Supervisor: Michael Page

Background: It is a common myth that the direction of rotation of bath water as it drains down a plughole is determined by the rotation of the Earth – in particular the hemisphere in which it is performed. Careful experiments have been performed to confirm that the rotation of the Earth has only a miniscule effect on the motion (Shapiro, 1962; Trefethen et al, 1965) and it can also be demonstrated theoretically using simple scaling arguments. More recently, Tyvand & Haugen (2005) published a computational study of the problem and also demonstrated that the influence of the rotation of the Earth is negligible. But what are the principal determinants of the direction? Why does the water often flow down in a consistent direction, and why is that direction opposite in the Australia to what is commonly observed in the United Kingdom, for example?