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.
While the plates lack the traditional hemispheric shape, what is gained from the shape change is significant.
By changing the area exposed to atmospheric pressure to a two dimensional circular surface, my 9th graders had no problem calculating the exact amount of pressure holding the plates together. In addition, because the plates are two dimensional it allowed the designer to provide three different sized grooves and “O”-rings to actually change this area. When the area is decreased, the force that holds the plates together is also decreased. Not only can my students do the calculations to determine the new areas and corresponding forces, but they can “feel” them as well. Using the largest groove and “O”-ring creates an area that requires roughly 170 lbs of force to separate. Using the smallest “O”-ring, it only takes little over 60 lbs.
The product also comes with a very nice manual with a suggestion that I had never thought of. Once I have my students calculate the force required for a given area, I have one student stand on a bathroom scale holding the upper handle of the evacuated plates while another student sits on the floor in front of the first student and pulls downward on the lower handle. The students then watch the scale and note the maximum weight it records before the plates separate. This weight, subtracted from the student’s weight, roughly approximates their calculated force.
I particularly like the fact that the vacuum plates come with their own hand pump. While I own both a classic large laboratory electrical vacuum pump and a smaller “squeeze-type” pump, I love the fact that the included pump has an obvious mechanism that students can see. The creation of the vacuum between the plates becomes something transparent and understandable rather than a magic “black box.” This same pumping system is used in Educational Innovations’ mini-bell jars, and I love those, too.
Finally, I am most impressed with the strength and durability of these plates. My set has been dropped, kicked, and beaten in every way imaginable by 9th graders over the past 5 years, and they still work like they did the day I took them out of the box. I used to guard my Magdeburg Hemispheres protectively. Now I pass these plates around the room and just let my students “have at them.” It’s nice to have the kind of durability that turns a quick “one-off” demo into a truly “hands-on” experience. Thank you EI!