Control in a bottle
A “diver” in a bottle apparently reacts to thought or spoken commands. In fact it is controlled by the operator pressing on the bottle.
- Large (1.5 L or larger) clear plastic bottle with lid.
- Sachet of vinegar or chilli sauce. It is also possible to use other things such as biro lids – anything that will float, is open in at least one place or is flexible and contains some air.
- Paper clips or putty.
The “diver” should be weighted so that it only just floats. It is convenient to do this in a bowl or a glass prior to placing the diver in a bottle. Place the diver in your bottle and fill the bottle with water. Pressing the bottle should then make the diver sink.
The Cartesian diver is based on Archimedes’ principle. The “diver” floats because it experiences an upthrust equal to the weight of water displaced. This is the weight of the water that has the same volume as the diver. Pressing on the bottle compresses the air trapped in the diver which reduces the volume and hence the weight of water displaced.
When filling the bottle, make sure that it is completely full. Any air left in the bottle will be compressed at the same time as the air in your diver so the bottle would need to be squeezed harder to make the diver sink.
If the diver is appropriately weighted the bottle can be used as a thermometer. If the water is warmed so that the pressure increases in the bottle this can be enough to cause the diver to sink.
Take care not to drop the bottle on your toes.