Magdeburg hemispheres

The Magdeburg hemispheres are a pair of large copper hemispheres with mating rims that were used in a famous 1654 experiment to demonstrate the power of atmospheric pressure. When the rims were sealed with grease and the air was pumped out, the sphere contained a vacuum and could not be pulled apart by teams of horses. Once the valve was opened, air rushed in and the hemispheres were easily separated. The Magdeburg hemispheres were invented by German scientist and mayor of Magdeburg,^[1] Otto von Guericke, to demonstrate the air pump that he had invented and the concept of atmospheric pressure.

Speculation varied about the contents of the sphere. Many thought it was simply empty, while others argued the vacuum contained air or some finer aerial substance drawn from the mercury. Sound did not transmit through the sphere, indicating that sound needed a medium in order to be heard, while light did not.^[2]

The first artificial vacuum had been produced a few years earlier by Evangelista Torricelli and inspired Guericke to design the world's first vacuum pump, which consisted of a piston and cylinder with one-way flap valves. The hemispheres became popular in physics lectures as an illustration of the strength of air pressure, and are still used in education. The original hemispheres are on display in the Deutsches Museum in Munich.

Aside from its scientific importance, the experiment served to prove the recovery of the city of Magdeburg, which only two decades earlier had undergone the Sack of Magdeburg - considered the worst atrocity of the Thirty Years' War - when 20,000 of its inhabitants were massacred, and only 4,000 remained at the end of the war in 1648. Von Guericke was concerned with both aspects of the experiment, in his double capacity as a leading scientist and as the mayor who worked tirelessly to restore the city's wealth.