January 1947 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
A few days ago I mentioned that
a popular early form of radio detector circuit involved the used of a
flame - yes, the flame of
a fire, not a romantic significant other. The subject arose in a couple articles
in the January 1947 issue of Radio-Craft magazine that celebrated the 40th
anniversary of Lee de Forest's Audion vacuum tube invention. This particular
piece was authored by de Forest himself, who was a personal friend of Radio-Craft
editor Hugo Gernsback. It is a very interesting autobiographical account of the
early days of experimentation and the evolution of what eventually became the world's
first mass producible signal amplifying device. You will also read that de Forest
created the designation of the "B" battery for a reason he makes obvious. Also,
although you have probably seen pictures of the old household type gas light fixture,
a look at the included photo of one shows from where the modern incandescent electric
lamp bulb socket gets it shape - including the switch twist knob as found on 3-way
sockets (see image below).
How the Audion Was Invented
By Lee de Forest
In the summer of 1900, I was working under the light of a Welsbach gas burner
in my hall bedroom in Chicago, experimenting with my so-called "Sponder," an anti-coherer
for the reception of electrical waves for use in wireless telegraphy. One night
I noticed that whenever the little spark from my transmitter coil was put in operation
the light from the Welsbach burner dimmed. When my transmitter key was lifted, (the
normal light of the burner was restored. Thus I was able to translate into light
variations the signals from my key. I was amazed and highly elated by this unexpected
phenomenon, and for several weeks played with it, believing that I had accidentally
discovered that incandescent gases were affected by Hertzian waves, and that here
I had discovered an absolutely new principle which might be of the utmost value
as a detector for wireless telegraphy. This illusion persisted until my assistant
and I put the spark coil in a closet and closed the wooden door; thereupon the fluctuations
of the gas burner were no longer observed. This proved conclusively that the effect
observed was not due to the electrical waves from the spark, but to the sound waves
therefrom. I had merely hit upon a new type of sensitive flame.
I was intensely disappointed by this outcome, but I was positive that there must
be, nevertheless, some change in the conductivity of incandescent gases resulting
from the passage therethrough of high-frequency electrical waves, and I determined
to investigate further and prove that my original conception had a basis in the
physics of gases.
Early Flame Detectors
It was not until 1903, when I was working in a small laboratory at 11 Thames
Street, in lower Manhattan, that I had leisure and opportunity to resume my work
in this direction. There I used a Bunsen burner, locating within the flame two platinum
electrodes, one of which was connected through the telephone receiver to a dry battery,
and thence to the other platinum electrode. I enriched the flame with sodium, or
I then found that when the electrodes were properly located in the gas flame
the signals from my spark transmitter were distinctly audible in the telephone receiver.
I made countless experiments with this phenomenon; and to prove definitely that
the effect was not acoustic but electrical, I connected one of the flame electrodes
to my antenna, the other to the ground, and actually obtained wireless signals from
ships in New York harbor.
Realizing that a gas-flame detector would be wholly unsuited for practical wireless
work, I thought of other means for heating the gases. I tried a small electric arc
- which was altogether too noisy to be of any use. After several futile attempts
to build such a device myself, I persuaded Mr. McCandless, a manufacturer of miniature
incandescent lamps, to build for me a tube containing a platinum plate and carbon
filament. The plate was connected to the positive side of the dry battery; the negative
terminal to the filament. In series was a telephone receiver. This device was not
the Fleming valve. It has always been quite impossible for me to understand the
confused idea, in the minds of some otherwise keen thinkers, that the audion differed
from the Fleming valve merely by the insertion of a third electrode therein. Without
the use of the B-battery the valve would be nothing but a rectifier with one too
many electrodes. The employment of the local battery in the plate circuit is just
as necessary an element to the success of the device as is the grid-itself. At the
time I was working on the two-element audion with B-battery, I had never heard of
the Fleming valve. My approach to this perfected device was by an entirely different
series of events, and began with the gas-flame detector.
This device was a genuine relay, in which the local energy of the plate battery
supplying the current through the remaining gas in the tube was controlled by pulsations
of the incoming high-frequency waves, which were picked up on an antenna connected
to the plate electrode, the filament being connected to the ground. This was the
same arrangement I had previously used with the gas flame detector. At that time
I had requested McCandless not to exhaust the tube to any high degree of vacuum,
because I then thought that the presence of gas was an essential element. This diode
detector, as stated above, was intrinsically very much more than a simple rectifier
of high-frequency current. The addition of the plate battery made a very great difference
in the intensity of the signals received, for I was employing the high-frequency
energy, not to actuate my telephone diaphragm, as Fleming had done, but to control
very much larger quantities of. energy from the local battery.
I argued that the above arrangement 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. To avoid
this difficulty and still improve the sensitivity of the detector, I wrapped a piece
of tin foil around the outside of the cylindrical-shaped gas envelope, and . connected
this third electrode to the antenna or to one terminal of the high-frequency device.
I then realized that the efficiency could be still further enhanced if this third
electrode were introduced with-in the envelope. I induced McCandless to construct
another "audion," as I then called it. This last device contained two plates with
a filament located between them, and, as before, a considerable amount of gas in
the envelope. This detector showed further distinct improvement over its predecessors.
The Grid Audion
Welsbach gas mantle, starting point of the series of experiments
which led to the audion.
It occurred to me that the third, or control, electrode should be located more
efficiently, between plate and the filament. Obviously, this third electrode so
located should not be a solid plate. Consequently I supplied McCandless with a small
plate of platinum, perforated by a great number of small holes. This arrangement
performed much better than anything preceding it, but in order to simplify and cheapen
the construction I decided that the interposed third electrode would be better in
the form of a grid, a simple piece of wire bent back and forth, located as close
to the filament as possible.
3-Way Incandescent Lamp Socket
At this time I was using a 6-volt filament energized from a dry or storage battery,
which I called the A-battery; the plate battery I called the B-battery - terminology
which has persisted to this day.
As the various experiments and improvements outlined took place during the period
1903 to 1906 and later, I applied for successive patents. At that time the Patent
Office was not glutted as it is today, and my applications were related to an entirely
new art, so that the Office issued my patents within only a few weeks or few months
Early in 1907 I conceived the idea that this remarkable wireless telegraph detector,
the three-element, or grid, audion - which had already covered itself with glory
in the minds of the hams and wireless telegraph operators - might also be useful
as an amplifier of audio-frequency or telephonic currents. I had made some experiments
in this direction, and I took out a patent containing very broad claims on the device
as an amplifier of currents without limitation of the frequency thereof. This patent,
No. 841,387, granted January 15, 1907, has since been acclaimed as one of the most
valuable patents ever issued by the United States Patent Office. The same, of course
can be truthfully said about the patent on the grid electrode, No. 879,532, filed
January 29, 1907.
In the summer of 1906 I presented a paper before the American Institute of Electrical
Engineers describing the audion, but only as a diode using the B-battery. I had
not then applied for a patent on the grid, or control-electrode, type, and therefore
I made only veiled reference in this paper to it. The grid patent was filed on January
Early Types of Audions
The first audions were of cylindrical form; later, in 1907 or 1908, McCandless
suggested that it would be easier for him to construct the device in the spherical
form. In the first audion the grid and plate electrodes were both brought out near
the base; but in 1907 the plate and grid electrodes were brought out through the
top of the tube. To distinguish readily between the two, I used a red sleeving over
the lead to the plate, and a green sleeving over the grid wire - "green for grid,"
to be easily remembered by the operator.
In my first experiments on the audion as an amplifier for telephonic currents
I added a third, or C-battery as I called it, in series with the control electrode.
Although, unfortunately, I did not specify the polarity of this C-battery, the circuit
diagram of the amplifier patent shows it with its negative terminal connected to
the control electrode. This was the way I always employed it; but due to this unfortunate
omission from my specification, Fritz Lowenstein was able, a few years later, to
obtain a very valuable patent covering the negative bias of the grid. He was, however,
by no means the first to apply this negative bias to the control electrode.
Later Audion Improvements
Evolution of the audion. Even the earliest flame detectors incorporated
the B battery, as shown in the left-hand figure (Patent No. 867,878). The circuit
at center (Patent No. 836,070) is essentially the same, with a heated filament in
a low-vacuum tube taking the place of the Bunsen burner flame. Thus the B battery
was also a feature of the earliest two-element audions. In the circuit at right
(Patent No. 841,387) we see the separation of r.f. and local circuits, as well as
the C battery (credit for the invention of which was lost to de Forest because polarity
markings were omitted from the patent drawing). Obviously the whole series of heat
detectors were thermionic relays, not simple rectifiers.
From 1906 to 1910 I made countless improvements or changes in the form of the
audion, such as the substitution of tantalum and then tungsten for the carbon filament;
the use of nickel for plate and grid instead of platinum; the double filament so
arranged that if one burned out the second one could be readily connected, thus
doubling the life of the detector. As early as 1907 McCandless began to pump my
tubes to the same vacuum he employed in his miniature incandescent lamps. Naturally
some of the tubes contained more gas than did others, and we found that a very small
amount of gas made the device a more sensitive detector than those of higher vacuum.
When used as a detector of wireless signals, the lack-of-linearity characteristic
was of course of no importance - maximum sensitivity was what we all were after.
But so long as only an "incandescent lamp vacuum" was employed, it was impossible
to use more than 22 or 30 volts in the B-battery without producing the "blue arc"
which at once rendered the device extremely insensitive.
My patents show types of audions employing two, three, or more grids, as well
as the "double audion" having one plate and grid on either side of the double filament.
The latter arrangement developed into a beautiful oscillator, the first push-pull
type in electronic history.
It was not until the summer of 1912 that I actually succeeded in developing the
audion and its accompanying circuits into a genuine amplifier of telephonic currents.
Seeking to make this amplifier more efficient and able to handle larger power, I
besought McCandless to exhaust the tubes to the highest possible degree, to permit
the use of more plate voltage. But the best that he could do still restricted this
voltage to about 45. Thereupon I took some of his tubes to a manufacturer of X-ray
tubes in San Francisco, who re-exhausted these to the best of his ability, using
mercury vapor diffusion pumps. (McCandless had used only mechanical pumps.) With
these re-exhausted tubes I was able to use as high as a 220-volt plate battery,
without causing the "blue arc." Three of these high-vacuum audions connected in
cascade gave amazing audio-amplifier effects, so that using, as my input source
a telegraphone wire on which music or speech had been recorded, and as my output
device a loudspeaker of the 1912 vintage, I was able to hear the reproduction of
voice and music over a distance of 100 feet or more in the open air. Thus it is
evident that the approach to the radio, or amplifier, tube possessing an extremely
high vacuum was merely a gradual, and perfectly obvious, result of the growing requirement
for more power from the amplifier or the oscillator. More power demanded higher
voltages, and it was obvious that higher voltages would necessitate higher degrees
of vacuum. I never considered for a moment that there was any invention involved
in the gradual evolution of the audion from a gaseous, or low-vacuum, to a high-vacuum
At this point in the development of the audion amplifier I was requested by my
good friend John Stone to bring my demonstration apparatus from Palo Alto, California,
to show to the engineers of the telephone company in the Western Electric laboratory
in New York. From that point on, the further development and refinement of the audion
amplifier to its present degree of ultra-perfection is well-known electronic history,
and requires no resume here.
Regeneration and Oscillation
During those epoch-marking experiments in Palo Alto, in the summer of 1912, I
accidentally hit upon the feed-back circuit, which made of the audion amplifier
an oscillator of currents of any frequency. Thereafter began the intensive development
of the "oscillion," as I then called it, in larger and larger sizes and degrees
of power, until in 1915 I was employing a 25-watt power tube for broadcasting from
my Highbridge, Bronx, laboratory steel tower. Simultaneously, the Western Electric
engineers were developing the oscillator along very similar lines to a point where
they were able, utilizing a battery of one hundred or more of these tubes, to transmit
the human voice across the Atlantic without wires - a feat the prediction of which
by myself, just two, years previously, had been considered a ridiculous improbability,
had almost resulted in my incarceration in Atlanta Penitentiary!
Posted March 3, 2020