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The arc converter, sometimes called the arc transmitter or Poulsen arc after its inventor Danish engineer Valdemar Poulsen, is a device that used an electric arc to convert direct current electricity into radio frequency alternating current. It was used in radio transmitters for a short period around 1920 before it was replaced by vacuum tubes, and was one of the first technologies that was used to transmit sound (amplitude modulation) by radio. It is now on the list of IEEE Milestones as a historic achievement in electrical engineering. [1]
The English engineer William Duddell discovered how to make a resonant circuit using a carbon arc lamp. Duddell's "musical arc" operated at audio frequencies, and Duddell himself concluded that it was impossible to make the arc oscillate at radio frequencies.
Valdemar Poulsen, who had demonstrated the 'Telegraphone' (the world's first magnetic recording device) at the Paris Exhibition of 1900, turned his inventive genius to the problem and succeeded in raising the efficiency and frequency to the desired level; Poulsen's arc could generate frequencies of up to 200 kilohertz and was patented in 1903.
After a few years of development the arc technology was transferred to Germany and Great Britain in 1906 by Poulsen, his collaborator Peder Oluf Pedersen and their financial backers. In 1909 the American patents as well as a few arc converters were bought by Cyril F. Elwell. The subsequent development in Europe and the United States was rather different, since in Europe there were severe difficulties for many years implementing the Poulsen technology, whereas in the United States an extended commercial radiotelegraph system was soon established with the Federal Telegraph Company. Later the US Navy also adopted the Poulsen system. Only the arc converter with passive frequency conversion was suitable for portable and maritime use. This made it the most important mobile radio system for about a decade until it was superseded by vacuum tube systems.
Unlike the spark-gap transmitter converter, the arc converter produces undamped or continuous waves (CW). This was an important feature as the use of damped waves resulted in lower transmitter efficiency and communications effectiveness, while covering the RF spectrum with interference. This more refined method for generating continuous-wave radio signals was initially developed by Danish inventor Valdemar Poulsen. The Poulsen arc converter can be likened to a continuous-duty-rated electric arc welder with a tuned circuit connected across the arc. The negative resistance characteristics of an electric arc permits the creation of a relaxation oscillator that converts direct current to radio frequency energy. The arc converter consisted of a water-cooled bronze chamber in which the arc burned in hydrogen gas between a carbon cathode and a water-cooled copper anode. Above and below this chamber there were two series field coils surrounding and energizing the two poles of the magnetic circuit. These poles projected into the chamber, one on each side of the arc to provide a magnetic field. This field helps to stabilize the arc and improve overall conversion efficiency. In today's world one can still find oscillators based on negative resistance devices; the tunnel diode is one of them.
Since the arc took some time to strike and operate in a stable fashion, normal on-off keying could not be used. Instead, a form of frequency shift keying was employed. The arc was left operating continuously and a portion of the tuned circuit was shorted out when the key closed. Therefore, the "mark" (key closed) was sent at one frequency; the "space" (key open) at another frequency. If these frequencies were far enough apart, and the receiving station's receiver had adequate selectivity, the receiving station would hear standard CW when tuned to the "mark" frequency. This emission of signals at two differing frequencies was eliminated by the development of uniwave keying by Lt. W. A. Eaton, USN. wherein the generated energy was dissipated in to a noninductive resistive absorbing circuit during open key periods.
It was most successful when made to operate in the frequency range of a few kilohertz to a few tens of kilohertz. The passive frequency multiplier was relied upon to bring the output frequency up to practical transmission frequencies.The frequency multiplier and antenna tuning had to be selective enough to suppress the high harmonic output of the arc converter.