The Magdeburg hemispheres were a pair of large copper hemispheres precisely cast so that their rims fit tightly together. Magdeburg was a small place in Germany.
When the rims were sealed with grease and the air was pumped out, the resulting sphere contained the world's first artificial vacuum.
To get the air out of the sphere, von Guericke designed the world's first vacuum pump, which consisted of a piston and cylinder with one-way flap valves. To power the machine, several people would turn a crank arm connected to the vacuum pump.
The Magdeburg hemispheres, a little over a foot (30 cm) in diameter, were designed to demonstrate the vacuum pump that von Guericke had invented. When the air was sucked out from inside them, they were held firmly together by the air pressure of the surrounding atmosphere.
Guericke greased the rims of the two hemispheres and carefully fitted them together. Then the local blacksmith began vigorously pumping the air from inside the sealed copper globe. After a while he was joined by his assistants, as the cranking of the pump gradually became more laborious.
As the cranking of the pump became more and more laborious a team of eight horses harnessed together was used on either end. At a signal from Guericke, the two teams of horses strained forward in opposite directions, attempting to pull the two hemispheres apart. But no matter how hard they pulled, the horses could not separate the spheres. The adjacent photograph shows the experiment conducted.
Guericke told the crowd that all that was holding the hemispheres together was the pressure of the air surrounding them.
The vacuum inside the globe meant there was no opposing pressure to balance this great outer force.
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