Sparks and Lightning: Electrical Theories from the "Electrician" Dufay to the Scientist Coulomb

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Sparks and Lightning: Electrical Theories from the "Electrician" Dufay to the Scientist Coulomb

Overview

To relate the history of electricity and magnetism during the eighteenth century is to give a good general outlook of how early modern physics matured from being a mostly qualitative topic of study to a demanding quantitative and mathematical science. Scientific instruments played a great deal in accomplishing this conceptual transformation, which is part of what the French call l'esprit géométrique des Lumières, or the quantifying spirit of the Enlightenment. Woven into this story is the emergence of a popular scientific culture and awareness that foregrounds the educational practices conducted today by our modern science centers.

Background

The most ancient reference pertaining to electricity that we know of comes from Antiquity. Thales of Miletus (c. 624-547 b.c.), one of the earliest Greek philosophers, noted in circa 600 b.c. that rubbing amber and a few other substances attracted feathers or bits of straw or leaves to them; the first written statement on this experiment, however, was recorded 200 years later in Plato's dialog entitled Timaeus. In Rome, Pliny the Elder (a.d. 23-79) wrote about similar experiments in his famous Natural History, where he also mentioned shocks given by torpedo fishes.

We cannot say much about the European Middle Ages, except for the appearance of the magnetic compass late in the twelfth century, originating from China. This device, along with the log-line and the hourglass, helped sailors find their way by making use of the navigational technique called "dead reckoning." A discrepancy had been observed between the geographic and magnetic north poles, as indicated by the compass, the difference in degrees being named "declination." Christopher Columbus (1451?-1506) was the first, however, to recognize during his 1492 epoch-making voyage of discovery that somewhere in the Atlantic the declination came to zero and then, by going further westward, increased again but in the opposite direction. (Edmond Halley [1656-1742] later tried to use this phenomenon in order to find a solution to the long-sought problem of the determination of longitude at sea, but to no avail.)

Even though some applications were found for electricity and magnetism, a theoretical explanation was still needed. The first one came in 1600 with the publication of a book entitled De Magnete ("On the Magnet"). The author, the English scientist William Gilbert (1544-1603), was in fact the first to distinguish what he called "the amber effect" (electricity) from magnetism. He worked out a complete theory on the latter, associating, for example, the Earth to a big lodestone and thus explaining the gravitational attraction between the planets. Gilbert discovered also many other "electrics"—substances like common glass, resin, sulphur, precious stones, and sealing wax that attract light objects after being rubbed—and "nonelectrics"—that is, substances that could not be electrified by friction. Today, these nonelectrics are named conductors.

During the seventeenth century electrical experiments were carried out on the whole by the Jesuits and by the members of the Italian Accademia del Cimento. While René Descartes (1596-1650) tried to understand electrical attraction while working on his scheme of ethereal vortices, Robert Boyle (1627-91) investigated with the help of vacuum pumps whether air was mechanically involved in electrical attraction or whether the action resulted from an independent electrical effluvium. It was Francis Hauksbee (1666?-1713), however, who inaugurated the Enlightenment's successful train of electrical researches with his study of the "barometric light" effect—an occasional flashing observed in the vacuum tube of a mercury barometer. He discovered later that neither the barometer nor the mercury was needed to produce the flashes; it is basically an electrical discharge through a gas under low pressure, like our contemporary neon signs. Although he could associate this effect to electricity, he was unable to explain it.

More than 20 years later, in 1729, Stephen Gray (1666-1736) became the first experimenter to ascertain that electricity could be communicated by contact. Helped by Granville Wheler (d. 1770), Gray used his orchard as an experimental test-bench—in other words, by suspending a wire from silk cords mounted on poles, he managed to carry electricity over more than 650 feet (198.12 m). The conduction of electricity gave rise to spectacular and entertaining demonstrations. Gray and Wheler, for instance, suspended a young boy from the ceiling and as soon as he became electrified by conduction, he started attracting objects with all parts of his body. Gray's experiments proved to be revolutionary. All that was needed now was a sound theoretical explanation for all of these phenomena.

Impact

In 1733 Gray and Wheler's reports caught the interest of Charles-François de Cisternay Dufay (1698-1739). In a more methodical and organized way than Gray, Dufay began a systematic survey of all the substances that could be electrified. Except for metals and materials too soft or fluid to be rubbed, he found that electricity was an almost universal property of matter. Furthermore, playing with electrified gold leaves led Dufay to the "bold hypothesis" of two kinds of electricity: a "vitreous" one, produced by vitreous substances like glass; and a "resinous" one, produced by resinous substances like amber and copal. He discovered that electricity of the same kind repelled while it attracted the opposite kind. This became the basis of a two-fluid electrical theory worked out by the most prominent French electrician during the Enlightenment, abbé Jean-Antoine Nollet (1700-1770).

In 1745 Nollet published in the Mémoires of the Paris Academy of Science (and more thoroughly a year later in a book) an electrical theory based on two electricities, not qualitatively different as Dufay discovered but distinct in the strength and direction of current flow. Attraction and repulsion were respectively explained by "affluent" and "effluent" fluxes, two opposing currents of electrical fluids emerging in jets from an electrified body. Until 1752 Nollet's system enjoyed the widest consensus any electrical theory had yet received, even though it could not account for one of the foremost discoveries of the century: the Leyden jar.

Devised by Ewald Georg von Kleist (1700?-48) but best described by Pieter van Musschenbroek (1692-1761), the Leyden jar—a glass flask filled with water—was the first condenser. A wire extends from the water inside through a stopper that seals the flask. The wire conducts electricity from another source into the jar, where it is held. When a conductor (like someone's hand) touches the wire, the electricity flows out. Depending on the charge stored inside the jar, this can create quite a force, as Musschenbroek discovered: "My whole body quivered just like someone hit by lightning. . . . The arm and the entire body are affected so terribly I can't describe it. I thought I was done for." Putting several Leyden jars in parallel increased the effect, which permitted Nollet to use the device in spectacular demonstrations, for instance by simultaneously jolting 180 soldiers for the entertainment of the king and court. These kinds of experiments anticipated the electrical shows given today in many science centers by Van de Graaf electrostatic generators.

The first person to give an explanation of the Leyden jar was Benjamin Franklin (1706-1790), one of the earliest American scientists. In 1747 he proposed a one-fluid theory of electricity based on accounting, meaning that if someone transfers electricity to a friend, the latter will have an excess of the fluid (be charged positively) while the former will have a lack of it (be charged negatively), the total sum being zero (or conserved). Therefore, this single electrical fluid could neither be created nor destroyed, only transferred from one body to the other. For Franklin, the effect of the Leyden jar was readily explained by the interior of the bottle being charged positively with electrical fluid while the exterior was charged negatively by a similar amount.

Franklin is also credited with the first useful technology associated to electricity, the lightning rod, used to slowly draw electricity from the sky before it could strike as lightning. Later, with his famous kite experiment (done by French experimenters before Franklin did it), the American proved (as did Nollet) that electricity naturally produced in the sky was identical to the one artificially created by friction with an apparatus. From 1752 on, Franklin's system made numerous proselytes in Europe, gradually pushing out of the way Nollet's system. Improved later by numerous "electricians," Franklin's system gave birth to our modern general principles of electricity.

Electricity was then recognized as a "subtle" or "imponderable" (weightless) fluid, which means it is a substance that possesses physical properties but is not like ordinary matter—heat, light, gravitation, and magnetism were also regarded as subtle fluids. Numerous electrical effects had been found, but how could they be quantified? Was there a mathematical law of electricity? Charles-Augustin Coulomb (1736-1806) attacked the problem in 1785. Using a torsion balance of his own conception, he made precise measurements of angles, enabling him to affirm that particles of electrical and magnetic fluids interacted in a manner identical to Isaac Newton's (1642-1727) law of gravitation. Called Coulomb's law, the principle he worked out states that the force between two electric charges is inversely proportional to the square of the distance between them. According to the historian of science John Heilbron, "It was this move [the transfer from celestial mechanics to terrestrial physics], and a similar simultaneous one in magnetic theory, that established a model, and presaged a future, for classical mathematical physics."

Besides Franklin's lightning rod, few applications for electricity were devised in the eighteenth century. These had to wait until the turn of the century and Alessandro Volta's (1745-1827) discovery of current electricity, first produced by his silver and zinc battery. The glorious years that saw Dufay, Nollet, and Franklin's success were soon replaced by a new era characterized by a scientific and instrumental ethos in the manner of Coulomb, whose rigor tried to master matter itself. The quantifying spirit tidal wave that broke against the shore of enlightened Europe required a level of mathematical and technical skills only within the reach of professional scientists. The artistic and gentlemanly work that had previously characterized European science gave way to a nascent modern science.

JEAN-FRANÇOIS GAUVIN

WHEN LIGHTNING STRIKES—ALMOST

In the eighteenth century getting shocked by static electricity was nothing short of a fad. However, even those who experimented with electricity—known as "electricator physicists"—having taken sizeable jolts were not aware how dangerous electric charge could be. Among the latter was American statesman/scientist Benjamin Franklin, admired by the French—the most avid experimenters—for his more realistic and practical experiments. Studying the point discharge phenomena, Franklin invented the lightning rod in 1747—some French even had lightning rod hats. Franklin was not the first, but he believed lightning was the same as the electrostatic spark. He wrote a paper describing an experiment to "draw down the electric fire" from a cloud. This entailed a rod or wire about 40 feet (12 m) long connected to a Leyden jar (an early capacitor) inside a supporting insulated enclosure, like a sentry box, to protect the observer.

Others took on the experiment before Franklin tried it himself. In May 1752 Thomas Francois D'alibard used a 50-foot (15 m) vertical rod and successfully drew sparks from the rod in Paris. A week later a M. Delor repeated the success also in Paris. Franklin's most ardent electrical theory supporter, John Canton, was able to duplicate the feat the same year in England. From the descriptions it appears these did not involve an actual lightning stroke—fortunately. Franklin's turn came in June, and he did not stand in the pouring rain with kite in hand as romantic paintings show. He stood under a shed roof with a portion of his silk kite's string kept dry. A key was tied to the string and from there connected to the Leyden jar. Sparks jumped from the key to Franklin's knuckles and did charge the jar. Franklin was lucky that is all that happened.

Then in mid-1753 a Swedish experimenter, Georg Wilhelm Richmann, tried the experiment in his room in St. Petersburg, Russia. Franklin had cautioned to ground the Leyden jar. Lightning actually traveled down an erected wire with Richmann and a servant waiting by the Leyden jar. But Richmann, with discharge test rod in hand, had not grounded the jar, and unfortunately became the target. A foot-long spark leapt from the jar to Richmann's head, killing him instantly and bowling over his dazed servant. In the same city a few days later, Russia's eminent chemist Mikhail V. Lomonosov performed the experiment successfully, but now it was known that playing with lightning could be fatal.

WILLIAM J. MCPEAK

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