When Henry Adams, the one-of-a-kind American historian, wanted to contrast the medieval with the modern, the emblems he chose for those two eras were the Virgin and the Dynamo. The earlier age, he said, expressed its highest aspirations in building cathedrals consecrated to a spiritual ideal; in our time we exalt the generation of electricity—another invisible but powerful essence. The votaries of the electric cathedral certainly agree with this assessment. A textbook for power-plant operators says of the generating station, “It is like a shrine or source of unfailing light which must be given ceaseless attendance once it is brought into being.”

Adams encountered the dynamo at the Universal Exposition in Paris in 1900. A century later the electric power plant is no longer a novel piece of machinery that can attract a crowd at a fair, but it is more central than ever to daily life. And the dynamo (or generator, or alternator—they are all terms for the same machine) still seems an apt symbol of both the hopes and fears invested in industrial progress.

Electricity has become the standard currency of the energy economy. It is not in itself a natural resource that you can dig out of the ground or pump from a well, but other forms of energy are converted into electricity for convenience of distribution and use, just as the body converts a variety of foods into a few simple sugars that circulate to all the tissues. Thus, the power plant doesn’t create energy; it merely transforms it. The chemical energy locked up in coal, for example, is captured in the heat and pressure of steam, then passed on to the kinetic energy of a spinning turbine, and finally converted into electric current in the generator.

Three kinds of power plants are scattered around the American landscape. Fossil-fuel plants, which burn coal, oil, or natural gas, make up almost two-thirds of the nation’s generating capacity. Nuclear plants tap energy from the disintegration of uranium atoms. Hydroelectric plants are found only where the water is—or more specifically where the water runs downhill.

A few other energy sources are also squeezed to produce the juice of electricity. Their contributions are smaller, but the machinery is no less interesting to look at. Wind power, in particular, creates haunting landscapes.