Magdeburg Hemispheres | Hacker News Reader

Magdeburg Hemispheres

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49 pointsrfreytag18 comments

I'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 :)

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.

> 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&oldi...) and 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.

It 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?

Simple 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

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).

I think it's more illustrative to think of atoms/molecules per cubic meter. If I remember correctly, 1 or 0.1 µbar is proportional to around 1 atom per cubic meter.

That’s a lot of pressure made by one tiny little atom. I can’t believe that.

3x10^22 atoms per cubic meter of air. That micro is going to make it 3x10^16 atoms.

So 1 atom per cubic micrometer is about right for 0.1 microbar.

One cubic micrometer is 1mm³ right?

One cubic micrometer is 1µm³.

One cubic millimeter is 1mm³.

Oh yeah, duh. Thanks.

Right, a cubic micrometer is 1E-18 cubic meter. So there are ~2.5 molecules per cubic micrometer at ~0.1 microbar.

I think there's a few digits missing here even. 1 mol = 6.022E23 particles = 22.4 L at STP for an ideal gas, and 1 m^3 = 1000 L

Of course real air isn't quite an ideal gas, but some reasonable approximations [1] come out to 2.53E25 molecules (N2 and O2) per cubic meter at 15 C and 1 atm.

[1] https://www.quora.com/How-many-air-molecules-are-present-in-...

Right. So 0.1 µbar (1E-7 bar) means ~2.5E18 molecules per cubic meter.

You have ~1 molecule per cubic meter at ~1E-25 bar.

Evvacuating 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%).

Thanks. Well, that is extremely counter-intuitive.

I 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.

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