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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. ==Vacuum bell jar== 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.〔(【引用サイトリンク】url=http://www.belljar.net/about.htm )〕 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.〔(【引用サイトリンク】url=http://www.belljar.net/about.htm )〕 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.〔(【引用サイトリンク】url=http://www.belljar.net/about.htm )〕 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.〔(【引用サイトリンク】title=Bell Jar Experiment )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bell jar」の詳細全文を読む スポンサード リンク
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