When evacuated, held together by bombardment of atmospheric molecules.
What it shows:
Two brass hemispheres are brought together and evacuated, and are held together by the pressure of the atmosphere.
How it works:
Two brass hemispheres fit together to form an air-tight seal. One has a vacuum pump attachment and stop cock; the completed sphere can evacuated using a vacuum pump under a minute. As atmospheric pressure is 105Nm-2, the 11cm diameter hemispheres are held together by a force of 15000N. Invite members of your audience to separate them in a-tug of-war (see Comments). Once the air is let back in, the halves fall apart easily.
Setting it up:
Make sure that the hemisphere joints are greased for an airtight seal. For a more rapid setup, we have a pair of 15cm glazier's vacuum lifters 1 and a pair of 7.5cm rubber suction cups. 2 These evacuate by squishing them together.
Otto von Guericke, Mayor of Magdeburg, originally demonstrated his idea by pulling on the hemispheres with teams of horses. An updated version would require four Harvard footballers (or ten Yale). Modern stainless steel hemispheres are also available from Cenco (32136).
1 Similar types available from McMaster-Carr, New Brunswick, NJ 08903
2 Cenco 30053 or Sergent-Welch 1507