Magdeburg pressure disks rvfm ph12184 p a practical vacuum pressure demonstration set consisting of two plastic 10cm diameter hemispheres p p each hemisphere has a wooden handle firmly attached to it one of the hemispheres has a raised edge to hold a rubber seal while the other hemisphere has a stopcock for attaching to vacuum tubing when the hemispheres are pushed together and evacuated using a vacuum pump they are hard to pull apart get 2 students to pull then open the stopcock