Another route to rectification was soon found, emerging from Thomas Edison's work on the electric lightbulb. Back in 1883 Edison had observed that if he placed a small metal plate in one of his experimental bulbs, it would pick up an electric current that somehow managed to cross the bulb's vacuum from the hot filament. Not long afterward, a British engineer named John Ambrose Fleming noticed that even when the filament carried an alternating current (which Edison hadn't tried), the current passing through the vacuum always traveled from the hot filament to the plate, never the other way around. Early in the new century Fleming devised what he called an "oscillation valve"—a filament and plate in a vacuum bulb. It rectified a wireless signal much more reliably than Braun's crystals.

By then the nature of the invisible current was understood. Experiments in the 1890s by the British physicist Joseph John Thomson had indicated that a flood of infinitesimally small particles—electrons, they would be called—was whizzing through the vacuum at the incredible speed of 20,000 miles per second. Their response to signal oscillations was no less amazing. "So nimble are these little electrons," wrote Fleming, "that however rapidly we change the rectification, the plate current is correspondingly altered, even at the rate of a million times per second."

In 1906 the American inventor Lee De Forest modified Fleming's vacuum tube in a way that opened up broad new vistas for electrical engineers. Between the filament and the plate he inserted a grid like wire that functioned as a kind of electronic faucet: changes in a voltage applied to the grid produced matching changes in the flow of current between the other two elements. Because a very small voltage controlled a much larger current and the mimicry was exact, the device could serve as an amplifier. Rapidly improved by others, the three-element tube—a triode—made long-distance telephone calls possible, enriched the sound of record players, spawned a host of electronic devices for control or measurement, gave voice to radio by the 1920s, and helped launch the new medium of television in the 1930s. Today, vacuum tubes are essential in high-powered satellite transmitters and a few other applications. Some modern versions are no bigger than a pea.