Thermal expansion of metals. Image 2 of 2. Heated copper ball failing to pass through a copper ring. This apparatus has been designed so that when the copper ball is heated, the metal will expand and the ball will be unable to pass through the ring. When it has cooled down, it will be able to pass through the ring again. This demonstrates the general principle of thermal expansion, seen in gases and liquids, and in some solids, such as metals. For the unheated copper ball passing through the ring, see image A305/022.