A bell jar is a piece of laboratory equipment used for creating vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

s.http://www.belljar.net/about.htm It can be similar in shape to a bell

Bell (instrument)

A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...

, 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

Vacuum pump

A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...

. By pumping the air out of the bell jar, a vacuum is formed.

Bell jars are generally used for classroom demonstrations or by hobbyists, when only a relatively low-quality vacuum is required.