3B Scientific Magdeburg Hemispheres - Manual

3
Instruction Sheet
3B SCIENTIFIC® PHYSICS
U30010 Magdeburg Hemispheres
7/03 ALF
®
The Magdeburg hemispheres are used to demonstrate
the effect of atmospheric pressure (historical experiment
according to Otto von Guericke).
1. Safety instructions
• Do not try to pry the two evacuated hemispheres
apart using tools.
• Do not try to separate the hemispheres using ropes
or similar, because when they do separate they can
turn into projectiles.
• When ventilating the hemispheres hold on tight to
the apparatus so that you can avoid any damage
from the hemispheres falling.
• Ample space should be left behind the persons try-
ing to separate the Magdeburg plates.
2. Description, technical data
Two plastic hemispheres with handles and inlaid rubber
sealing rings can be joined vacuum tight. One hemisphere
is equipped with stopcock or ventilating valve and hose
connection. A short hose completes the apparatus.
Vacuum connection: 8 mm
Diameter: 120 mm
Hose length: 110 mm
3. Operating principle
A protective mantle of air called the atmosphere sur-
rounds the earth. The air molecules like all other materi-
als are subject to the force of gravity and thus become
concentrated at the earth’s surface. The atmospheric pres-
sure, which is exerted by the air mass, is highest at sea
level and gradually decreases with increasing distance
from the earth’s surface. Air pressure like any liquid ex-
erts equal and constant pressure on all sides of an object.
In an open object equilibrium always prevails between
internal and external pressure. If the internal pressure
drops more than the external pressure, the state of equi-
librium is reestablished by letting air in. Conversely, if the
internal pressure becomes greater than the prevailing
external pressure, there is the same tendency to escape.
In an enclosed object the force resulting from the differ-
ence between the internal and external pressures affects
the object’s surface either causing it to compress or in
the latter case causing the object to explode.
The physicist and mayor of Magdeburg Otto von Guericke
was the first to demonstrate the effect of atmospheric
pressure. His vacuum experiments commenced around
1650 and reached their zenith in 1654 in a spectacular
experiment attended by Kaiser Ferdinand III in the town
of Regensburg: the famous Magdeburg hemispheres. To
do this von Guericke evacuated the air out of two copper
hemispheres of 42 cm diameter and sealed with an oil
and wax saturated leather lining. The air pressed the hemi-
spheres together with such force that even 16 horses
were unable to pull them apart.
4. Operation
• Before starting the experiment check ensure that the rub-
ber seal ring is correctly situated and that it is undamaged.
1 Vacuum hand-operated pump
2 Hose
3 Stopcock or ventilation valve with hose nipple
4 Hemisphere
5 Handle
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4
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