Bell Jar

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

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.