Magdeburg hemispheres

The Magdeburg hemispheres were a pair of large copper hemispheres precisely cast so that their rims fit tightly together. When the rims were sealed with grease and the air was pumped out, the resulting sphere contained the world's first artificial vacuum. The Magdeburg hemispheres were designed by German scientist Otto von Guericke in 1650 to disprove Aristotle's famous supposition that 'Nature abhors a vacuum'. To get the air out of the sphere, von Guericke also designed the world's first vacuum pump, which consisted of a piston and cylinder with one-way flap valves. To power the machine, several people would turn a crank arm connected to the vacuum pump.

Overview

The Magdeburg hemispheres, a little over a foot (30 cm) in diameter, were designed to demonstrate the vacuum pump that von Guericke had invented. When the air was pumped out from inside them, they were held firmly together by the air pressure of the surrounding atmosphere.

Famous demonstration

Guericke's most famous performance was carried out on 8 May 1654 before the Emperor Ferdinand III, and attracted crowds from all over Saxony. Before the emperor and a large crowd, Guericke greased the rims of the two hemispheres and carefully fitted them together. Then the local blacksmith began vigorously pumping the air from inside the sealed copper globe. After a while he was joined by his assistants, as the cranking of the pump gradually became more and more laborious.

The crowd then watched as a team of eight horses harnessed together was led into the square and attached to one hemisphere of the copper globe. Another team of eight horses was then attached to the other hemisphere. At a signal from Guericke, the two teams of horses strained forward in opposite directions, attempting to pull the two hemispheres apart, but no matter how hard they pulled, the horses could not separate the spheres. Guericke told the crowd that all that was holding the hemispheres together was the pressure of the air surrounding them. The vacuum inside the globe meant there was no opposing pressure to balance this great outer force.

Other demonstrations

Later, the demonstration was presented to the Reichstag and the Emperor Ferdinand III in 1654 in Regensburg, where 30 horses, in two teams of 15, could not separate the hemispheres until the vacuum was released. In 1656 he repeated the demonstration with 16 horses (2 teams of 8) in his hometown of Magdeburg, where he was mayor. In 1657, Gaspar Schott was the first to describe the experiment in print in his Mechanica Hydraulico-Pneumatica. In 1663 the demonstration was given in Berlin to the Elector of Brandenburg (or according to some sources in 1661 before Elector Friedrich Wilhelm) with 24 horses.

Reports of the number of horses in each experiment vary widely. This confusion is due in part to the multiple repetitions of this experiment, but also to the unnecessary use of two opposing teams of horses. Two teams of n horses pulling against each other cannot generate any more force than a single team of n horses pulling against an immovable object.

The original hemispheres are maintained by the Deutsches Museum in Munich. Many copies of them (usually smaller) have been made to illustrate the principle of air pressure to students. Re-enactments of von Guerike's 1654 experiment are performed in locations around the world by the Otto von Guericke Society. The experiment has been commemorated on at least two German stamps.

Guericke's experiment soon became so famous that he was demonstrating it all over Germany. Tales of his feat with the hemispheres soon spread all over Europe. Soon news of this phenomenon reached the likes of Robert Boyle, the founder of modern chemistry. Within four years, in 1654, Boyle and his assistant Robert Hooke had designed an air pump, which they attached to a vacuum flask. From this, through various experiments, they formulated what is called Boyle's law, which states that the volume of a body of an ideal gas is inversely proportional to its pressure. Soon the ideal gas law was formulated.

Based on these concepts, in 1679, an associate of Boyle's named Denis Papin built a bone digester, which is a closed vessel with a tightly fitting lid that confines steam until a high pressure is generated. Later designs implemented a steam release valve to keep the machine from exploding. By watching the valve rhythmically move up and down, Papin conceived of the idea of a piston and cylinder engine. He did not, however, follow through with his design. Nevertheless, in 1697, based on Papin's designs, engineer Thomas Savery built the world's first steam engine.

Trivia

A variation of Schott's sketch featuring a single pair of horses trying to separate a pair of jeans appears on the tag of Levi Strauss & Co. jeans.

References

published in print Summer 2004.

External links

Magdeburg Hemispheres