The Cartesian diver

The baking oil bottle lowers when pressure is applied and rises again as soon as the pressure is released. The water level in the baking oil bottle rises when the bottle sinks and rises when the bottle rises. The photos on the side show the initial situation without pressure and the situation where the bottle is just starting to sink.

Fill the plastic bottle to the brim with water. Fill the baking oil bottle about a fifth full with water. Now put the baking oil bottle into the bottle with the opening first. It should float just above the surface. Now screw the bottle on and press on the side of the bottle.

• A sturdy plastic bottle,

• A baking oil bottle (the photo shows a baking oil bottle with a colored adhesive strip),

• The water level in the bottle must be well adjusted.
• The bottle must be filled to the brim with water.
• The bottle must not be too tight, but also not too flexible.

What is technically behind the experiment

The experiment is a variant of the Punika submarine. By applying pressure to the bottle, the volume of air in the baking oil bottle is reduced. This changes the buoyancy of the baking oil bottle.

There are two different ways to explain the experiment:

1. the baking oil bottle and the air it contains are considered as a whole. The volume of this body made up of glass and air is reduced by external pressure. As a result, the body displaces less water and experiences less buoyancy. The weight of the body is independent of the pressure.

2. consider the baking oil bottle, the air it contains and the water it contains as a single body. This body always has the same volume, but different masses depending on the pressure. The buoyant force therefore remains the same (because the same amount of water is always displaced) but the weight increases with external pressure.

What the experiment has to do with everyday life

Cartesian divers can be bought in fancy toy stores under the name "devil in a bottle". They have the shape of a devil. The water enters the devil through an opening in the tail. The tail is wrapped around the body as a tube. When water is expelled from the devil, i.e. when it rises, the devil turns. This is a consequence of the recoil principle, which is explained in the balloon car.

The experiment can be easily turned into a magic trick by not mentioning the pressing on the bottle during the experiment. The diver can be moved as if by magic at the teacher's command. In contrast to comparable experiments with divers in glass bottles, where pressure is applied from above, the experiment shows that the direction of the pressure is not important at all.

Rita Wodzinski