Magdeburg hemispheres

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The Magdeburg hemispheres were one of Otto von Guericke's most interesting experiments, designed to demonstrate the air pump he had invented. They were two hollow copper hemispheres a little over a foot in diameter which fitted together so well that they could hold a vacuum. The air was pumped out from between them, and they were then held firmly together by the air pressure of the surrounding atmosphere.

The demonstration was presented to the Reichstag and the Emperor Ferdinand III in 1654 in Regensburg, where 30 horses, in two teams of 15, could not separate the hemispheres until the vacuum was released. In 1656 he repeated the demonstration with 16 horses (2 teams of 8)in his hometown of Magdeburg, where he was mayor. In 1657, Gaspar Schott was the first to describe the experiment in print in his Mechanica Hydraulico-Pneumatica. In 1663 the demonstration was given in Berlin to the Elector of Brandenburg with 24 horses. (disputed: 1661 before Elector Friedrich Wilhelm?)

Reports of the number of horses in each experiment vary widely. This confusion is due in part to the multiple repetitions of this experiment, but also to the unnecessary use of two opposing teams of horses. Two teams of X horses pulling against each other cannot generate any more force than a single team of X horses pulling against an immovable object.

The original hemispheres are maintained by the Deutsches Museum in Munich. Many copies of them (usually smaller) have been made to illustrate the principle of air pressure to students. Re-enactments of von Guerike's 1654 experiment are performed in locations around the world by the Otto von Guericke Society . The experiment has been commemorated on at least two German stamps.

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