A bell jar during a low-pressure test
|Uses||Enclosing objects, containing gases or a vacuum|
A bell jar is a glass jar, similar in shape to a bell, and can be manufactured from a variety of materials (ranging from glass to different types of metals). Bell jars are often used in laboratories to form and contain a vacuum; they may also serve as display cases or transparent dust covers.
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.
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. In the absence of their medium, the sound waves cannot travel.
Decorative or preservative
Purely decorative bell jars were commonly used in the Victorian period to display and serve as transparent dust covers for clocks and taxidermy. Decorative bell jars were made of thin glass, with more care being taken regarding their optical clarity, and 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.
Similar glass domes were used as cheese domes or garden cloches.
In popular culture
- The Bell Jar (1963) is a roman à clef by Sylvia Plath, originally published under the pen name "Victoria Lucas"
- "The Jar" (November 1944) is a short story by Ray Bradbury, published in Weird Fiction