- Before the invention of the transistor,
the most important piece of electronic equipment
was the vacuum triode.
All radios used them,
early television used them.
Heck,
even early computers had rooms
full of these jacked-up light bulbs,
but how do they work
and how
and why
were they invented?
This is a story of a smudge on a light bulb,
an assistant with a good memory,
and a con man working around a pack.
Ready?
Let's go!
It all started in 1880 with the regular light bulb.
Thomas Edison had noticed that,
when his light bulb (glass shuttering) broke,
they would often create black smudges
on the inside of the bulb,
near the positive end of the filament.
He had worked very hard to make good vacuums
so, he was pretty sure that they were not created
by any substance inside the bulb.
Therefore,
he deduced
that they must be something coming off
of the filament itself.
He then added a plate to the bulb
with a hope that whatever was emanating from the filament,
would stick on the plate,
and possibly keep the filament
from breaking in the first place.
The plate turned out to be not much help.
However,
he noticed
that if he added another battery
between the filament
and the plate,
something strange happened.
The current could flow
if the plate was positive,
but would not flow
if the plate was negative.
Edison patented as a volt meter,
although he didn't really know what was going on,
or why anyone would want this expensive
and ineffective volt meter.
What was going on?
Well, it all has to do with the movement
of negatively charged electrons.
In the light bulb.
When the filament got hot,
the electrons could become free.
When the filament broke,(shuttering glass)
the electrons would fly towards the positive end
and leave that black smudge.
If before the filament broke,
you added an extra voltage
between the filament and the plate
and the filament was negatively
charged and the plate positive,
the negatively charged electrons could easily jump off the
filament and onto the positively charged plate,
and let the current flow.
If, however,
the plate was negatively charged,
then the electrons would stay on the plate
as it was cold,
and not concentrated on the thin filament.
This was called the Edison effect.
Although aside from patenting it,
Edison wasn't much interested in it.
However,
Edison had a
young English technical advisor
named John Ambrose Fleming who was interested.
Fleming was there to help Edison with alternating current,
which Edison quickly decided to reject
and sent Fleming back to England.
Jump forward 24 years to October
of 1904.
By this time, Fleming had become a technical advisor
to Guglielmo Marconi,
helping him
with sending wireless telegraph signals.
Marconi's biggest problem in 1904 was in his receiver.
Marconi used a tube with little filaments
that would stick together or cohere when a
radio wave went through it
and then would be tapped when the signal was complete.
It was slow,
and awkward
and inefficient.
According to an engineer at the time,
the coherer quote was publicized as wonderful,
and it was
wonderfully erratic and bad.
It would not work when it should,
and it worked over time when it should not.
The year before in 1903,
a Canadian named Reginald Fessenden,
had made an electrolytic detector,
that used a chemical reaction
to rectify or filter out
one direction of the current,
So that it could be heard on headphones.
Fleming felt,
that the detector did not work well at high frequencies.
Also, they didn't hold the patent.
He was thinking about the problem when he had,
according to him,
a sudden, very happy thought.
Why not try the lamps?
He wrote to Marconi,
quote
"I found a method of rectifying electrical oscillations"
that is,
making the flow of electricity all in the same direction.
By November,
Fleming filed for a patent
for what was called a Fleming valve,
or a vacuum diode
di for two,
ode for path.
Note that the Fleming valve,
was exactly the same
as the Edison effect valve.
What was different was how he used it.
So, let's talk a little bit about what Fleming did.
Fleming used an antenna
with a coil to receive the radio signal.
He then had another coil
so that the alternating
current in the first coil,
would induce an alternating current in the second coil.
He then had that signal go through a sensitive
current meter,
called a mirror galvanometer,
and had one end of the signal go to the plate,
and one to the filament of his valve,
heated by a battery.
The valve thus made
the signal one way,
so that the meter turned in one direction
and could be recorded.
Fleming did not use headphones,
possibly because he was partially deaf.
This worked pretty well,
but it was not very popular
as it was expensive,
and most people started using semiconductors,
and what were called crystal sets
as rectifiers instead.
However,
one person was very interested in the Fleming valve
and his name was Lee de Forest.
At the time,
Lee de Forest was being sued by Reginald Fessenden
for copying his electrolytic detector.
So, de Forest was on the hunt for a new detector.
We know de forest had read about
the Fleming valve,
because in December of 1905,
De Forest filed for a patent.
They used what he describes as quote,
"an electric valve, which has been fully described by
J.A Fleming",
then five weeks later,
deforest files for a patent
for a new detector,
that is strikingly similar to the Fleming valve.
They both had a filament
and a plate in the vacuum tube,
and he did the filament with a battery.
They also both took a radio wave from an antenna
and passed it through the valve
between the plate and the filament.
In fact,
there are only four very minor differences.
First, de forest used headphones
instead of a current meter.
Second, he added an extra battery
that didn't seem to do much.
Third,
He didn't use parallel coils,
at least in this circuit.
And forth,
he incorrectly
thought the trace amounts of gas in the bulb
were ion nodes,
and that
was what was causing the current to flow.
So, he insisted that the bulb
not be a perfect vacuum.
Because this device was used to make sound,
and he incorrectly thought it had to do with the ionization,
of gas.
He dubbed this an audion.
Not surprisingly,
Fleming sued.
Meanwhile, de Forest started adding
pieces of metal all over the place.
He wrapped his valve in tinfoil.
He circled the outside of it with wire.
He had multiple connected plates inside the bulb.
Then on Christmas day of 1906,
deforest ordered the bulb with three connections,
the filament,
the plate,
and between them,
a wire that was bent into a zigzag shape
that he called a grid.
Adding the third wire in the
middle was described in 1922.
As quote, "the most important single step
taken in the whole development of radio communication"
So, what did the wire do?
Well, imagine you set up a vacuum triode
by heating the filament with a battery,
and attaching a separate voltage
so that the filament is negative
and the plate is positive.
In this case, the electrons would
jump off the heated filament,
past the grid wire,
and onto the plate.
Now imagine,
adding a weak signal to the grid wire.
If you add a signal where
the grid wire has a positive charge on it,
many more electrons would be attracted to the grid,
and jump off the filament,
and onto the plate.
This would vastly increase
the current on the plate.
If you add a signal to the grid wire
with a negative charge, however,
it would block the negative electrons
from jumping onto the plate
drastically reducing the current.
In this way,
small changes in the grid wire
can make big changes in the current
coming out of the plate.
This was both an electric amplifier,
and an electric rectifier,
or one-way valve.
Unfortunately,
Lee de Forest,
couldn't get his device to work that well,
and he mostly just used it
as a complicated rectifier.
In addition de Forest was convinced,
that you needed a little bit of gas
to make them work.
Possibly to distinguish it
from the Fleming valve,
which led to some low quality triodes,
because the audion was expensive,
and complicated,
and had variable quality,
very few people used it.
It wasn't until five years after its invention,
When a tenacious, undergraduate named
Howard Armstrong figured out how to make
the triodes sink,
by feeding the signal back to it
in a system he called regeneration.
This is what made the vacuum triode,
the Swiss army knife for the next 50 years.
Meaning they used it for everything.
- Yeah.
If it weren't for these babies,
there wouldn't be any radio.
Yes,
and a million others
owe their jobs to the vacuum tube,
including myself.
As a matter of fact,
practically everybody
has been benefited by it in some way.
- And that story starts next time on
the secret history of electricity.
Thanks for watching my video.
Please remember to give it a thumbs up,
and if you haven't yet subscribed,
it's always a fun thing to do.
If you're interested in learning
more about Fessenden,
and how he made his electrolytic detector,
and the first am signals.
I have a video about that.
If you're interested in learning more about
Lee De Forest and his colorful life,
I have a video about that,
and make sure you check out
the next one about regeneration
and positive feedback,
and Howard Armstrong.
It's going to be a good one.
Okay.
Have a good day.