I got a wonderful present from Mrs B at the weekend - a Max Min Thermometer. This is also known as Six's Thermometer after its inventor. It is a U-shaped thermometer which records the same temperature on both sides. If you look carefully you'll see that the number scales go in opposite directions on each side. Each side also has a small steel marker in the tube. These record the maximum and minimum temperatures in a given time period. They stay where they are and you have to use a magnet to pull them back to the end of the coloured liquid to allow the process to start again.
It works because there is a clear liquid in the tube that records the minimum temperature (left hand side above and right hand side below). This expands and contracts with temperature pushing the blue temperature recording liquid round. Above the blue liquid on the other side is a vacuum - nothing to stop the blue liquid moving round the tube.
