Thermal expansion of liquids. Laboratory apparatus designed to demonstrate the thermal expansion of liquids. Here, a reservoir of black-coloured liquid is in the round-bottomed flask at bottom. Running upwards from the flask is a microbore tube made of glass. The microbore is the narrow opening running up the centre of the tube. As the liquid is heated or cooled (here in a beaker of water), it will expand or contract and move up or down the microbore. The expansion is relatively small, and it can only be seen when a narrow tube is used. The same principle is used in mercury thermometers to measure temperature changes as the mercury moves up and down a microbore.