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Cartesian Diver

Difficulty Level: 1/5 very simple

Parts and Tools

  • Disposable plastic pipette
  • Large screw
  • Empty 2l soft drinks bottle
  • Large glass
  • Water
  • Scissors

Instructions

The Cartesian Diver is a classic science demonstration which helps demonstrate the principles of buoyancy and the ideal gas law.

A small 'diver' is placed into a bottle which has been filled with water. The diver is an object which is weighted, but has a bubble of air enclosed within which makes it almost neutrally buoyant.

When the sides of the bottle are squeezed, the bubble of air in the diver is compressed, causing a small amount of water to enter the open end. This in turn causes the diver to sink to the bottom of the bottle. When the sides of the bottle are released, the air bubble in the diver returns to its original volume and the diver again floats to the surface.

This guide shows how to make a very simple version of the Cartesian Diver requiring very few parts. This version is perfect for use in the classroom.

Fig 1: Components required
Fig 1: Components required

Fig 1 shows the components required for this model. A two-litre soft drinks bottle needs to be filled to the brim with water. Ensure there is no air in the top of the bottle. A disposal plastic pipette is also required. This needs to be cut in half with a pair of scissors. Also required is a large screw. The head of the screw needs to be very slightly wider than the neck of the bulb end of the pipette.

Fig 2: Screw inserted into the neck of the pipette
Fig 2: Screw inserted into the neck of the pipette

Fig 2 shows the screw which has been inserted into the neck of the pipette. The screw should be fairly secure but will also allow water to flow into the bulb when the bottle is squeezed. This forms the 'diver'.

Fig 3: Diver in a glass of water
Fig 3: Diver in a glass of water

Fig 3 shows the diver in a glass of water. Squeeze the bulb to allow water to enter the diver. Keep doing this until the diver just about floats. If the diver is too buoyant you will not be able to apply enough pressure to the bottle to enable it to sink. Not enough buoyancy and the diver will remain at the bottom of the bottle.

Fig 4: Diver in the bottle
Fig 4: Diver in the bottle

Fig 4 shows the diver in the bottle. You will need to carefully remove the diver from the glass and place it into the bottle without it losing any water. The bottle should be topped up to the brim with water and the cap firmly screwed on. Once again, ensure there is no air in the bottle. If there is, this air pocket will be compressed instead of the air bubble in the diver and the demonstration will not work.

If you squeeze the sides of the bottle, the diver should now sink to the bottom. A little trial and error will allow you to get the diver to float in the middle of the bottle. If you look at the diver closely when the bottle is squeezed, you will see the size of the air bubble change.

Disclaimer

Before attempting any of the construction projects featured on this website, ensure you have, and know how to use, the appropriate tools, components and safety equipment and are competent to undertake the project. These guides are for information only and we hold no responsibility for any accidents, injuries or damage caused by the use or misuse of any equipment, project or information contained within this website. In short - use common sense and stay safe!