1590s, "do by turns" (transitive), from Latin alternatus, past participle of alternare "do one thing and then another, do by turns," from alternus "one after the other, alternate, in turns, reciprocal," from alter "the other" (see alter). Replaced Middle English alternen "to vary, alternate" (early 15c.). Transitive meaning "interchange reciprocally" is from 1850; intransitive sense "follow one another in time or place" is from c. 1700; that of "pass back and forth between actions, conditions, etc." is by 1823. Related: Alternated; alternating.
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Harper, D. (n.d.). Etymology of alternator. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/alternator