What Did de Forest Really Invent?
September 1961 Radio-Electronics

September 1961 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Upon the demise of Lee de Forest in 1961, Radio-Electronics magazine ran a series of articles on the life and times of the great inventor. If you are not familiar with the inventive path which led de Forest to his Audio vacuum tube, I advise reading some of the articles listed at the bottom of the page. Begin with How Audions Were Built, from the January 1947 issue Radio-Craft magazine (also published by Gernsback), where you will discover that early attempts at modulating and demodulating audio signals used a candle flame that responded to an electric field. It was from there that de Forest evolved the Audion. He was the first to utilize a glass housing evacuated of air, in order to prolong the life of the cathode and to eliminate noise caused by air current. Interestingly, in 1946, when de Forest was 73 years old, Radio-Electronics editor Hugo Gernsback, who was a good friend of Lee's, convinced him to allow his hands to be photographed for posterity's sake. de Forest died in 1961 at 88.

What Did de Forest Really Invent?

By Fred Shunaman, Managing Editor

De Forest did not "add a grid to the diode": he developed an entirely new electronic device.

A profound misconception about the nature of de Forest's fundamental invention has prevented many from appreciating the full importance of his contribution to communications and electronic science. Indeed, if he had simply "interposed a grid between the cathode and anode of a two-element vacuum tube" as some seem to think, he would have no claim to the title "Father of Radio" which he has borne without challenge for several decades.

Yet, in spite of their superficial resemblances, the de Forest Audion and the Fleming valve do not belong in the same family of detection devices. De Forest started out with the Responder, an electrolytic detector. A steady current from a local battery flowed through it. Arrival of a radio signal reduced the conductivity of the solution and interrupted the current. Thus it was, like the Branley coherer, a relay, in which very small amounts of radio-frequency power controlled much larger quantities of power supplied by a local source. The Responder was the subject of de Forest's first patent.

While working with the Responder, de Forest was led by an accident into the study of flame or hot gases as a means of detecting wireless signals. The Audion was the last of a long series of heat-operated devices. All of them used a local or B-battery (de Forest's term - abbreviated from "booster battery"). The leads from this battery terminated in two electrodes, placed in different parts of a flame (in the earlier models). The battery current was to be controlled by the wireless signal, picked up by an antenna attached to one of the electrodes of the device, which in its simplest form is shown in Fig. 1. Another electrode was grounded, to complete the antenna circuit. The telephones were in the battery circuit.

Several of these devices, using a Bunsen burner as the chief element in the detector, and with different types of elements variously positioned, were patented by de Forest.

But the inventor realized that his flame detector, though improving with each new model, was fundamentally unsuited for practical work, especially on shipboard. A breath of air or slight list of the craft could change the relative positions of flame and electrodes, making the detector intermittently useless. "So," he said, "I thought of other means of heating the gases." After trying and abandoning a small arc, he had a number of small lamps - with the essential elements built in - constructed (Fig. 2).

It would be naive to suppose that Fleming's valve might not have given him a suggestion. On the other hand, he might have got his inspiration from an ordinary electric light bulb. In any case, the elements sealed into those earlier tubes - and most especially the circuitry surrounding them - were definitely those of the de Forest gaseous detectors. (Even the final grid Audion is reminiscent of a much earlier device which used a number of rings in a Bunsen burner flame.)

De Forest argued that the arrangement of Fig. 2 was imperfect "because it permitted part of the high-frequency energy to pass to earth through the telephone and B-battery circuit, instead of concentrating it upon the ions between the plate and filament." He therefore tried attaching the antenna to a piece of tinfoil wrapped around the glass of the lamp. The next step was to insert it in the tube, at first as an additional plate or "wing" on the opposite side of the filament from the plate.

The new model was "a further distinct improvement" on previous detectors, and work was intensified to find the best shape and position for the new element. The Audion was practically invented. Already a system was in existence that isolated the signal and local circuit, and finding the ideal placement of the control element was a matter of a few experiments. De Forest was convinced that it would have the greatest effect on the conductive stream (of ions, as he envisioned them) if placed between cathode and "wing" or plate. The plate was perforated as a first experiment, then de Forest handed tube maker McCandless a piece of wire, bent in zig-zag fashion into the shape of a grid about 5/8 inch square. Instructions were to put this grid between the wing and filament, and "as close to the filament as possible."

The new tube, delivered with a batch of others of different forms, had a life of barely 3 minutes. Yet that was long enough to prove a device of an entirely new order of sensitivity was born. Thus the Audion, which differed from all previous detectors of electric waves in that it could amplify, was born.

In contrast, the Fleming valve was a simple rectifier. It is related to the crystal diode or the imperfect-contact type of detector rather than to the family of the coherer. Putting a third electrode in it - while retaining its method of operation - would simply make it "a rectifier with one too many electrodes." It could never have advanced electronics beyond the crystal-diode stage.

In recent years even the British - who tended to defend Fleming during his lifetime - have pretty well recognized de Forest as the inventor of a new device. De Forest's own attitude, which the British considered sporting, was partly responsible for the swing in his favor. Fleming consistently attacked de Forest as a charlatan and cheat, while he took a far more tolerant attitude toward his British colleague. According to one British author, de Forest went so far as to remark that "Fleming went to his death convinced that he was the inventor of the vacuum tube."

Now that both great pioneers of radio are dead, and personalities no longer enter into the problem, it is clear that what de Forest invented was not just another and more efficient "oscillation detector" but a revolutionary new device, a thermionic relay in which a very small electric charge can be made to control the flow of much larger charges, in short a device that can amplify. It is this ability to amplify all types of electric signals, from direct current to the kilomegacycles, without changing their form, that made the Audion and its descendants creators of the Electronic Age. De Forest's confidence in the relay principle and the "booster battery", which could supply large amounts of power, was justified and helped him to invent the device on which depends not only modern radio, but television, radar, countless control devices, including those that make modern aviation possible, our great new computers and data-processing devices and a host of other equipment and apparatus. The term "Father of Radio" dates from a day when the potentialities of his invention could not be realized or imagined. Today he may more correctly be hailed as the Father of the Electronic Age.

Dr. de Forest's picture appearing on page 33 was taken from an oil painting by Geo. L. Camarero in 1952. It now hangs in the home of H. Gernsback.

In 1946, editor Hugo Gernsback asked permission to have de Forest's hands photographed. De Forest, in a letter to Gernsback, objected to this because he thought "it was not dignified." We convinced him that the picture was necessary for a historical record. The photograph above is printed here for the first time; it shows the hands that fashioned the Audion, which ushered in the electronic age.

Posted August 30, 2024