The Vacuum Tube is a Century Old

Forrest M. Mims III

As we immediately realize each time the power fails, life as we know it would not exist without electricity. And electricity would be much less useful if the vacuum tube, transistor and integrated circuit had never been invented.

All three of these devices are used to control the flow of electricity. This ability makes possible everything from the digital watch on your wrist to cell phones, televisions and computers.

Figure 1. Vacuum tubes like this one revolutionized radio and sound recording and made possible television. This particular tube is from a rare example of a scientific instrument that was in continuous use monitoring carbon dioxide at Hawaii's Mauna Loa Observatory for some 48 years. Replacing the tubes in this instrument was a common maintenance chore. Photograph by Forrest M. Mims III.

Today the first of these inventions, the vacuum tube, is used only in specialized applications and by music enthusiasts who prefer the sound of vacuum tube amplifiers over those that employ transistors and integrated circuits. But there was a time when vacuum tubes were used in countless radios, TV sets and even hearing aids small enough to fit in a shirt pocket.

In 1880 Thomas Edison paved the way for the first vacuum tube when he installed a metal plate inside one of the early electric lamps that he invented. He found that some of the electricity flowing through the heated filament of the lamp would mysteriously travel to the plate through the vacuum inside the lamp.

In 1904, British scientist John Fleming improved on Edison's discovery and invented a vacuum tube in which electricity would flow in only one direction between the filament and the plate. His invention was called a valve or a diode (meaning two electrodes), and it provided an ideal way to detect radio signals.

In 1906, Lee de Forest modified Fleming's diode by placing a metal screen, called a grid, between the filament and the plate. For the first time, it was possible to control the electricity flowing between the filament and the plate by a small voltage connected to the grid.

De Forest called his invention the Audion, and he described it in Scientific American magazine. Its most important role was as an amplifier in radio transmitters, home and car radios, record players, televisions and hearing aids.

In all these gadgets the Audion tube allowed a very small voltage from a radio signal or a microphone to control a much larger flow of electricity. For example, a hearing aid converted a tiny sound at the microphone into a much louder sound at the earphone.

You've probably heard the annoyingly loud squeal in an auditorium when a microphone is placed too close to its speaker or the volume of the amplifier is set too high. This effect can be put to use by connecting some of the output from an amplifier back to its input. The result is an oscillator that can generate a pure musical tone. String together an array of oscillators, each with a different tone frequency, and you have a primitive electronic organ.

Vacuum tubes revolutionized technology and made possible the first digital computers. Because each tube produced heat, the thousands of tubes in a computer had to be air conditioned. The tubes burned out frequently, and they were large and fragile.

For these reasons vacuum tube technology peaked in the 1940s. Further advances required an entirely new discovery, and one was waiting in the wings 60 years ago. It was the transistor, and it transformed the world of electronics.