Magdeburg Hemispheres are a classic way to teach your students about vacuums! In a modern application of the famous 17th-Century experiment, you can demonstrate the force of air pressure without a vacuum pump. When the two cups are pressed together, the air is evacuated and a vacuum is created, separating the discs will be an interesting challenge for your students. Made of rubber. They are 3-1/4" in diameter and come in a set of 2.