it floats) if its relative density is less than the density of the fluid that is surrounding it. According to Archimedes' principle, an object will be buoyed up by a force that is equal to the weight of water that it displaces. A boat may be heavy but it also pushes a lot of water out of the way, making its relative density low so that it floats. The relative density of a brick is more than the density of water because it does not displace very much of the water. As a result, a brick sinks in water.

[Materials:]
[Kids working with a pop bottle]

[Physics Girl holding the experiment]

[Procedure:]
  1. Decorate an eye dropper using materials that are not very heavy. The black end of the eye dropper can be the head of the diver.
  2. Attach two paper clips to the spout of the eye dropper. Bend one of the paper clips so that it can be used to pick things up.
  3. Fill an empty 2L pop bottle with water. Add some food colouring to the water if desired.
  4. Bend the ends of 3 fasteners so that they make a loop. Drop them into the pop bottle.
  5. Place the eye dropper into the bottle with the spout facing downwards.
  6. Close the lid of the bottle tightly.
  7. Squeeze the sides of the bottle and watch the diver sink to the bottom. Let go and watch the diver float to the top. Attempt to pick up the fasteners using the diver.

[Conclusions:]
According to Pascal's principle, when we push on the sides of the plastic bottle, the pressure pushes on all of the water and on the air inside the diver. Air can be compressed much more easily than water, therefore the air inside the Cartesian diver becomes more dense than the water as we push on the sides of the bottle. Due to the increase in the relative density of the Cartesian diver, the diver sinks to the bottom of the bottle.

Why is this diver called "Cartesian"?
"Cartesian" comes from the name of the French mathematician and philosopher Rene Descartes, who lived from 1596 until 1650. The harder question to answer is WHY the diver is named after Descartes. Nothing written by Descartes about the diver has ever been found. It is one of Galileo's students, Raffaelo Maggiotti, who first described the Cartesian diver in writing (he called it the Cartesian Devil). In a short pamphlet he wrote, he refers to the device as "my invention". Another mystery comes from the fact that the French do not even refer to the diver as Cartesian. They call it "ludion," a word derived from the Latin meaning actor, jester, wandering entertainer. Some students have suggested that the diver is now called "Cartesian" because it makes you think. As the philosopher who became famous for the sentence "I think therefore I am," Descartes may have been the best, if not the most accurate, choice for a namesake.

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Produced by Galactics.
Comments: galactics@spacesim.org.
Last updated on 14 August 1998.