Teaching the basic concepts of air pressure has always been one of my favorite units in Physical Science. There are so many great demonstrations, some with long colorful histories. One classic standby is the use of the famed Magdeburg Hemispheres. The Magdeburg Hemisphere demonstration was invented in 1656 by Otto von Guericke, then mayor of Magdeburg, Germany.
Having just invented the world’s first vacuum pump, Von Guericke set to work creating a device to demonstrate its valuable contribution to science. That device was the Magdeburg Hemispheres. Von Guericke’s original spheres were much larger than those commonly available today and made of thick metal. He used them to dramatically demonstrate the pressure of the atmosphere by evacuating them and using two teams of 15 horses to attempt and pull them apart. Of course the horses failed to separate them.
Most spheres commonly sold today are made of cheap black plastic and meant to be evacuated with a typical classroom vacuum pump. They do a reasonable job of demonstrating the basic concept, but, in my own experience, do not hold up well to normal classroom use. Over the span of my 20 year career I have probably had to replace these hemispheres at least five times.