Arc Lamp into Arc Transmitter

First things first: Edison's light bulb had a filament, it was not an arc lamp. The Arc lamp was crucial to the invention of the arc transmitter. Actually early on they were more or less the same device. the key difference was that a true transmitter transmits on a specific frequency (more or less) and that ideally it be designed to convey data. early designs had trouble with both of these things.   The 1888 book Municipal Lighting has a nice quote to start us off.

"For many years it has been known that an extremely brilliant light could be produced by slightly separating two pencils of carbon, through which a powerful current of electricity, was passing, as the mysterious force spans the gap with an “arc” of intense light. The one insuperable bar to the general introduction of this light, was its great cost, due to the necessity of producing the current by the consumption of zinc in the galvanic battery. In spite of this expense the arc light early found a limited application to lighthouses, and other important government works."

The British chemist Humphrey Davy is usually credited with inventing the arc lamp. But we don't know exactly when Davy made his discovery. Different sources cite anywhere from 1802 to as late as 1809. (The later dates are more credible) But in all versions he used charcoal strips as electrodes and connected them to a battery. The arc he then generated between those two terminals  produced an intense light. But the effect had been known prior to him. It was probably encountered by several other electrical experimenters but it was documented at least as early as 1802 by Vasily Vladimirovich Petrov. He was a Russian scientist working on copper-zinc batteries he described his arc as a "special fluid with electrical properties." Davy's light was brighter, and his work is what made artificial lighting seem practical, and commercially viable.

Here's how it works. The arc is the discharge that occurs when a gas is ionized. In modern arc lamps that can be neon, mercury,  argon, xenon, krypton, or sodium.then high voltage is applied and the tips of the carbon rods are heated to incandescence, generating light. Modern arc lamps use tungsten instead of carbon. In Davy's arc lamp, the electrodes are carbon rods in free air, his electrodes need to be in contact to arc. The electric current actually heats the terminals and maintains the arc across the teeny tiny gap. In modern lights it's different because we use a ballast to match the current etc.

The first arc lamp was not suitable for street lighting; it had a few problems. The electrical requirements were too high. Generator improvements reduced that problem but the electrical arc heating up the carbon electrode burned it away. Eventually the gap was too wide for the arc to jump. That was resolved by Paul Jablochkoff. In 1870, he devised a lamp that used two parallel carbon rods lengthening their life.

The only real remaining problem after that was this damn humming noise—a byproduct of the generated sparks. Several inventors seized on those implications: Nikola Tesla in 1886, Elihu Thomson in 1892, William Duddell in 1897, Reginald Fessenden as early as 1900... the history of radio rolls on from there. You can revisit this 19th century technology courtesy of a how-to video from MAKE:

Duddell's singing voice

The first AM transmitters were arc transmitters aka arc converters. The first model to generate a continuous wave (CW) were invented separately by Elihu Thomson and William Du Bois Duddell. These models generate a carrier wave by continuously shocking a tuned circuit. This was the best we had until vacuum tube technology appeared on the scene.

Previous to these models transmitters were all spark gap transmitters. These just emitted a series of short arcs hence the term. These sparks were noisy and emitted broadband noise. Real improvements to these transmitters just timed the sparks so instead of emitted a series of damped waves, they edged closer to transmitting a continuous wave. This is what Marconi was using from about 1895 forward. He was still using them in 1901 for his public transatlantic experiments even when there was superior technology available.

Elihu Thomson had beat the hell out of that spark gap idea by 1892 (patent 500630). He discovered that a carbon arc with a tuned circuit would "sing" i.e. emit audio frequencies as well as radio frequencies. This is only a year after Tesla's invention HF coupled oscillatory circuit (patent 454622) in 1891. Tesla's was for arc lights, but it's actually closely related. Thomson's work also extended into arc lighting. This is the transmitter Reginald Fessenden used in his experiments.

It's worth noting that one of the reasons that it is believed that Fessenden may have broadcast voice and even music is that this arc converter couldn't be stopped and started (make/break) fast enough for practical Morse code. (This is widely debated) But it had other more serious problems. The intensity was variable as was the frequency. Fessenden made numerous improvements of his own mostly between 1900 and 1903. after that he moved onto rotating spark-gap transmitters that were generally inferior concepts.

But before Fessenden and after Thompson is where we squeeze in William D. Duddell. e was a a British physicist and electrical engineer. Like Thompson and Tesla he was working on arc lighting. In 1897 he invented an Oscillograph sensitive enough to see the shape of a wave with a frequency of up to 100 Hz. Tools matter. It was the first time we could see AC waveforms. In 1899 he discovered that arc lamps could emit frequencies of up to 1 MHz. His own oscilloscope couldn't quantify that, nor could any until after 1920. Around 1900 he invented an arc transmitter based around a carbon arc (like Thompson) but with a shunt to a resonant tuned circuit. This cancelled out the resistance of the RLC circuit. RLC stands for  Resistor, Inductor, and a Capacitor. [Engineers use the character "L" for inductance, in honor of the physicist Heinrich Lenz. But also because we are poor typists. ]  More here.

Duddell's Singing Arc Transmitter was only able to emit waves at about up to 15,000 Hz. Most of it's output was still audible sound. He had doubts that it could emit RF at all. He played "God Save The Queen" at a demonstration in 1901. This was the device that Valdemar Poulsen began improving up on in 1903 (patent 789449), and commercially feasible in 1908. The Poulsen arc transmitted continued to be used until about 1920. More here.

Valdemar: Great Dane

Valdemar Poulsen is chronically under-rated by history. He was Danish, which is probably part of the problem. Popular radio history is very anglo-centric. We Americans carve out exceptions for Marconi, Hertz and Tesla, but little more. So this leaves Valdemar Poulsen on an island so to speak. But he was indisputably brilliant and far ahead of his time. In 1902 (30 years before Edwin H. Armstrong) Poulsen invented FM. In September of 1902 he patented an RF arc oscillator that used FM. He wasn't the first, that was Cornelius Ehret, but that's beside the point. Ahead is ahead whether it's 30 years or 30 minutes.

Here's the difference: while Ehret imagined an FM system and patented an FM system, he failed to make a working prototype. Poulsen succeeded where Ehret failed. He imagined and patented a working model. If that's not impressive enough, he also invented magnetic recording. So... why don't you know who this guy is? More here.

He was a poor student, and was particularly bad with mathematics. He family pushed him into medical school and he didn't do so well there either. He found work at the Copenhagen Telephone Company where he had time to experiment. He may or may not have read Oberlin Smiths Electrical World article on theoretical Magnetic recording. But his world was along those lines. Oberlin thought wire recording was impractical and was thinking more aline the line of a threat embedded with ferrous particles. Smith turned out to be right in the long run. But Poulsen managed to get it to work with wire, a battery, a magnet and some telephone parts. On 1st December 1898, he filed a patent in Denmark for the Telegraphone, the first magnetic recording device in history. In a variation of the wire method he also patented a 4.5 inch diameter steel disk with an embossed magnetic recording and playback was achieved by spinning the disc... like a hard drive.

In 1908 he developed an arc converter that we now call the Poulsen Arc Transmitter.  It was capable of transmitting at frequencies up to 100 kHz. This was widely used in early experimental radio before the rise of the vacuum tube. It was a huge improve ment on Duddell's Singing Arc Transmitter which only could achieve 10kHz. By 1920 some Poulsen Arc transmitters were operating as high as 1000kW!

He died on 23 July 1942. He was 73 years old. Germany had fully occupied Denmark by 1940, but they allowed the Danish government to function until 1943. So Poulsen never saw Denmark liberated, but he never saw it dominated either.