### William Duddell's Singing Sparks

May 15, 2012

Electricity was known to the ancient Greeks. They produced triboelectricity by rubbing amber against other materials, such a wool. In fact, the word, "electricity," is derived from the Greek word for amber, "ηλεκτρον" (elektron). I wrote about triboelectricity in a previous article (Triboelectricity, July 17, 2007).

 Portion of "On Stones," a discourse on minerals (c. 300 BC) by Theophrastus, mentioning the attractive property of amber.[1]

The triboelectric effect was first noted by Thales of Miletus, who was remarkable in his age for attempting to explain natural phenomena without intervention of the gods. He was also a mathematician, and he is credited with Thales' Theorem that any angle inscribed in a semicircle is a right angle (see figure).
 Proving Thales' TheoremAOB and BOC are isoceles triangles(α + β) + α + β = 180°2(α + β) = 180°α + β = 90°(Rendered using Inkscape).
The ancient Greeks were also aware of pyroelectricity. Theophrastus wrote about the ability of the mineral, tourmaline (λυγγουριον), to attract straw and ash when heated. Since straw and ash are lightweight insulators, they are easily moved by a static electrical charge, much as when pieces of paper are attracted to a plastic comb. The pyroelectric effect can be used for energy-harvesting, as I wrote in a previous article (Pyroelectric Energy Harvesting, October 15, 2010).

After the experiments of Benjamin Franklin, the study of electricity was revitalized as experimenters tried to apply this physical phenomenon to everything in the world. One mechanical method of generating electricity after Franklin's time used the triboelectric effect. The Wimshurst machine, developed by the British engineer, James Wimshurst (1832-1903), contains two disks that rub together in a contrary circular motion by a hand crank. Metal patches transfer the generated charge to outer electrodes.

Static electricity was a limited resource for electrical experiments. Volta's invention of the battery, and Michael Faraday's electrical generator, helped electrical experimenters by supplying ready sources of voltage.

Faraday's generator required permanent magnets, but electrical generation was advanced with the invention of the electromagnetic dynamo by Werner von Siemens and Charles Wheatstone. Dynamos were able to supply both high voltages and high currents, and their only external power requirement was a rotary steam engine. Siemens employed his dynamos to power electric arc furnaces for melting metals.

 An Early Dynamo.Fig. 1 of US Patent No. 284,110, "Dynamo-Electric Machine," by Karl Zipernowsky and Maximillian Déri, August 28, 1883. (Via Google Patents).[1]

Siemens used electric arcs as a heating method, but London used carbon arc lamps for street lighting, replacing all of its gaslights by 1900. Incandescent lights, although suitable for indoor use, were too dim and too short-lived for street use. Humphry Davy, a British chemist, invented the arc lamp in 1809. Davy's arc lamp was powered by a battery, and it had charcoal strips for electrodes.[3] About a decade after London was lighted by carbon arc lamps, Charles Steinmetz patented his mercury arc light (US Patent No. 1,025,932). I wrote about Steinmetz in a recent article (Charles Proteus Steinmetz, May 3, 2012).

There were two problems with London's carbon arc lighting system. It required a small army of technicians to keep it operating, since the carbon electrodes are consumed.[3] Another problem was that the arc lamps produced sound as well as light. In 1899, William Duddell a British physicist and electrical engineer, was asked to look at this noise problem.

Not surprisingly, the noise was found to be caused by the nonlinear nature of the arc that resulted in a negative resistance. Duddell found that adding a parallel inductance-capacitance circuit across the arc formed an oscillator, and the frequency of oscillation could be tuned.[3]

Interestingly, the oscillations could be heard as tones when the circuit was tuned for an audio frequency. Similar tones were observed by a German scientist, a "Dr. Simon," a few years earlier, when he modulated the current supply of an arc lamp,[4] but it does not appear that Duddell knew of this work.

Duddell wired a keyboard to an arc lamp, and he played some tunes at a meeting of the Institution of Electrical Engineers in 1899. This is reported by Nature to have been in 1898. Perhaps the optical character recognition for this paper is to blame.[4]

Duddell, were he working today, would have received a poor performance review by his manager. He never did eliminate the carbon arc noise, but he showed how it could be made much worse! Duddell never patented his invention, but in 1902, Valdemar Poulsen a Danish electrical engineer and his assistant, Peder Pedersen, realized that such a device would function as a radio transmitter if the circuit was tuned for radio, rather than audio, frequencies.[3]

Poulsen and Pedersen's arc converter was used for a decade before this technology was supplanted by vacuum tubes. The arc converter was able to transmit audio signals via amplitude modulation in an era when continuous wave transmission using Morse code was predominant. Poulsen also invented the magnetic wire recorder.

Duddell was later named president of the Institute of Electrical Engineers, and he was elected to membership in the Royal Society in 1907.

### References:

1. Earle Radcliffe Caley and John F.C. Richards, "Theophrastus on Stones: Introduction, Greek Text, English Translation, and Commentary," Ohio State University (Columbus, Ohio, 1956). Greek text, p. 23; English translation, p. 51, as follows:
"... And since amber is also a stone—for the kind that is dug up is found in Liguria—the power of attraction would belong to this too. The stone that attracts iron is the most remarkable and conspicuous example. This also is rare and occurs in few places. This stone too should be listed as having a similar power."
2. Karl Zipernowsky and Maximillian Déri, "Dynamo-Electric Machine," US Patent No. 284,110, August 28, 1883.
3. December 20, 1900: Nature reports on William Duddell's 'musical arcs', APS News, vol. 19, no. 11 (December, 2010), p.2.
4. Some Experiments on the Direct-Current Arc, Nature, vol. 63, no. 1625 (December 20, 1900), pp. 182-183.
"On Thursday last, December 13, Mr. W. Duddell read before the Institution of Electrical Engineers a paper on "Rapid Variations in the Current through the Direct-Current Arc," which he illustrated by experiments. Members of the Institution have already learnt from the experimental demonstration given by Mr. Duddell in 1898, when he read the paper by Dr. Marchant and himself on the alternate current arc, to expect from him most interesting experiments. Nor were they disappointed last Thursday. It is perhaps too much to say that the experiments then shown excelled in beauty and interest those exhibited on the former occasion, but they fully maintained the same high level of excellence."

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