Two-liquid Cartesian diver

It is quite easy to make a version of the well known Cartesian diver experiment that uses two immiscible liquids. This allows students to test their knowledge of density and pressure in explaining the diver’s behaviour. Construction details are presented here together with a mathematical model to explain the observations. M This article features online multimedia enhancements The Cartesian diver is one of the most popular simple experiments. It is always attractive even if you have seen it many times before. In addition, this simple experiment offers various possibilities for interesting inquiry questions for students, therefore encouraging discussion and a search for the right answers. For these reasons we decided to build a large-scale version of the Cartesian diver for our hands-on science centre in Ljubljana. But, as in any other science centre where people build their own exhibits, we also wanted to add ‘something new’ to our exhibit. The following idea emerged from discussion: let’s try with two liquids of different densities that do not mix and two or more divers that will initially float at different liquid boundaries. As usual, the idea was first tested on a simple prototype made from material that one can find easily. While playing with a prototype it appeared to one of us that the two-liquid Cartesian diver may be useful as a demonstration experiment in school. In this article we present the construction of the experiment and some suggestions for how to use it at the advanced level in secondary school. There are numerous references in the literature and on the web on how to construct a simple Cartesian diver, how to explain its behaviour and how to use it in building students’ ideas about the observed phenomena. When searching for previous reports on multi-liquid Cartesian divers we found only a report on the Cartesian diver using continuously varying liquid density obtained with salted water [1]. One drawback of such a diver is that even if we do not touch the bottle the density of the liquid will eventfully become uniform due to convection and diffusion.