12 Electricity and Magnetism
Studying electricity was difficult under such conditions. It was also hard to imagine very many useful applications for electricity. All that rubbing was a bit of a drag, one might say. What was really needed was a source of steady electricity—and a way to control its path. Then in 1800, an Italian physicist, Alessandro Volta (1745– 1827), developed a steady source of electricity. Volta invented the first electric battery. It was not quite like modern batteries, but the idea for both is the same. With Volta’s battery, a steady flow of charges became available. Now electricity could really take off. To control the path of charges, people began building circuits, as described in the sidebar on page 11. The best material with which to make the circuits was obviously metal, a superb electrical conductor. Not all metals are equally good conductors, however. One of the best is gold. But gold is far too expensive to use for everyday circuits—to wire a house would cost millions of dollars. Because copper is cheap and a relatively good conductor, it is the metal most often used. Circuits make a complete, closed path for electric charges to flow. Current flows all the way around the loop. If there is an open anywhere in the circuit—a break in the path—a steady current cannot flow. The charges flow in a circuit like water falling downhill. They move naturally down their gradient; the gradient in this case is an electrical gradient called the potential difference. This is similar to the discharge of static electricity, as separated charges move together again because of electrical forces. But unlike in static electricity, in steady currents the charge must be continuously separated so that it keeps flowing. Separating charges requires work, by friction (as in static electricity), by a battery, or by some other means (described later). As the charges travel around the circuit they move down their electrical gradient, and they must to be “lifted up” again before they will make another trip. This is the job of the battery or other power source in the circuit. In the late 19th and early 20th centuries, electric power companies formed. Generating electricity at many different locations