A dilatometer is used to measure changes to the length and width of rods made of different metals which are subjected to heat. It was the Dutch physicist Petrus Van Musschenbroek (1692-1761) who first invented and described an apparatus of this type. In 1731 he called it a pyrometer, although today it is known as a dilatometer. The dilatometers of the 18th century were in demand by clockmakers for studying the thermal properties of metal rods, and were also used in laboratories in physics experiments. The model in the photograph is similar to the one found in the book Leçon de Physique experimentale (1764) by Nollet (1700-1770). Our piece dates from the second half of the 18th century (the year 1772) and was built by one of the most significant Spanish instrument makers of his time, Juan González.