
Magdeburg Hemispheres - rfreytag
https://en.wikipedia.org/wiki/Magdeburg_hemispheres
======
darekkay
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 :)

~~~
rerx
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.

~~~
wolfgke
> 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...](https://en.wikipedia.org/w/index.php?title=Otto_Waalkes&oldid=874260876))
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
_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?

~~~
mirimir
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](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).

~~~
posterboy
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.

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

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

~~~
jonsen
So 1 atom per cubic _micro_ meter is about right for 0.1 microbar.

~~~
TheSpiceIsLife
One cubic micrometer is 1mm³ right?

~~~
jonsen
One cubic micrometer is 1µm³.

One cubic millimeter is 1mm³.

~~~
TheSpiceIsLife
Oh yeah, duh. Thanks.

------
kahlonel
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.

