The Barrage de la Rance was the first hydroelectric power plant to generate energy using tidal power. (Corbis-Bettmann)

Hans Christian Oersted, the son of an impoverished pharmacist, made the great discovery that electricity and magnetism are related. Oersted was born on the small Danish island of Langeland, about halfway between Copenhagen and Hamburg. There was no school in Langeland, so he and his younger brother, Anders Sandoe, went to the homes of neighbors who taught the boys to read and write. Later the town surveyor taught them mathematics, and the mayor taught them English and French. When he was twelve, Hans began to help his father in the pharmacy, and the work stimulated his interest in science.

In 1794 Oersted and his brother Anders matriculated at the University of Copenhagen. Hans studied the sciences, and Anders, who eventually became a leading jurist and...