bell jar vs protective cover

bell jar and protective cover both are nouns.

Word	Noun	Adjective	Verb	Adverb
bell jar	Yes	No	No	No
protective cover	Yes	No	No	No

As nouns, protective cover is a hypernym of bell jar; that is, protective cover is a word with a broader meaning than bell jar:

bell jar: a bell-shaped glass cover used to protect and display delicate objects or to cover scientific apparatus or to contain gases
protective cover: a covering that is intend to protect from damage or injury

Other hypernyms of bell jar include protection, protective covering.

bell jar (noun)	protective cover (noun)
a bell-shaped glass cover used to protect and display delicate objects or to cover scientific apparatus or to contain gases	a covering that is intend to protect from damage or injury