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.

The Transformer (Part 4)

Elihu Thomson was a geeks geek. It has been said that as a boy, his first invention was an electric friction generator built from a wine bottle. He became a science professor at Philadelphia's central High school. This wasn't a public high school, this was a different type of institution they did, and still do, grant BAs in addition to high school diplomas. I explain that because he was a professor, not just a teacher. In 1880 with another teacher, Edwin Houston, he started the Thomson-Houston Company. He was 23 years old, and Houston was 33. They were focused on the new hot item: arc lamp systems. More here. This set a huge number of modern events in motion.

The Thomson-Houston Electric Company was founded in 1883 by a group of shoe manufacturers based in Lynn, Massachusetts. Those investors, led by Charles A. Coffin, bought out Elihu Thomson and Edwin Houston's American Electric Company from their New Britain, Connecticut investors. Like the Gaulard and Gibbs transformer, the Thomson-Houston had a number of patent conflicts. But unlike Gaulard and Gibbs, they had the backing to pull a Google and just buy up the damn patents. In 1889 they bought out Charles Brush and ended their biggest problem. More here.

Their 2nd  biggest problem was Westinghouse. In 1886 Westinghouse took it's improved Stanley transformer and installed it's first arc light system in Buffalo at an existing Brush Co. station. That same year Westinghouse won out on a patent for AC distribution over Thomson-Houston. To his credit, rather than fight Westinghouse endlessly patent-by -patent, Thomson convinced them to share. By 1887 Thomson-Houston was manufacturing Westinghouse arc systems, and Westinghouse had a license to sell Thomson-Houston arc lighting gear. The deal lasted 2 years, ending when the Westinghouse patent was declared invalid. It didn't matter, Thomson had been hard at work trying to circumvent the patent anyway. The MBAs call it coopitition now.

Thomson was on a 2-year tear, patenting  more transformer models; different shapes, different laminations, different cores and eventually a core that sat insulated in an oil bath. When the Westinghouse patent died, Thomson-Houston patents were unavoidable. Westinghouse will owned the Albert Schmid improvements to the Stanley design, but they were otherwise in a sea of Thomso's patents. In 1890, Fleming wrote that there were 100,000 lamps using the AC dynamo in the Thomson-Houston system. He went on to describe their transformer.

"The field magnets are of cast iron, and are made in two parts, the lower one bearing the pedestals for the armature bearings. Each casting carries projecting inward from it one-half the total number of radial pole pieces. The number of polar projections varies from eight in some of the smaller sizes to twenty-two in the largest. The armature is a laminated iron cylinder, and has applied to its surface a set of flat coils which form one layer of double-insulated copper wire of square cross section. This armature winding consists of a number of flat oval coils, equal in number to the number of polar projections, wound in a lathe on special formers, and curved so as to fit the cylindrical surface of the iron core. There is a space of three-quarters of an inch or more In the centre of each coil which, in some machines, is filled with an insulating slip or core, and in others with a second separate coil used for exciting the field of the dynamo. The armature coils are securely bound to the armature core by German silver wire, and are covered with insulating strips of mica. These coils are either connected all in series and in series with the external circuit..."

You can see from that description that this transformer also draws heavily on those that came before it.Nonetheless the Thomson-Houston Co. patents were legit because they were narrow and specific. In 1892, Thomson-Houston merged with the Edison General Electric Company and history rolled on. In 1909, Thomson became the first recipient of the Edison Medal, bestowed by the American Institute of Electrical Engineers AIEE, what we now call the IEEE. He was president of the organization from 1889-90, and acting president of MIT from 1920-1923. He died in 1937. Edwin J. Houston was president of the IEEE twice actually, he took a different path, forming a consulting firm in 1894 with Arthur Kennelly.


There is a lot more to be said about Thomson-Houston and GE. I recommend the book Innovation as a Social Process: Elihu Thomson And The Rise of General Electric by W. Bernard Carlson. There were other important transformers that followed but most of them were a part of the natural progression to make components smaller. Here's a short list of models I didn't get into for those of you who are obsessive enough to read more.

• Charles Brush Transformer - 1881
• Lowrie-Hall Transformer - 1886 
• Mikhail Dolivo-Dobrovolsky  Three-Phase transformer - 1888
• Swinburne-Hedgehog Transformer- 1890
• Oerlikon Transformer - 1891
• Siemens Cable Transformer - 1891
• Frankfort-Laffen Transformer - 1892
• Schuckert Transformer 1894
• Elkhart Transformer - 1894