Positive Feedback (Regeneration) How a College Student Invented it in 1912!

Table of Contents

Edwin Howard Armstrong

The Start of Armstrong’s Invention


Edwin Howard Armstrong

It all started with an extremely smart and tragically stubborn college student named Edwin Howard Armstrong.   Howard (he went by his middle name) grew up in Manhattan when it was just a relaxed suburb of New York to middle class family.  In 1904, when he was 13, his father gave him “The Boy’s Book of Inventions” and young Armstrong was hooked. 

He was especially taken with a romantic description of Guilermo Marconi and his wireless telegraph system and became determined to be an inventor himself[1].  Soon, he had setup his own wireless system in the attic of his house with a terrifying 125-foot antenna that he climbed regularly for fun[2]

When he was 19, Armstrong entered the engineering program at Columbia University under professor P.  When he was a Junior, a friend gave him an Audion (triode) to play with[3].  At the time, most people used crystals instead as they were both cheaper and worked better. 

Armstrong found that if he re-routed the signal into the triode with a capacitor he could get a “very definite increase[4]” in the strength of the signal.  He decided to add an inductor (coil) and a capacitor to the circuit, just as Pousen did with his arc lamp.  Armstrong was shocked to find that it could produce almost one hundred times the strength! 

Armstrong “played” with changing the length of the coil (the inductance) to see what would happen in the system.  To his shock the more inductance he had the stronger the signal he got back.  He called this a “regeneration circuit” where the signal was in a “feedback loop”. 

With de Forest’s Audion, and a series of capacitors, inductors (coils) and resistors, Armstrong could drastically improve the strength of a radio signal (see figure 7-12).  Armstrong’s sister Ethel said that he woke her on the night of September 2nd, 1912 crying, “I’ve done it! I’ve done it![5]”  Suddenly, he was receiving signals from as far away as San Francisco and Ireland[6]

The Start of Armstrong’s Invention

The diode was useful but what made a revolution (and I do not use that word lightly) was an extra piece of metal grating or a wire put in a zigzag between the filament and the plate.  This new object, called an Audion at the time but eventually called a triode was invented by a real character and con artist named Lee de Forest.

So what did this wire do? Well, imagine that the filament had a negative charge and the plate a positive charge, then the electrons would “jump” off the filament, past the wire and onto the plate.  If the wire had a positive charge on it, more electrons would jump between the filament and the plate. 

If the wire had a negative charge, however, it would block the negative electrons from jumping on to the plate.   In this way, small changes in the current in the wire could make big changes in the current coming out of the plate.  This was the first amplifier.  De Forest used it in his radio detector where it worked basically as well as Fleming’s valve. 

De Forest didn’t realize it, but his little grate was the basis of all electronics for the next fifty years: “The triode vacuum-tube is one of a small number of technical devices, such as the printing press and the internal-combustion engine, that have radically changed human culture.[7]” 

On that very same day, Armstrong found another odd development with the Audion.   There was a limit to the amplification, and at a certain point, according to Armstrong’s notes, the “signals changed from clear to hissing[8]”.  Armstrong wondered if he was no longer picking up the Radio signal in the air, but instead with his batteries, coils, capacitors, and the triode, he was actually creating waves with his receiver.  He was right. 

Armstrong didn’t patent his device until 1913 as his father wouldn’t give him the $150 he needed for the patent until he graduated from college[9].  After graduation, he stayed on at Columbia as a graduate student working with his former professor named Michael Pupin who told everyone about his student’s incredible discoveries. 

On January 31st, 1914, Armstrong demonstrated his new amplifying device to three representatives from the Marconi telegraph company including a young Marconi executive named David Sarnoff.  They ended up spending the entire cold night, thirteen hours straight[10], picking up radio signals from incredibly far away.  

Sarnoff and Armstrong formed an instant camaraderie.  Twenty years later Sarnoff wrote to Armstrong about the “thrill which came to me at hearing for the first time signals from across the Atlantic and across the Pacific.[11]”  Afterwards, they would send each other telegrams to mark the anniversary of that date for decades to come[12]

Although Sarnoff wanted to purchase Armstrong’s patents, Marconi wasn’t interested.  They had a system that worked and they had the majority of its business.  Also, they had sued De Forest over the Audion due to it’s similarity to their Fleming valve and were hesitant to use a device that demonstrated that the Audion was superior. 

Meanwhile, de Forest immediately tried to patent “his” regenerative Audion.  When he was told that Armstrong had beat him to it, De Forest claimed that he had figured out these uses for the Audion in 1912 and sued Armstrong. 

In April of 1917 World War 1 had started and by the fall of that year Armstrong had joined the Army and worked for the Army Signal Corps in Paris.  The patent lawsuit was put in stasis for the length of the war and Armstrong gave his patents to the government for free for the duration as well. 

Also, during the war, Armstrong had come up with a new idea. This discovery, called the superheterodyne, would make Radio a household item and both Armstrong and Sarnoff great fortunes, at least for a while.


[1]p 56-7 “Empire of the Air” Lewis

[2]p 63 ibid

[3]p 9 “A History of the Regeneration Circuit: From Invention to Patent Litigation” Sungook Hong, School of Biological Sciences… Seoul, Korea

[4]p 9 “A History of the Regeneration Circuit: From Invention to Patent Litigation” Sungook Hong, School of Biological Sciences… Seoul, Korea

[5]p 70 “Empire of the Air” Lewis

[6]p 11 “A History of the Regeneration Circuit: From Invention to Patent Litigation” Sungook Hong, School of Biological Sciences… Seoul, Korea

[7]p 15 “Dawn of the Electronic Age…” Fredrick Nebeker

[8]p 71 “Empire of the Air” Lewis

[9]p 12 “A History of the Regeneration Circuit: From Invention to Patent Litigation” Sungook Hong, School of Biological Sciences… Seoul, Korea

[10]p 4 “The Boy Genius and the Mogul” Stashower

[11]p 113 “Empire of the Air” Lewis

[12]p 126 “The Master Switch: The Rise and Fall” Tim Wu

Love Kathy Loves Physics? Share This To Your Friends:

Leave a Comment

Your email address will not be published. Required fields are marked *