Carpool lanes, like this one on a California freeway near the Westwood neighborhood in Los Angeles, were designed to encourage more people per car. (Corbis-Bettmann)

A transformer is an electrical component used to connect one alternating current (ac) circuit to another through the process of electromagnetic induction. The input current travels through a conductor (the primary) wound around a conductive core. The current traveling through the primary windings creates an alternating magnetic field in the core. An output conductor (the secondary) is wound around the same core so that the magnetic field cuts through the secondary windings, inducing the output electrical current. For most transformers, the primary and secondary windings never come into direct electrical contact with each other. Instead, the transfer of energy from primary to secondary is accomplished solely through electromagnetic induction.

Transformers were developed through a series of scientific discoveries in the nineteenth century. Most...