The hemisphere of Magdeburg

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Atmoshperic pressure of around 100 000 N/m² acts on the external surface of the sphere. When the valve of the sphere is open the pressure on the inside and on the ouside of the sphere is equal (see picture below).

When air is being sucked out of the sphere and the valve remains closed there is a vacuum on the inside of the sphere. Since there is hardly some air on the inside of the sphere and air pressure is about zero, nearly no force is exerted on the internal surface of the sphere. On the outside nothing changed the same pressure of about 1 atm acts in the external surface of the sphere (see picture below).
This pressure pushes both hemishperes aganst each other. To separate the hemispheres one has to apply a greater force than the force pressing the two halfs together.