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Magdeburg Hemispheres
- kahlonel 8y agoI have been in Magdeburg for 4 years, and these hemispheres are a big deal here. Fun fact: If you graduate from the CEE department of OvG university as a PhD, people put you on a cart and roll you towards the site where these hemispheres are displayed and celebrate once you are there.
- darekkay 8y agoI'm from Magdeburg and both the experiment and Otto-von-Guericke is known to everyone here. Apart from places being called after him (like our university), there are hemisphere statues all over the city. The experiment is also repeated/recreated from time to time on bigger events. I still did not expext my home town to ever appear on HN frontpage :)
- rerx 8y agoThe city has a weird fascination with people called "Otto". Equally revered is Otto I, Holy Roman Emperor, who a 1000+ years ago made Magdeburg his city of residence.
- wolfgke 8y ago> The city has a weird fascination with people called "Otto". Equally revered is Otto I, Holy Roman Emperor, who a 1000+ years ago made Magdeburg his city of residence. Also the German comedian Otto Waalkes (https://en.wikipedia.org/w/index.php?title=Otto_Waalkes&oldid=874260876 https://en.wikipedia.org/w/index.php?title=Otto_Waalkes&oldi...) and Otto von Bismarck (https://en.wikipedia.org/wiki/Otto_von_Bismarck https://en.wikipedia.org/wiki/Otto_von_Bismarck) - even though in opposite to Otto I and Otto von Guericke these two are not from Magdeburg.
- yesenadam 8y agoIt is unclear how strong a vacuum Guericke's pump was able to achieve, but if it was able to evacuate all of the air from the inside, the hemispheres would have been held together with a force of around 20 000 N (4400 lbf, or 2.2 short tons) I'm no expert, but if it was able to evacuate all of the air from the inside sounds unlikely - impossible, even. What's the best we could do nowadays? How total are (practical) vacuums?
- jessriedel 8y agoEvvacuating 99.9999% of the air is technologically much harder than merely 95% of it, but the difference in the force exherted on the spheres is small (only 5%).
- yesenadam 8y agoThanks. Well, that is extremely counter-intuitive.
- mirimir 8y agoSimple mechanical vacuum pumps can reach ~1 mbar (0.001 atmosphere).[0] I doubt that Guericke's pump could reach even 100 mbar, but that's only 10% of atmospheric pressure. So the force to separate the hemispheres would have been only ~2 short tons. That's still a lot, however. The best multistage pumps can reach ~0.0001 mbar.[0] Generally, attainable vacuum by pumping depends on leakage rate vs pump speed, and also on the vapor pressures of components. Such as oil used for lubrication, grease for sealing, rubber or plastic hoses, etc. So for a high vacuum system, you want just metal and glass, and you want them to be very clean. Fingerprints are a huge issue. Also, you typically put a cold trap between your pump and the working volume. Both to get lower pressure, and to avoid contamination. For low vacuum, dry ice in acetone is OK. Then liquid nitrogen, and finally liquid helium. 0) https://cds.cern.ch/record/1046850/files/p43.pdf https://cds.cern.ch/record/1046850/files/p43.pdf Edit: It's important to avoid contamination of the working volume (say, by pump oil) and of the pump (say, by water, or corrosive substances).