Short answers, pulled from the story.
Alternating current is an electric flow that periodically reverses direction and changes its magnitude continuously with time. Most power circuits use a sine wave where the positive half-period corresponds to the forward direction of the current.
French instrument maker Hippolyte Pixii constructed the first alternator in 1832 based on principles discovered by Michael Faraday. This early device produced pulses rather than a smooth sine wave until later modifications improved the output.
Thick cables used for high-current transmission often become hollow to reduce mass and cost since the interior metal carries little electricity due to the skin effect. At 60 hertz, the skin depth of copper is approximately 8.57 millimeters which shrinks the usable cross-section significantly compared to direct current flow.
In autumn 1834 three Hungarian engineers named Károly Zipernowsky, Ottó Bláthy, and Miksa Déri determined open-core devices were impractical for reliable voltage regulation. They patented closed magnetic circuits where copper windings wrapped around iron cores in either toroidal or shell forms.
Electrical energy travels efficiently as alternating current because transformers can increase or decrease voltage levels without complex mechanical parts. Power lines operate at hundreds of kilovolts to minimize heat loss caused by wire resistance during long-distance transport.