bell jar vs protective covering

bell jar and protective covering both are nouns.

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

As nouns, protective covering is a hypernym of bell jar; that is, protective covering 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 covering: a covering that is intend to protect from damage or injury

Other hypernyms of bell jar include protection, protective cover.

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