|This article relies largely or entirely upon a single source. (March 2011)|
A bell jar during a low-pressure test
|Uses||Enclosing objects, containing gases or a vacuum|
A bell jar is a piece of laboratory equipment used to contain vacuum. It is similar in shape to a bell, and can be manufactured from a variety of materials (ranging from glass to different types of metals). A bell jar is placed on a base which is vented to a hose fitting that can be connected via a hose to a vacuum pump. By pumping the air out of the bell jar, vacuum is formed.
The lower edge of a vacuum bell jar forms a flange of heavy glass, ground smooth on the bottom for better contact. The base of the jar is equally heavy and flattened. A smear of vacuum grease is usually applied between them. As the vacuum forms inside, it creates a considerable compression force, so there is no need to clamp the seal. For this reason, a bell jar cannot be used to contain pressures above atmospheric, only below.
Purely decorative bell jars were common in the Victorian period for the display of clocks and taxidermy, as well as transparent dust covers. These are made of thin glass, with more care being taken regarding their optical clarity. They did not have a thickened base flange. For this reason, they are not suitable for vacuum use and would usually fail if pumped down. Other similar glass domes were also used as cheese domes or garden cloches.
Bell jars are generally used for classroom demonstrations or by hobbyists, when only a relatively low-quality vacuum is required. Cutting-edge research that needs an ultra high vacuum requires a more sophisticated form of vacuum chamber. However, several tests may be completed in a chamber with an effective pump and low leak rate.
An example of a classroom science experiment involving a bell jar is to place a ringing alarm clock under the bell jar. As the air is pumped out of the sealed bell jar, the noise of the alarm clock fades, thus demonstrating that the propagation of sound is mediated by the air. Deprived of its medium, the sound waves cannot travel.
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