Directly below is a picture of the original hemispheres
in the Deutches Museum in Munich.
| Most physicists have seen the woodcut at the
right, in which eight horses on the right and eight horses on the left
are trying to pull apart two evacuated hemispheres. The experiment was
performed in 1657 at Magdeburg, Germany by Otto von Guericke (1602-1686),
the Burgomeister (mayor) of Magdeburg. The hemispheres were made of bronze
and were about 1.2 feet across. Von Guericke calculated that a force of
almost 2700 pounds would be needed to pull the two hemispheres apart.
Directly below is a picture of the original hemispheres
in the Deutches Museum in Munich.
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Otto von Guericke studied at Leipzig and Helmstedt, and
studied law at Jena 1621-1622. He then studied law at Leiden, and attended
lectures on mathematics and engineering. During the rest of his long career
he served as an engineer and a diplomat, and kept abreast of developments
in science.
The pump at the left was constructed by von Guericke ca. 1650 to study the production of vacua and experiments with them. He was the first to show that a bell cannot be heard in a vacuum, that animals will die in a vacuum, fruit can be preserved if they are placed under the evacuated bell jar, and a flame cannot be maintained in a vacuum. In his early experiments with evacuated vessels, von Guericke
used casks, but soon realized that a spherical, metallic vessel was necessary
to prevent implosion due to air pressure.
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| Horses are not readily available in lecture halls, and so smaller-scale demonstrations like the one shown at the right are commonly used. The two hemispheres have been fitted together and pumped out with the double-barrel vacuum pump, and the two boys are unable to pull them apart. Indeed, a large and strong person typically cannot separate the two halves. |  |
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Magdeburg hemispheres from the latter part of the nineteenth
century and the early part of the twentieth century were rarely marked
with the name of the maker. The handle on the side where the valve is located
unscrews, allowing the hemispheres to be threaded onto the opening in the
base plate of the vacuum pump. If you use a hemisphere in a demonstration,
be sure to smear vacuum grease onto the matching faces. A number of years
ago I omitted to do this, and gave what I thought was an evacuated pair
of hemispheres to a large, premed football player at Kenyon. He gave a
mighty heave, met no resistance, and fell backward spectacularly!
The pair of hemispheres at the left is at the United States Military Academy at West Point, New York. |
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University of Texas at Austin |
| And now a variation on the theme. Jack Judson, the curator of the Judson Collection at the Magic Lantern Museum in San Antonio, Texas, pointed out that the bottom support of this Magdeburg Hemisphere pair is cupped and ground flat on the bottom, thus allowing it to be placed directly on top of the hole in the pump plate. The usual technique is to make a screw connection to this hole, or, in the Texas example directly above, use a rubber hose to make the connection. |  |