This article is about the laboratory equipment. For the novel by Sylvia Bell jar book pdf, see The Bell Jar. This article relies largely or entirely on a single source. Relevant discussion may be found on the talk page.
Please help improve this article by introducing citations to additional sources. A vacuum 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. A vacuum is formed by pumping the air out of the bell jar.
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.