What is a circuit?

A flow of electric charge constitutes an electric current. In order for a current to exist in a conductor, there must be an electromotive force (emf) or potential difference between the conductor's ends. An electric cell, a photo-voltaic cell, and a generator are all sources of emf. An emf source with an external conductor connected from one of the source's two terminals to the other constitutes an electric circuit . . .

(from "Electricity" The Concise Columbia Encyclopedia, 1983 page 258. Levey & Greenhall, Eds.)

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