Magdeburg Hemispheres (1654): Otto von Guericke's Vacuum

The Magdeburg hemispheres were two copper half-spheres that Otto von Guericke, mayor of Magdeburg, used to demonstrate the force of air pressure. After he pumped the air from between them, teams of horses could not pull them apart. Guericke first showed his vacuum experiments to the emperor at Regensburg in 1654, and the horse demonstration became one of the most famous experiments in the history of science.

Background: an engineer-mayor and the vacuum

Otto von Guericke was born in Magdeburg on 20 November 1602 into a patrician family. He studied law and mathematics at Leipzig, Jena and Leiden and became an engineer. He survived the Sack of Magdeburg in 1631, when imperial troops destroyed most of the city, and later helped to rebuild it. From 1646 he served as one of its burgomasters for about three decades.

Guericke was drawn to an old question: can empty space exist? Aristotle and most scholars said no, and Descartes argued that space was always full of matter. Guericke decided to settle the question by experiment, independently of Torricelli’s mercury tube of 1643.

The air pump

Around 1650 Guericke built the first air pump. He first tried to pump water out of a sealed barrel, and then air out of a copper sphere, which at one point collapsed with a bang under outside pressure. His later pumps used a piston and valves, worked by hand, to draw air from a vessel stroke by stroke.

With it he showed that a candle flame went out in the evacuated vessel and that animals could not survive without air. He also showed that air has weight by weighing a vessel before and after pumping.

Regensburg, 1654

In 1654 Guericke was at Regensburg as a delegate to the Imperial Diet, the assembly of the Holy Roman Empire, which was meeting with Emperor Ferdinand III. There he demonstrated his pump and his vacuum experiments to the emperor and the assembled princes. The archbishop-elector of Mainz, Johann Philipp von Schönborn, bought one of the devices.

The Jesuit scholar Kaspar Schott, a professor at Würzburg, studied the apparatus and published the first description of Guericke’s experiments in his Mechanica hydraulico-pneumatica of 1657. This book spread news of the work across Europe.

The horses

For his most dramatic experiment Guericke had two copper hemispheres made with carefully fitted rims, sealed with a leather ring soaked in wax and turpentine. Sources usually give their diameter as roughly 40–50 centimetres. When the air was pumped out, they held together firmly.

Teams of horses were harnessed to rings on each side and driven apart, but they could not separate the hemispheres. When a valve was opened to let air back in, the halves fell apart by hand. Guericke’s own book of 1672 illustrates the experiment with 16 horses, eight on each side.

The dates and details of the horse demonstrations are not fully certain. Many accounts place the first at Regensburg in 1654, but the best evidence points to a public demonstration in Magdeburg around 1656–1657 and another at Berlin in 1663 for Frederick William, Elector of Brandenburg.

The science: why the hemispheres held

The hemispheres did not stay together because the vacuum “pulled” them. They were pushed together by the weight of the atmosphere, which presses on every surface at sea level with about one kilogram-force on each square centimetre. With most of the air removed from inside, there was little pressure pushing back from within.

The total force depends on the area of the opening. For hemispheres about half a metre across, a perfect vacuum would mean a force of roughly two tonnes holding them shut. Guericke’s pump could not remove all the air, so the real force was somewhat lower, but still far beyond what the horses could overcome. In physical terms, pulling with horses on both sides gave no advantage over tying one side to a post; the second team mainly made the show more impressive.

Consequences

Schott’s book reached Robert Boyle in England, who with Robert Hooke built an improved air pump in 1658–1659. Boyle’s experiments led to his New Experiments Physico-Mechanicall (1660) and to Boyle’s law relating the pressure and volume of a gas. The air pump became the emblem of the new experimental science.

Guericke described all his experiments in Experimenta Nova (ut vocantur) Magdeburgica de Vacuo Spatio, published in Amsterdam in 1672. It also described a spinning sulphur globe that produced static electricity when rubbed, an early electrical machine. He was raised to the nobility in 1666, adding “von” to his name, and died in Hamburg on 11 May 1686.

Timeline

• 1602, 20 November: Otto Gericke is born in Magdeburg.
• 1631, May: He survives the sack of Magdeburg.
• 1646: He becomes a burgomaster of Magdeburg.
• c. 1650: He builds his first air pump.
• 1654: He demonstrates his vacuum experiments at the Imperial Diet in Regensburg.
• 1657: Kaspar Schott publishes the first account of the experiments.
• 1663: A horse demonstration is staged in Berlin.
• 1672: Guericke’s Experimenta Nova is published in Amsterdam.
• 1686, 11 May: Guericke dies in Hamburg.

Why it matters

The Magdeburg hemispheres made the invisible pressure of the atmosphere dramatically visible to rulers and the public. Guericke’s air pump opened up the study of the vacuum and gases, and his demonstration remains a classic of science teaching.