Engines of Our Ingenuity

No. 23:
THE VACUUM TUBE

by John H. Lienhard

Click here for audio of Episode 23.

Today, we find a rejected invention that changed our world. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The "Edison effect" was the name given to a phenomenon that Edison observed in 1875 and refined later, in 1883, while he was trying to improve his new incandescent lamp. The effect was that, in a vacuum, electrons flow from a heated element -- like an incandescent lamp filament -- to a cooler metal plate. Edison saw no special value in the effect, but he patented it anyway. Edison patented everything in sight. Today we call the effect by the more descriptive term, "thermionic emission."

Now the Edison effect has an interesting feature. The electrons can flow only one way -- from the hot element to the cool plate, but never the other way -- just like the water flow through a check valve. Today we call devices that let electricity flow only one way, diodes.

In 1904, the Edison effect was finally put to use, but not in a light bulb. Radio was in its infancy, and the British physicist John Fleming was working for the British "Wireless Telegraphy" Company. He faced the problem of converting a weak alternating current into a direct current that could actuate a meter or a telephone receiver. Fortunately, Fleming had previously consulted for the Edison & Swan Electric Light Company of London. The connection suddenly clicked in his mind, and he later wrote,

To my delight I ... found that we had, in this peculiar kind of electric lamp, a solution ...
Fleming realized that an Edison-effect lamp would convert alternating current to a direct current because it let the electricity flow only one way. Fleming, in other words, invented the first vacuum tube. Of course, most vacuum tubes have been replaced with solid-state transistors today; but they haven't vanished entirely. They still survive, in modified forms, in things like television picture tubes and X-ray sources.

Fleming's discovery reveals an aspect of the creative process that comes at us again and again. The creative inventor takes ideas out of their original contexts and uses them in new contexts. He turns bread-mold into penicillin, coal into electricity -- or, I suppose, lead into gold -- because he isn't constrained to keep each thought in its own container.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


This episode has been considerably revised as Episode 1323.


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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