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A voltaic cell.

Voltaic cell

A voltaic cell.

"Just as a head of water supplies a hydraulic pressure that causes the liquid to flow through a pipe in spite of friction, so there is an electrical pressure that forces a current through a conductor in spite of its resistance. The unit of electrical pressure is called the volt." — Avery, 1895

Voltaic cell

"Just as a head of water supplies a hydraulic pressure that causes the liquid to flow through a pipe…