Between 1907 and the 1920s most AM radio was sent out using something called an arc transmitter (or a Poulsen arc). In fact, the tragedy of the Titanic in 1912 was partially caused because they didn’t use the arc transmitter. But how does an arc transmitter work, why was it created and what does that have to do with the Titanic? This is the story of a chemist with a basement full of batteries, a typo, a trailblazing female scientist, her husband’s musical grad student, a Dane who made the impossible possible, and an overworked and testy telegraph operator. Ready? Let’s go!
In the very early 1800s, a handsome English chemist named Humphry Davy filled a basement with batteries to conduct electrochemical experiments. In 1809, he noticed that if two carbon rods had a large voltage between them they would create a bright and constant spark. Davy wanted to call it an arc lamp as the light made an arch, but it was quickly called an arc lamp due to a typo. It wasn’t until the 1850s that they had generators to use these “arc-lamps” to light public spaces but it was very popular in the 1860s and 70s. In the 1880s arc lamps began to be replaced with incandescent light bulbs although they were still used even in the 1920s for bright lights for film or searchlights.
Now we come to a phenomenal English woman named Hertha Ayrton. She was born in extreme poverty but with some help from early suffragettes managed to get an education in Math and Science. When she was 30, she took an evening class in electricity and fell in love with the teacher William Ayrton and they married the next year. William Ayrton was conducting experiments on the physics of arc lamps as he wondered why they would sputter and hiss. In 1893, his work was destroyed in a fire, and, according to Hertha he “had neither the courage nor the inclination to rewrite the paper”. Hertha then took over the research and quickly produced twelve papers in The Electrician on the Physics properties of arc lamps. She became the first woman to give a talk at the Institution of Electrical Engineers as well as their first female member. [She was also nominated to be a fellow of the Royal Society but was rejected because as a married woman she was the property of her husband and property could not be a fellow!] She determined (among many other things) that arc lamps hiss because of the oxygen in the air. Shockingly, she also found that for certain arcs increasing the voltage actually decreased the current. What was going on? Hertha’s husband William became convinced from Hertha’s results that the arc-lamp must have negative resistance! William convinced others to conduct experiments whose results, “together with the curves found by Mrs. Ayrton, lead to the conclusion that the arc has a negative resistance”. This created a HUGE controversy. William Ayrton said, “had [we] lived in the Middle ages we should undoubtedly have been burned in the solid carbon arc.” Put a pin in that, it’s important.
William Ayrton had a graduate student named William Duddell who also began studying arc lamps. He felt that Hertha had left him a window to study what happens to arc lamps when you change the current “more or less rapidly”. While playing with changing the current across a lamp, he added a condenser (also called a capacitor) next to the arc lamp powered by a battery. To his surprise, the system, which was powered by direct current, created alternating current, which made the whole system ring musically. Duddell said, “here then was a puzzle – a direct-current solid arc… became intermittent and gave out a musical note on simply shunting the arc with a condenser. ” Duddell straitened the lead wires and the musical note disappeared but if he had the wires go through a coil “the sound was greatly magnified.” What was going on? Duddell quickly realized that the capacitor and the coil oscillated and the arc lamp first started and then amplified the signal.
Let me explain. A capacitor is made of two conductive materials with an insulator between them, which can store charges on its surfaces. If those charges are discharged through a coil then the changing field in a coil will create more charges to flow causing the capacitor to become charged in the opposite direction. This will then cause charges to flow in the opposite direction. Therefore, if you discharge a capacitor through a coil it will create an oscillating wave, however, the wave will die down due to friction. Duddell realized that the arc itself played two roles. First, the arc lamp with a capacitor and coil converted some of the direct current into alternating current. It did this because when you first connect the circuit the battery would charge up the capacitor until it reached a certain voltage and then the electricity could jump over the arc. When the current jumps over the arc it also discharges the capacitor through a coil creating the alternating current. Secondly, the alternating current from a capacitor and a coil is kept from dying out by the arc (with its negative resistance) which converts some of the energy from the battery to the alternating current to keep it “ringing”. Duddell had created a smooth AC generator with no moving parts and one of the first (if not the first) electrical resonant circuits! This is the backbone of almost all radio! Duddell was convinced, however, that his device could never create more than 10,000 waves per second, way below the frequency of radio waves.
As a side note, Duddell knew that the frequency his system generated depended on the size of the capacitor and the length of the coil. He actually adjusted the connection to the coil to make the lamp play “a distinct and not unmusical rendition of ‘God Save the Queen’” which also makes him the inventor of the first electronic keyboard.
A Danish scientist named Valdemar Poulsen heard about Duddell’s “singing arc lamp” and he thought it would be perfect for making a device that would “sing” at radio frequencies. Poulsen said that radio waves that were produced before were like “the sound waves produced by a pistol shot” while his continuous waves were like “the waves of sound produced by a tuning fork.” How did Poulsen succeed where Duddell failed? Poulsen started with Hertha Aryton’s statement that the hissing of arc lamps had to do with reactions with oxygen in the air, Poulsen put his lamp near a machine called a “spirit-vapor” that emitted hydrogen gas. This “ate up” the oxygen which made the signal more constant and caused the amplifier to work better, especially at higher frequencies. He also found that if he made his oscillations too fast, sometimes the capacitor would spark and glow. He attempted to solve this problem by adding a magnet or an electromagnet in all sorts of orientations and found that if he placed a magnetic field over the gap in the arc lamp, he could successfully create very high frequencies. The magnetic field would stop most of the direct current from flowing between the two carbon rods but it would not stop the high-frequency alternating current. These two things alone, a magnet and a vapor lamp would create a simple and crude radio transmitter that was basically at a constant frequency.
Poulsen refined it further and found that a copper-carbon arc worked better than a carbon-carbon arc (although the copper had to be kept cool with water). Finally, he found that if they rotated the carbon electrode it would help keep the signal steady. Because this machine would produce radio waves at a constant frequency you could send several signals at different frequencies at the same time. Also, smooth radio waves are required for transmitting sound wirelessly on AM radio.
Poulsen had an idea that his arc transmitter could be, “used as an electric generator for wireless telegraphy and telephony.” And he was right! Unfortunately, Guillermo Marconi was not interested in new technology, especially one for that he did not have exclusive patents! This was tragically played out in 1912 when the Titanic fitted with Marconi’s old-fashioned shotgun “spark gap” transmitters that could only send one message at a time. When the Titanic passed the point where they could send messages to the US, the sole wireless operator was overwhelmed with messages from and to his wealthy passengers. At the same time the SS Californian stopped 20 miles away due to an ice field and sent the Titanic a message that said “Say, old man, we are surrounded by ice and stopped.” However, the Titanic operator probably didn’t hear what the Californian’s operator said as he was blown out by the volume of the nearby boat, and, irritated, the Titanic operator replied, “shut up, shut up, I am busy.” The operator of the Californian was offended and sent a blast of wireless noise and, at 11:35 pm, went to bed. Fifteen minutes later, the Titanic hit an iceberg. The officers on the Californian misunderstood what was happening and stood by as the Titanic sank. If they had used arc transmitters, they could have had multiple people sending multiple signals simultaneously at different frequencies and Titanic’s operators might not have been so overworked. Also, like today, they could have special frequencies just for distress signals and warnings. Marconi eventually learned his mistake and by 1922, 80% of all commercial wireless signals were from arc transmitters.
Back in 1904, Poulsen demonstrated his transmitter at a conference in Saint Louis. One person in attendance who was very interested in sending sound wirelessly was the self-described “father of radio”: Lee de Forest. De Forest knew that his “arch rival” Reginald Fessenden had been sending sound wirelessly since 1900 and was working on an alternator to create smooth radio waves. However, the alternator was a feat of engineering, nearly impossible to copy. The crude Poulsen arc, however, was far easier to make at home. How this con artist stole his way into Radio history is next time on the secret history of electricity.
Also, if you want to know more about the amazing Hertha Ayrton I will produce a video about her as well and will provide a link at the end of this video.
 Aryton, Hertha “The Electric Arc” The Electrician Vol 34. 1895 p. 335
 Frith and Rodgers, “On the Resistance of the Electric Arc” Proceedings of the Physical Society, v. 14 (1895-6) p. 311
 Dudell, William “Current Through Direct-Current Arc…” Journal of the Institution of Electrical Engineers, v. 30 1900-1901 p. 247
 p 62 “The Poulsen ‘Wireless’” The Graphic, January 12, 1907
 Poulsen, V. “System for Producing Continuous Electric Oscillations” Transactions of the International Electrical Congress, St. Louis 1904 p. 971