One of two types of electrical flow, direct current (DC) predates alternating current. Unlike alternating current, direct current flows in only one direction. Current is measured in amperes, frequently shortened to "amps," which is the amount of electrical charge that flows over a given time span.

Benjamin Franklin, who first suggested the terms "positive" and "negative" electrical charges, believed the flow of current went from positive to negative. In fact the opposite is true, because current is carried by electrons, which are negatively-charged and so attracted to positive polarities. The electric battery that Alessandro Volta invented in 1800 produced direct current and was an immediate success. It, its successors, and direct current generators were used to operate a variety of electrical devices during the next eighty years.

Then, in 1881, serious competition arose in the form of a brilliant Croatian electrical engineer named Nikola Tesla. Employed by the...