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We live at the bottom of an ocean of air and almost never notice its weight. In the 1600s many scholars still held that nature would not allow empty space at all. A German engineer and city mayor settled the argument with a show: two metal half-spheres, a pump, and teams of horses that could not pull them apart. This post covers what Otto von Guericke did, why the sources disagree on the details, a worked calculation of the force, and the traps students fall into.
Who was Otto von Guericke?
The 1911 Encyclopaedia Britannica entry gives the outline. He was born in Magdeburg on 20 November 1602, studied law at Leipzig, Helmstedt and Jena and mathematics at Leiden, became an engineer, and in 1646 became mayor of Magdeburg. He first tried to make a vacuum by pumping water out of a barrel, found that air leaked in through the wood, and switched to a copper globe. That made him, in the entry's words, "the inventor of the air-pump (1650)". He also built an electrical machine using a rotating ball of sulphur. He died in Hamburg on 11 May 1686.
He set out his results in a book published in Amsterdam in 1672, Experimenta nova (ut vocantur) Magdeburgica de vacuo spatio, "New Magdeburg experiments on empty space". A scan is held by the Biodiversity Heritage Library.
What happened with the Magdeburg hemispheres?
Two hollow metal hemispheres were fitted rim to rim and the air was pumped out from inside. With no air pushing outward from within, the air outside pressed them together so hard that horses pulling in opposite directions could not separate them. Open the valve, let air back in, and they fell apart by hand.
Here the sources disagree, and it is worth seeing how. The 1911 Britannica places the show before Emperor Ferdinand III at the Imperial Diet in Regensburg in 1654, says the hemispheres were copper, and states that "it is recorded that thirty horses" failed. The Deutsches Museum in Munich, which holds a set of hemispheres, describes 16 horses and brass hemispheres, dates that demonstration to 1661, and says its air pump matches Guericke's third design from about 1662. These two sources cannot settle which account is right, so this post reports both. When you write about him, say which source you are using.
Why did Guericke care about a vacuum?
The Deutsches Museum explains that his questions were partly about the cosmos. If the planets are not carried by crystal spheres and the stars are very far away, what fills the space between them? The traditional view was that nature avoids emptiness altogether. Guericke wanted to show that empty space could exist and be made with the right instruments. He argued from what the museum calls "the testimony of things themselves" rather than the authority of earlier scholars.
Worked example: how hard does the air push?
The NCERT Class 11 chapter on fluids gives atmospheric pressure at sea level as 1 atm = 1.013 × 10^5 Pa. Take sample numbers: hemispheres with a rim diameter of 0.50 m, and assume a perfect vacuum inside.
Find the right area. The net force acts across the circle where the rims meet, so A = πr² = π × (0.25)² = 0.196 m².
Force. F = ΔP × A = 1.013 × 10^5 × 0.196 ≈ 1.99 × 10^4 N, about 20 kN.
Make it concrete. That equals the weight of roughly 2,000 kg, since 1.99 × 10^4 ÷ 9.8 ≈ 2,030 kg.
A realistic pump. If the pump removes only half the air, the inside pressure is about half an atmosphere, ΔP halves and the force drops to about 10 kN.
What mistakes do students make with this experiment?
Using the curved surface area. The hemisphere's curved area is 2πr², but the pushes on the curved surface partly cancel sideways. The net pull apart depends only on the flat circle, πr².
Forgetting the inside pressure. The force comes from the difference between outside and inside. A poor vacuum means a smaller force.
Thinking two teams double the force. If eight horses pull on each side, the tension is what eight horses produce. Tie one hemisphere to a strong post and eight horses on the other side give the same pull. The second team mainly makes a better show. This is Newton's third law in action.
Mixing units. Keep pressure in pascals and area in square metres to get newtons.
The JEE (Main) 2026 syllabus lists "pressure due to a fluid column" and Pascal's law, so force-from-pressure questions like this are fair game. Check the current year's syllabus. The Deutsches Museum notes that Blaise Pascal was experimenting on empty space at the same time; our post on Blaise Pascal and pressure tells that side of the story.
What can a student take from how Guericke worked?
Change the method when it fails. Wood leaked, so he moved to metal.
Make the result impossible to ignore. A public test with horses persuaded people that an argument alone could not.
Build the tool you need. His pump went through several designs.
Write it down. His 1672 book let others check and repeat the work.
A practice routine: redo the example for a 0.30 m rim and for a pump that leaves 20% of the air inside, then explain in two sentences why a second team of horses is unnecessary. If forces from pressure are hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Next step: press a small suction cup onto a smooth tile, measure its diameter, and estimate the most force air could hold it with.
