A Magdeburg sphere consists of a pair of hollow hemispheres, historically made from copper or brass, with a seal around the edge that’s tight enough to allow air to be pumped out of the assembled object. When air is evacuated from the sphere, the pressure inside of it is lower than the atmospheric pressure outside pushing on it from all directions, so the halves stay together. The inventor, Otto von Guericke, used such a sphere to perform a dramatic demonstration of the existence of atmospheric pressure. During the experiment, air was pumped out of the sphere, and a team of horses was attached to either side. They strained in opposite directions yet were unable to pull the hemispheres apart until air was let back in.
The demonstration was not only a scientific endeavor but a political move. Von Guericke, then mayor of the city of Magdeburg, in what is now...
