An odd convection experiment

I was given this piece of equipment a decade ago but never got round to sorting it out. I looked at it today and it worked! You put tap water into the narrow middle tube and ice in the bit that goes round the middle on the outside. Thermal energy is transferred from the water into the ice by conduction through the metal walls (remember the free electrons!). This means that there is colder water half way down the inner tube. Water contracts and becomes more dense when it gets cold at these temperatures (not true if you are very close to 0 degrees Celsius!). The cold water sinks and thus the thermometer at the bottom reads about 5 degrees Celsius lower than the one at the top. It took about half an hour to work. The aim is show that cold fluid sinking is a type of convection. It's why my fridge has a freezer box inside it at the top not at the bottom (there has been a previous post about this)