Heat the brass ball. Try to pass it through the ring. That is it. That is 300 years of physics education in a single object.
Dutch scientist Willem 's Gravesande designed this apparatus in the early 18th century to demonstrate thermal expansion, and it has been in science classrooms ever since. At room temperature, the ball passes through the ring freely. Heat the ball, and it does not. Cool it down, and it does again.
What makes this apparatus still worth owning is the same thing that made it worth building: the principle is invisible until you hold it. Reading about thermal expansion is one thing. Watching a brass ball you just heated refuse to fit through a ring it passed through ten seconds ago is another.
Details: Brass ring and ball. Wooden handle. Classic demonstration apparatus. Named after Willem 's Gravesande (1688-1742), Dutch mathematician and physicist.
