Past tense of 'alternate': to switch back and forth between two or more things repeatedly.
From Latin 'alternatus,' the past participle of 'alternare' (to do by turns, to alternate), itself from 'alternus' meaning 'every other' or 'alternate.'
Alternating currents literally power our modern world—Thomas Edison's 'War of Currents' against Nikola Tesla's AC proved that alternating patterns are more efficient for transmitting electricity over long distances than direct current.
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