A bell jar is a glass jar, similar in shape to a bell (i.e. in its best-known form it is open at the bottom, while its top and sides together are a single piece), 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. It is a common science apparatus used in experiments. Bell jars have a limited ability to create strong vacuums; vacuum chambers are available when higher performance is needed. They have been used to demonstrate the effect of vacuum on sound propagation.

In addition to their scientific applications, bell jars may also serve as display cases or transparent dust covers. In these situations, the bell jar is not usually placed under vacuum.

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 done at ultra-high vacuum requires a more sophisticated vacuum chamber. However, several tests may be completed in a bell jar chamber having an effective pump and low leak rate.

Some of the first scientific experiments using a bell jar to provide a vacuum were reported by Robert Boyle. In his book, New Experiments Physico-Mechanicall, Touching the Spring of the Air, and its Effects, (Made, for the Most Part, in a New Pneumatical Engine), he described 43 separate experiments, some of which were carried out with Robert Hooke, investigating the effect of reducing the air pressure within the bell jar on the objects contained within.

One of the best known of these experiments involved placing a ringing bell inside the jar, and observing that upon pumping out the air, the ringing disappeared. This experiment demonstrated that the propagation of sound is mediated by the air, and that in the absence of the air medium, the sound waves cannot travel. This experiment is often used as a classroom science experiment, where the experiment is repeated with an item such as an alarm clock placed under a bell jar, and the noise of the alarm clock fading as the air is pumped out being used to demonstrate the effect. By additionally placing a microphone inside the bell jar and observing that the sound detected by the microphone reduces as the air is pumped out, the effect of absorbance of the sound by the glass of the jar itself can be excluded.

Another common experiment using a bell jar involves placing a jar over a lit candle, and observing that the flame goes out, demonstrating that oxygen is required for combustion. A common variation of this experiment is to place the candle and bell jar over water, and to observe that when the candle extinguishes, the water level will rise inside the bell jar. The explanation for this observation is that the heating of the air inside the jar by the candles causes it to expand, and when the candle has exhausted the oxygen supply and extinguishes, the air will cool and contract, leading to the water being drawn up to fill the space. A common misconception is that the water level rises to replace the consumed oxygen, but since the combustion reaction produces carbon dioxide gas as a product, this explanation is not correct.