- Positive feedback is when you take a signal
and you feed it in a loop so it amplifies itself,
and it is a vital part of electrical engineering.
But where does it come from and who thought of it first?
Well, I happened to know the answer to that.
It was invented by a college student named Howard Armstrong
in his parents' attic in 1912.
This is a story of a inspirational children's book,
a neighbor who broke all the rules,
an immigrant with an inspiring story
and a very, very important coil of wire.
Ready? Let's go!.
♪ Electricity ♪
Edwin Howard Armstrong grew up in Manhattan
and Yonkers to an upper-middle-class family.
In 1904, when he was 13 years old,
his father gave him "The Boys Book of Inventions".
And after reading about Guillermo Marconi,
young Armstrong became enamored of all things wireless.
Soon, he was the neighborhood wireless expert.
He even built 125-foot antenna
that he climbed regularly for fun.
When he was 16,
he met a local engineer named Charles Underhill.
And for the next two years,
Armstrong would bike to Underhill's house after school
to be instructed on the fundamentals of wireless.
Armstrong said, however,
that the most important lesson was not in the books,
but in Underhill's insistence,
that Armstrong was an original thinker
and didn't need to be tied down
by what was conventionally known or done.
Later in life, Armstrong supposedly like to say,
"It ain't ignorance that causes all the trouble
in the world.
It's all the things people know that ain't so"
Armstrong's tutelage with Underhill ended
when he entered Columbia University
to study electrical engineering
under Professor Michael Pupin.
Pupin was an amazing individual.
He had immigrated by himself to America
from Serbia 35 years earlier
with only 5 cents to his name when he was only 16 years old.
He had sold almost all of his possessions
to pay for the ticket, including his warm winter coat,
figuring that he didn't need a coat in America
as the Native Americans in the drawings
we're so little clothing.
He then barely he survived
the passage by clinging to the smoke-stack for warmth.
Despite knowing no one, having no marketable skills
and not even knowing English, he managed to get a job
and within five years he had taught himself English, Greek,
and Latin and manage to get a scholarship to go to Columbia.
He then got his PhD in Germany from Hermann von Helmholtz,
and in 1899 returned to Columbia University
as the founder of their electrical engineering department.
At the same time, he sold the patent to AT&T
for cool half a million dollars.
Despite his new found wealth,
he continued to research and study and keep up to date
with the latest in electrical technology.
For that reason, in 1906 Pupin went to a talk
by a man named Lee de Forest
who demonstrate a vacuum tube device
that he had invented called the Audion.
Now, de Forest's original Audion was very, very similar
to another device called a Fleming Valve.
And for that reason,
he was being sued by Fleming for patent violation.
Because de Forest was interested
in distinguishing his devices from the Fleming Valve,
he started inventing new valves
with stray wires all over the place.
On January 29th of 1907,
de Forest filed for two patents
with a total of 12 new tubes.
One of them had three inputs, the plate, the filament,
and between them a zigzag of wire
called the grid.
De Forest called all his tubes Audions,
but this one was quick called vacuum triode,
tri for three, ode for path.
The triode was revolutionary
because it wasn't just a one-way valve.
It was an amplifier and a detector.
Let me explain how it worked.
First, you heat up the film and of the bowl with a battery,
and then you add a separate battery between the filament
and the plate to create current
were negatively charged electrons
jump off the heated filament past the wire
and the onto the positive plate.
The signal is then placed between the grid wire
and the filament.
If the grid is positive,
many more electrons will escape the filament
increasing the current coming out of the plate
called the wing circuit.
And if the grid is negative,
many electrons will be blocked from jumping to the plate.
In this way,
small changes in the grid wire make big changes
in the current in the wing circuit,
which can be detected by headphones.
How did de Forest get a radio signal in the first place?
Well, anyone can catch a radio wave with an antenna,
however, by the 1900s it was common practice
to tune your receiver with two items:
two metal plates separated by glass or air and a coil.
The two metal plates are called a capacitor
as the metal surfaces have the capacity
to store charges on its surfaces
and the glass or air between them keeps the charges
from directly flowing from one plate to the other.
A coil of wire is important for two reasons:
One, if current goes through a wire in a coil
it creates a magnetic field like that from a bar magnet.
And two, if the magnetic field inside a coil changes
it induces a new current in a coil.
Now, if a capacitor has charges on it surfaces,
and then it's discharged through coil,
it will cause the coil to have a change in current in it
and thus, create a changing magnetic field,
which induces more current in the coil.
This extra current charge up the capacitor
in the opposite direction,
the capacitor then discharges in the other direction
starting the process all over again.
The frequency of the oscillation
depends on the properties of the coil and the capacitor.
So, if you want to tune your frequency,
you just change the length of the coil
or the area of the capacitor to make it resonate
with your radio waves.
To recap, de Forest received a signal with an antenna,
tuned his receiver with a coil in the capacitor,
until it resonated at the same frequency
as his incoming radio wave,
and then fed that signal into the grid of a triode.
Finally, he listened to the amplified signal with headphones
that were in the wing circuit between the plate
and the filament.
De Forest's trial was an improvement over the Fleming valve.
However, de Forest never utilized the wing circuit
and he was convinced that you needed trace amounts of gas
in the tube,
which actually made it not work well at radio frequencies.
In addition, the tubes broke easily,
they're confusing the setup
and they were very expensive, a whopping $5,
so they're quite unpopular.
However, Michael Pupin had a few
in his well-stocked radio laboratory at Columbia University.
Now we go back to young Howard Armstrong.
Through Pupin, Armstrong got his hands on a triode
in around 1911,
and he started playing around with it on the side,
although he's mostly busy with schoolwork.
One day in 1912,
he added the capacitor across his headphones
and the signal in the headphones got a little louder.
Armstrong started to wonder if the wing circuit
had high-frequency vibrations in it.
And if so, what to do about it?
While hiking in the mountains with his family,
he was struck by the adage,
"Wherever there are high-frequency oscillations
tune the circuit",
and he decided to try to tune the wing circuit
of the receiver by adding a coil.
As soon as their family returned home from vacation
Armstrong sprinted to his attic laboratory and tried it out.
Instantly, he got much stronger signals.
- But then uncle Howard burst into the room,
waking my mother up and he was dancing round
and round the room with this box and he was saying,
"I've done it, I've done it, I've done it."
- And done it he had,
this was a revolution in radio technology.
After just a few months
of playing around with coils and capacitors,
Armstrong manage to up the signal so much,
he could listen to signals from San Francisco, Brazil,
and Ireland, all from his attic and Yonkers.
The radio signal was fed to the grid
and to the filament at the same time with the same phase
so they amplify each other.
Then the amplified output was taken from the plate
and fed right back into the filament to be amplified
by the signal from the grid again.
This process is called positive feedback or regeneration.
It changed the amplification from a single tube
from around 20 times to around a 1,000
to a 100, 000 times stronger.
See the power of a coil, well, right?
Armstrong notice something else
on that fateful night in 1912.
When he amplified the signal
by changing the length of the coil,
after awhile, the amplification cut out
and was replaced by loud hissing noise.
He quickly realized
that his system was not just receiving signals,
but was creating its own radio waves.
Years later, Pupin said that,
"Armstrong system maintained its pitch
with a degree of accuracy never before obtained
by any apparatus constructed by man."
This consistency of note
was vital for using radio waves to transmit sound.
Once again, from Michael Pupin,
"It goes without saying
that long-distance radio communication
and radio broadcasting would be impossible
without this invention."
On January 30th, 1914,
a young executive from the Marconi Telegraph company
named David Sarnoff met with Armstrong for demonstration.
They ended up receiving signals
throughout the entire cold night, 13 hours straight.
The 22-year-old Sarnoff felt that Armstrong had,
"The most remarkable system in existence",
and soon Sarnoff tried to get his superiors interested
in using vacuum tubes to transmit music
into a "Radio music box."
It was Sarnoff's drive
as well as a new invention of Armstrong's
called the super heterodyne.
There was to make radio a household item
and both Sarnoff and Armstrong's great fortunes,
at least for awhile.
And that story is next time,
on The Secret History of Electricity.
♪ Electricity ♪
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