English: Ring and ball apparatus in a 1912 scientific instrument catalog for performing Gravesande's experiment, a simple experiment demonstrating thermal expansion used in physics education from the 1800s. It consists of a small metal ball suspended by a chain from a stand, with a metal ring below it. The ring is made just big enough so when both pieces are at the same temperature, the ball fits through the ring. However, when the ball is heated by dipping it in boiling water, or playing a spirit lamp flame over it, the metal expands and it can no longer fit through the ring. When it has cooled down, it fits through the ring again. In this example, the ring and ball are made of brass, because it has a greater coefficient of expansion than iron.