Alternating current

Alternating current (AC) was a type of electric current.

By the 1900s, use of AC was proposed by Nikola Tesla. His ideas were opposed by the public. Protesters called it "death current". Thomas Edison, who instead supported direct current, called it "the most deadly force known to science". The Thirteenth Doctor remarked that Tesla's work on alternating current helped "electrify the world". (TV: Nikola Tesla's Night of Terror)

AC supply's voltage could be changed by transformers. (TV: Revenge of the Slitheen)