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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' Theorem
AOB 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:
- 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."
- Karl Zipernowsky and Maximillian Déri, "Dynamo-Electric Machine," US Patent No. 284,110, August 28, 1883.
- December 20, 1900: Nature reports on William Duddell's 'musical arcs', APS News, vol. 19, no. 11 (December, 2010), p.2.
- 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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