An article recently appeared in the Toronto Star extolling the virtues of the vacuum tube. If you ever took a close look at one, you’ll see what an amazing device it really is. The article gets a little sidetracked by its discussion of tube vs. solid state guitar and stereo amplifiers, but it’s interesting nonetheless.
Without tubes, there is no radio. Nor are there any computers. At least not as early as we had them. Tubes were the first devices to give us the control over electron flow that was needed to make both radio and computers possible.
Tubes are also an interesting example of how the discovery of a basic phsyical phenomenon can lead to the development of an entire technology. In developing the light bulb, Edison when you stuck a metal strip into an incandescent light bulb, electrons given off by the hot filament would be attracted to the metal strip. Even though this property was noted by physicist Frederick Guthrie several years earlier, we now call this the Edison effect.
What’s interesting about this is that Edison didn’t realize the importance of the phenomenon. The reason he didn’t is that he was trying to increase the life of a light bulb and not invent electronics. It may have been an interesting effect, but since it did not significantly help him make better light bulbs, he wasn’t very interested in the effect.
It was up to John Fleming, a British scientist, and American inventor Lee de Forest to realize how important this was and to develop practical devices that used the Edison Effect. Fleming invented a “valve” by placing a metal tube around the filament inside the bulb. The Fleming Valve acts as a diode. When the voltage between the filament and the tube, or plate, is positive electrons flow; when the voltage is negative no electrons flow. Fleming used his valve as a rectifier in a detection circuit.
de Forest inserted a third element into the tube. Called the grid, this element allows the designer of a circuit to control the flow of electrons between the filament and the plate. With a grid, you can not only get an electron tube to rectify a signal, but also to amplify it. A small voltage on the grid controls a large flow of electrons through the tube.
What’s perhaps more amazing is how far we’ve come in 100 years. In the last century, we’ve gone from a device that’s inches across to putting hundreds of millions of electron control devices (transistors) in the same space. It couldn’t have been done however, without the tube.
Links
- IEEE Virtual Museum – Fleming Valve 1904
- Marconi Calling – Fleming’s Thermionic Valve
- About.Com – The History of Vacuum Tubes
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