Editing Cartesian diver (section)

== Experiment description ==
[[File:cartesian_diver.svg|thumb|left|upright|A Cartesian diver toy made from a drinking straw, paperclip and plastic bottle
{{Ordered list
| The trapped air in the straw makes the diver slightly buoyant, and it thus floats.
| Squeezing the bottle increases the water pressure; as the bubble shrinks, the diver's density increases and it sinks.}}]]
The experiment requires a large water-filled bottle, inside which is a "diver":  a small, rigid tube, open at one end, very similar to an [[Pipette|eyedropper]] with just 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. Two alternative "divers" can be constructed. One sealed but a flexible bulb, and the other a sealed glass bulb (flashlight minus metal base) with wool threads trailing below. The flexible one will compress reducing volume, and the solid glass one will not change, but air bubbles will be trapped in the fibres, and be exposed to the pressure – thus will change volume.

The "diving" occurs when the flexible part of the larger container is pressed inwards, increasing the pressure inside the larger container, causing the "diver" to sink to the bottom until the pressure is released, when it rises back to the surface. If the container is rigid, as with a glass bottle, the cork sealing the bottle would be pressed inwards or drawn outwards.

{{multiple image
| header = Inside an oval bottle
| total_width = 300
| image_gap = 5
| thumbtime1 = 5
| caption1 = A reverse diver<ref name="Panov" />
| image2 = W150 V1.webm
| caption2 = A double action diver<ref name="Panov" />
}}
There is just enough [[air]] in the diver to make it positively [[buoyant]]. Therefore, the diver floats at the water's surface. As a result of [[Pascal's law]], squeezing the airtight container increases the pressure of the air, part of which pressure is exerted against the water that constitutes one "wall" of the airtight container.  This water in turn exerts additional pressure on the air bubble inside the diver; because the air inside the diver is compressible but the water is an incompressible fluid, the air's volume is decreased but the water's volume does not expand, such that the pressure external to the diver ''a'') forces the water already in the diver further inward and ''b'') drives water from outside the diver into the diver.  Once the air bubble becomes smaller and more water enters the diver, the diver [[displacement (fluid)|displaces]] a weight of water that is less than its own weight, so it becomes negatively buoyant and sinks in accordance with [[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 positively buoyant and floats.

It might be thought that if the weight of displaced water exactly matched the weight of the diver, it would neither rise nor sink, but float in the middle of the container; however, this does not occur in practice. Assuming such a state were to exist at some point, any departure of the diver from its current depth, however small, will alter the pressure exerted on the bubble in the diver due to the change in the weight of the water above it in the vessel.  It is an unstable [[Mechanical equilibrium|equilibrium]]. If the diver rises, by even the most minuscule amount, the pressure on the bubble will decrease, it will expand, it will displace more water, and the diver will become more positively buoyant, rising still more quickly. Conversely, should the diver drop by the smallest amount, the pressure will increase, the bubble contract, additional water enter, the diver will become less buoyant, and the rate of the drop will accelerate as the pressure from the water rises still further. This positive reinforcement will amplify any departure from equilibrium, even that due to random thermal fluctuations in the system. A range of constant applied pressures exists that will allow the diver either to float at the surface, or sink to the bottom, but to have it float within the body of the liquid for an extended period would require continuous manipulation of the applied pressure.

Divers inside an oval plastic bottle acquire new interesting properties. Indeed, an oval bottle can increase in volume when it is compressed, and if this happens, the drowned diver can ascend.<ref name="Panov">Panov (2018), pp. 11−16</ref>