| Modern space epics leave students with the impression that
space ships make sounds that can be heard to an outside observer as they
zoom through the interplanetary void. We have known that this is not true
since the time of Otto
von Guericke, the mid-seventeenth century burgomeister of Magdeburg
in southwest Germany. In addition to developing the first vacuum pumps,
von Guericke devised a number of demonstrations of phenomena in a vacuum,
one of which was placing a bell in a vacuum, ringing it, and showing that
the sound could not be heard by an outside observer.
The apparatus collection at Dartmouth College includes this example of a bell that can be placed under the bell jar of a vacuum pump.
| At the left is a complete set-up for doing the bell in
a vacuum demonstration. In April 2000 it was at Kennedy Antiques in Durham,
North Carolina. Today we use essentially the same idea, except that the
bell is replaced by an electric bell or buzzer.
The bell-in-vacuo device at the right, at St. Patrick's College in Maynooth, County Kildare, Ireland, is quite different from the other devices. The small bell is suspended from the vertical stalk inside the glass sphere. With the air still inside the sphere, the apparatus is shaken, and the bell can be heard. The apparatus is then pumped out, the stopcocks closed, and the system is removed from the vacuum pump. Now, shaking produces no sound. This system eliminates the problem of passing a sliding mechanical motion through the wall of the evacuated space.
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