The transformer makes modern power transmission possible, changing voltages up or down with very little loss of power. Its development dates back to the early nineteenth century, when, in 1820, Hans Christian Oersted (1775-1851) discovered that an electric current in a conductor created a magnetic field and started a flurry of research into the area of electromagnetism. Michael Faraday decided to reverse Oersted's discovery by using a magnetic field to create an electric current. In 1831, he discovered that the current sent through a wire coil set up "magnetic lines of force" that expanded outward and affected a nearby second coil. By alternating the expansion and collapse of the lines, he could induce an alternating current (AC) of electricity in the second coil. Experimenting with the number of turns of wire in his coil, Faraday discovered that if the secondary coil had twice as many turns as the primary, the...