It may require complex calculations to give a mathematical description of phase transitions like melting and crystallisations. To avoid these calculations, Kamerling Onnes devised a clever gimmick. He had plaster models built in which the energy of a system was represented as a function of the volume and the entropy. In order to determine the transitions he rolled a plate of glass over these models. Metal wires were stretched across the folds where the glass did not touch the model. These indicated the transition from one phase to another.
