To demonstrate Atmospheric Pressure. Consists of two metal hemi-spheres 4 inch (10cm) in diameter, each with a sturdy metal handle.
One of the spheres has a stopcock to attach vacuum tubing 8 mm ID.
Join the two hemi-spheres and connect to a hand pump or vacuum pump and evacuate the air in the hemi-spheres.
Try pulling the hemi-spheres apart, you will find it very hard, if not impossible to pull them apart.
Upon opening the stopcock and letting the air in, the hemispheres can be easily pulled apart.
Thereby showing the presence and effects of atmospheric pressure.