A piece of Laboratory Equipment used to contain vacuums.
- Jars can be similar in shape to a bell
- 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.
- 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 the seal.
- For this reason, a bell jar cannot be used to contain pressure 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 the propagation of sound is mediated by the air.
- Deprived of its medium, the sound cannot travel.
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