Meanwhile, other experimenters were seeking ways to generate radio waves steadily rather than as sparkmade pulses. Such continuous waves might be electrically varied—modulated—to convey speech or music. In 1906 that feat was achieved by a Canadian-American professor of electrical engineering, Reginald Fessenden. To create continuous waves, he used an alternator, designed by General Electric engineer Ernst Alexanderson, that rotated at very high speed. Unfortunately, the equipment was expensive and unwieldy, and Fessenden, in any event, was a poor businessman, hatching such unlikely profit schemes as charging by the mile for transmissions.

Fortune also eluded Lee De Forest, another American entrepreneur who tried to commercialize continuous-wave transmissions. In his case the waves were generated with an arc lamp, a method pioneered by Valdemar Poulsen, a Danish scientist. De Forest himself came up with one momentous innovation in 1906—a three-element vacuum tube, or triode, that could amplify an electrical signal. He didn't really understand how it worked or what it might mean for radio, but a young electrical engineer at Columbia University did. In 1912, Edwin Howard Armstrong realized that, by using a feedback circuit to repeatedly pass a signal through a triode, the amplification (hence the sensitivity of a receiver) could be increased a thousandfold. Not only that, but at its highest amplification the tube ceased to be a receiving device and became a generator of radio waves. An all-electronic system was at last feasible.

By the early 1920s, after further refinements of transmitters, tuners, amplifiers, and other components, the medium was ready for takeoff. Broadcasting, rather than point-to-point communication, was clearly the future, and the term "wireless" had given way to "radio," suggesting omnidirectional radiation. In the business world, no one saw the possibilities more clearly than David Sarnoff, who started out as a telegrapher in Marconi's company. After the company was folded into the Radio Corporation of America (RCA) in 1919, Sarnoff rose to the pinnacle of the industry. As early as 1915 he wrote a visionary memo proposing the creation of a small, cheap, easily tuned receiver that would make radio a "household utility," with each station transmitting news, lectures, concerts, and baseball games to hundreds of thousands of people simultaneously. World War I delayed matters, but in 1921 Sarnoff demonstrated the market's potential by broadcasting a championship boxing match between heavyweights Jack Dempsey and Georges Carpentier of France. Since radios weren't yet common, receivers in theaters and in New York's Times Square carried the fight—a Dempsey knockout that thrilled the 300,000 gathered listeners. By 1923 RCA and other American companies were producing half a million radios a year.

Advertising quickly became the main source of profits, and stations were aggregated into national networks—NBC in 1926, CBS in 1928. At the same time, the U.S. government took control of the spectrum to deal with the increasing problem of signal interference. Elsewhere, some governments chose to go into the broadcasting business themselves, but the American approach was inarguably dynamic. Four out of five U.S. households had radio by the late 1930s. Favorite network shows such as The Jack Benny Program drew audiences in the millions and were avidly discussed the next day. During the Depression and the years of war that followed, President Franklin D. Roosevelt regularly spoke to the country by radio, as did other national leaders.