The Edison Effect

This article on tube development is meant to be a very brief overview of the subject. It hits what I believe are the most important points, but only enough to make my points. Anyone interested in learning this in depth and detail should be able to find plenty of information on the internet.

I have primarily organized and presented this as the development of the amplifier, partly because that's the way I learned it, and partly because it neatly covers the information I want to include. If you understand a pentode amplifier, you have the fundamentals for most other vacuum tubes made; it's not much of a leap to learn about cathode-ray tubes, magic eyes, ballast tubes, and so on.

If the narrative here has an American point of view, it may be because I'm an American and this is the way I've learned the material. I'm not a xenophobe and I don't care about the nationality of an inventor; but I find it more direct to tell a story through Edison, for instance, than through Guthrie, because I believe Edison's path is the more direct one. At least I believe it was as tubes developed in the U.S.A. Parallel development of tubes in Europe may lead to different milestones and inventors—with the same end results.

The Problem with Light Bulb Filaments

Thomas Edison's search for a good light bulb filament is a popular story; of trying thousands of materials that would produce an adequate amount light and last enough to be of practical use. The filament was placed inside a glass dome with a base, sealed so that a hard vacuum could be placed inside.

While he tried a number of other materials and variations, many of the filaments (including the ultimately successful one) had carbon on the outside. One of the carbon-clad materials he tried seemed promising, but left a carbon deposit on the inside of the glass, which darkened the glass and at least partially dampened the light. Edison introduced another electrode inside the lamp and hooked to one side of the battery or the other, hoping to attract the carbon to the electrode instead of the glass. It didn't work.

What he found instead was that if he hooked the electrode to the negative side of the battery, nothing happened. But if he connected it to the positive side, he could measure an electric current flowing through the circuit. He did not know why he would get current flow this way, but there it was.

Edison measured the current with a Galvanometer, which is another name for milliammeter or microammeter (large currents are measured on ammeters, but tiny currents are measured on galvanometersor at least they were in the 19th and early 20th centuries). The concept of the electron particle and the role it played in electric current was still being developed at that time, so Edison was at a loss to explain why he had a current. If you can demonstrate something but don't know what causes it, you call it an effect, so he referred to this as The Edison Effect.

Later on, this would be explained as thermoionic emission. If you apply an electrical current through a vacuum tube filament, you get light and heat, which are emitted as valence shell electrons boil off the molecules of the filament and float free in a cloud around the filament. This is called a space charge, and it carries a negative charge. If you make a nearby electrode negative, nothing happens; but if you give it a positive charge, the electrons will be attracted to it and an electric current will flow.


Thermionic emmission is the basis of how vacuum tubes work. The filament gets hot and becomes a source of eletrons (electrically known as a cathode). The positive electrode, which was originally a flat plate, got the name plate, though electrically it's known as an anode. There are a few specific tube types that have anodes, but most use the old term, plate.

A filament can the source of electrons itself, which makes it a cathode. Tubes where the filament is the source of electrons are called directly heated, and the filament is typically called the filament, not a cathode.

On later tubes, another electrode was added, and it was placed very very close to the filament. The materials in the filament and this new electrode were such that the filament only provided heat, and as the electrode got hot, it provided the electrons. This electrode was called a cathode, and this single-purpose filament was renamed the heater, and tubes that use these are called indirectly heated tubes.

That's the end of Edison's part of the story. He was interested in making a practical light bulb, not figuring out why current flowed in this particular case, and how it could be put to good use. That would be left to others, like J. Ambrose Fleming.

©opyright by James Ollinger. All Rights Reserved.