Magdeburg hemispheres

HOW

1. You need help from another person to perform this experiment.
2. Move the hemispheres to the centre of the device.
3. Close the valve located on the line leading to the pump.
4. Ask the other person to press the two hemispheres together.
5. Make several movements with the pump piston.
6. Check if the hemispheres can be easily separated. If not, you can stop pressing them.
7. Perform several more movements with the pump piston to pump the air outside the hemispheres.
8. Pulling on the ropes from both sides, try to disconnect the hemispheres. When the experiment is complete, open the valve.

WHY

Air molecules exert equal pressure on both the inner and outer sides of the hemispheres. However, if you pump out some of the air from inside the hemispheres, the pressure exerted from the inside is lower. As a result, the hemispheres are pressed against each other. After about 50 pump piston movements, the pressure in the hemispheres is about 1/2 the atmospheric pressure, after 80 – 1/3, and after 120 – 1/5.