How Howard Armstrong Invented Positive Feedback

Positive feedback in electronics, or when a small change is feed into a loop to magnify itself is a vital part of electrical engineering.  But where did it come from and how does it work?  I actually know the answer to that: it was created by a 20-year-old college student named Howard Armstrong in his parent’s attic on a summer night in 1912! This is a story of a childhood book, a rule-breaking neighbor, an inspiring immigrant, and a very, very, very important coil of wire. 

Table of Contents

Edwin Howard Armstrong’s Early Life

The Audion

How the Audion Works

How Armstrong Created the Positive Feedback

Edwin Howard Armstrong’s Early Life

Edwin Howard Armstrong (who went by his middle name) grew up in Manhattan and Yonkers to an upper-middle-class family.  In 1904, when he was 13, his father gave him “The Boy’s 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 a 125-foot antenna that he climbed regularly for fun! When he was 16, Armstrong 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 powerful 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 commonly done or known.  Latter in life, Armstrong supposedly liked to say: “It ain’t ignorance that causes all the trouble in the world.  It’s the things people know that ain’t so.”

Armstrong’s tutorage with Underhill ended when he entered Columbia University to study electrical engineering under Professor Michael Pupin.  Pupin had immigrated by himself to America from Serbia35 years earlier with only 5 cents to his name when he was only 16 years old. 

He had sold almost all of his possessions for the money for the ticket (including his coat which he figured wasn’t needed as the Native Americans in the drawings wore so little clothing)!  He then barely survived the passage by clinging to the smoke stack for warmth. 

Despite knowing no one, having no marketable skills, and not speaking English, within five years he had taught himself English, Greek, and Latin and managed to get a scholarship to go to college at Columbia (He chose Columbia because Alexander Hamilton went there and Pupin was a big fan after he read that this Founding father was also an immigrant!). 

The Audion

He then got his Ph.D. 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 a patent to AT&T for a cool half a million dollars! Despite his new wealth, Pupin continued to teach, research, 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 demonstrated a vacuum tube device that he had “invented” called an Audion.   Pupin was unimpressed and felt that de Forest didn’t seem to understand how it worked saying, “Why does it operate? If Dr. de Forest cannot explain it, I certainly cannot.”

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 eager to distinguish his devices from the Fleming valve, he started “inventing” valves with stray wires all over the place! 

For example, on January 29th of 1907, de Forest filed 2 patents with a total of 14 new tubes.  One of them had three inputs: the plate, the filament, and between them, a piece of zigzag wire called the grid.  De Forest called all of “his” tubes Audions, but this one was quickly called a three-element Audion or a vacuum triode (tri for three, ode for path). The triode was revolutionary because it wasn’t just a one-way valve it was actually an amplifier. 

How the Audion Works

Let me explain how it worked.  First, you heat up the filament of the bulb with a battery and then you add a separate battery between the filament and the plate to create a current where negatively charged electrons jump off the heated filament, past the wire and onto the positive plate.  

The signal is then placed between the grid wire and the filament.  If the grid is negative, many more electrons will be attracted, increasing 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, reducing the current in the wing circuit. 

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 the radio signal in the first place?  Anyone can catch a radio wave with an antenna, however, by the early 1900s it was common practice to tune the receiver with two items: two metal plates separated by glass 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 between them keeps the charges from directly flowing from one plate to the other. 

A coil of wire is important for two reasons: 1) if current goes through the wire in a coil it creates a magnetic field like that from a bar magnet and 2) if the magnetic field inside a coil changes it induces a new current in the coil. Now, if a capacitor has charges on its surfaces and then is discharged through a coil, it will cause the coil to have changing current in it and thus create a changing magnetic field, which induces more current in the coil. 

This extra current charges 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 capacitor and the coil, so if you want to tune the frequency, you can just change the length of the coil or the shape of the capacitor to resonate with your radio waves.

To recap: de Forest received a signal with an antenna, tuned his receiver with a coil and a 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 triode was a slight improvement over the Fleming valve or other one-way valves.  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, were confusing, and were very expensive (a whopping $5), so they were quite unpopular at the time.  However, Michael Pupin had a few in his well-stocked radio laboratory at Columbia University.   

How Armstrong Created the Positive Feedback

Now we go back to young Howard Armstrong.  Through Pupin, Armstrong got his hands on a triode in 1911 and started playing around with it on the side, although he was mostly busy with schoolwork.  One day in 1912, he added a 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 oscillations in it, and if so, what to do about it.  While hiking in the mountains with his family he was struck with 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” to test it out.  Immediately, he got significantly stronger signals and ran to his sister’s room [clip from “Empire of Air”] and done it he had, this was a revolution of radio technology. 

After just a few months of playing around with coils and capacitors, Armstrong managed to up the signals so much that he could listen to signals from San Francisco, Brazil, and Ireland: all from his attic in Yonkers! 

Michael Pupin called it, “epoch-making”.  The radio signal was fed to the grid and to the filament at the same time with the same phase so they amplified 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 louder to between 1,000 and 100,000 times louder! 

Armstrong noticed something else on that fateful night of 1912.  When he tuned the system by increasing the length of the coil, the incoming signal would get louder and louder until suddenly it would cut out and be replaced by “a loud hissing tone,” Armstrong quickly realized that his system wasn’t just amplifying the signal – it was creating a radio signal of it’s own!  

Pupin said that this system maintained, “its pitch with a degree of accuracy never before obtained by any apparatus constructed by man.”  This consistency of the note was vital for using radio waves to send 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 in a shack in New Jersey for a demonstration. They ended up receiving signals throughout entire cold night, thirteen 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 idea of Armstrong’s called superheterodyne that would make Radio a household item and both Armstrong and Sarnoff great fortunes, at least for a while.  And that is next time, on the secret history of electricity

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