A bell jar is a piece of laboratory equipment used to contain vacuums. It can be similar in shape to a bell, and can be manufactured out of 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, which can be connected via a hose to a vacuum pump. By pumping the air out of the bell jar, a vacuum is formed.
The lower edge of a vacuum bell jar is a substantial flange of heavy glass, ground smooth on the base. Its base is equally heavy and flattened. A smear of vacuum grease is usually applied between them. As the vacuum inside gives rise to a considerable compression force, there is no need to clamp this 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 a transparent dust cover. These are of thin glass, with more care taken over their optical clarity. They do not have a thickened base flange. For this reason they are not suitable for vacuum use and will 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 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 cannot travel.
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