Magdeburg Hemispheres

a celebrated invention designed to show atmospheric pressure. About 1650 Otto von Guericke, burgomaster of Magdeburg, prepared two hollow hemispheres of copper. Their edges were ground and polished, so that when placed together, oiled, and twisted slightly, they formed an air-tight globe. One of the hemispheres was fitted with a stopcock through which the inventor exhausted the air by means of an air-pump. He exhibited his apparatus before scientific men of the day and before the emperor, Ferdinand III, at the Diet of Ratisbon. When full of air the hemispheres fell apart of their own accord. When the air was exhausted they could not be pulled apart by two men. The experiment is still a familiar one in the school laboratory. The force with which the hemispheres are held together may be obtained readily. It is fifteen pounds for every square inch of the cross-section of the united hemispheres.