Three Scientific Revolutions: How They Transformed Our Conceptions of Reality (14 page)

Without going into the details of the experiments, I shall just relate what he and his collaborators contributed that included the significant introduction of the terms “positive” or “plus” and “negative” or “minus” electrics. As he states:

Hence have arisen some new terms among us. We say
B
(and bodies like circumstanced) is electrized
positively;
A
,
negatively
. Or rather,
B
is electrized
plus; A
,
minus
. And we daily in our experiments electrized [objects]
plus
or
minus,
as we think proper. To electrize
plus
or
minus
, no more needs to be known than this: that the parts of the [glass] tube or sphere which are rubbed do, in the instant of the friction, attract the electrical fire, and therefore take it from the thing rubbing; the same parts immediately, as the friction upon them ceases, are disposed to give the fire they have received to any body that has less. (pp. 340–41; brackets in original)

Despite the fact that Franklin and his associates benefited from the research of others in England and Europe, that in less than four years they were able to formulate a conceptual framework that generally accounted for the experimental results was a remarkable achievement, especially its quantifiability. This was illustrated in their explanation of the function of the Leyden jar. Because on their theory the total electrification was conserved, they showed experimentally that when the inner coating of the jar was positively electrified the outer coating was equally charged negatively, the flow always going from the greater to the lesser amount of electrification, but if connected by a wire equilibrium was instantly established.

Then, in seeking a more fundamental explanation, in a paper entitled “Opinions and Conjectures Concerning the Properties and Effects of the Electric Matter, arising from Experiments and Observations made at Philadelphia, 1749,” he attempted to explain the “electric matter” or “fluid” as consisting of very subtle particles since it penetrated all substances including hard metals. Moreover, if these particles are pliable and repel each other, then the repelling effect can be attributed to them. But while normally repellent, if they come in contact with a neutral object they will be distributed uniformly to maintain the neutrality, while if a conductor loses particles by their being attracted by another object, the remaining particles will attract additional particles to maintain equilibrium.

In answer to a criticism as to how objects can acquire an excess of electric matter if they can retain only a quantity equal to their own particles, Franklin claimed that “in common matter there is as much electric matter as it can contain; therefore, if more be added it can not enter the body but collects on its surface to form an ‘electric atmosphere,' in which case the body ‘is said to be electrified.'”
⁴³
Yet there was a remaining objection. While it was obvious why two positively electrified objects repel each other, when it was discovered that two negatively charged objects also repel, there was no immediate explanation. Why should two bodies possessing less electricity resist sharing some electricity?

As usual when one encounters an anomaly in a theoretical explanation something either has to give or be added. In this case it was German natural philosopher Franz V. T. Æpinus who introduced a resolving assumption.

The revolutionary idea of Æpinus was that in solids, liquids, and gases the particles of what Franklin called “common matter” repel one another just like the particles of the electric fluid in Franklin's theory. Æpinus's revision introduced a complete duality. The particles of common matter and of electric matter each have the property of repelling particles of their own kind while each kind of particle has the additional property of attracting particles of the other kind. (p. 343)

Attributing additional electrical charges to the natural particles of a body came to be known as the “two fluid system” analogous to Dufay's earlier hypothesis of two electric fluids, one vitreous and the other resinous.

However, as usually occurs with scientific explanations, while Æpinus's resolution explained the anomaly of negatively charged bodies repelling each other despite having fewer electrically charged particles, this explanation raised a further problem, as Æpinus realized. If the particles of common matter also repel each other this conflicts with Newton's universal law of gravitation that all material objects exert a gravitational attractive force on each other. How could the repulsive force of the negatively charged common particles generate the attractive gravitational forces? Æpinus proposed a counter-explanation to no avail; the resolution was beyond an explanation at the time that would have to await the discovery in atomic physics of variously charged particles.

But despite the theoretical impasse there was sufficient truth in Franklin's conception of electricity that he was able to draw practical consequences from it that enhanced his international acclaim. Although he was not the first to suggest that there was an affinity between electricity and lightning, he was the first to establish their identity. In an entry in his “experimental notebook” he indicated that there were twelve ways the “Electric fluid agrees with lightning:”

(1) giving light; (2) color of the light; (3) crooked direction; (4) swift motion; (5) being conducted by metals; (6) crack or noise in exploding; (7) subsisting in water or ice; (8) rending bodies it passes through; (9) destroying animals [he has killed fowls by the discharge of several Leyden jars connected together]; (10) melting metals; (11) firing inflammable substances; (12) sulfurous smell.
⁴⁴

In the essay on “Opinions and Conjectures Concerning the Properties and Effects of the Electric Matter” mentioned earlier, he had indicated that pointed objects attract an electrical force at a greater distance and with greater ease than a blunt object. He also expressed his belief that clouds were electrified as seen in bolts of lightning. But never satisfied with just conjectures, these combined documents led him to devise means of testing whether lightning was truly electrical and that clouds too were electrified, along with inventing ways of avoiding being struck by them.

Thus with the help of his son he undertook his famous kite experiment to prove that lightning was indeed a form of electricity. After attaching a wire as the detector to the front of a kite, he then tied to it a long kemp cord that reached the ground on the end of which was fastened a key and a silk ribbon for insulation. At the outbreak of a storm they raised the kite and ran into a shed after allowing the cord to become wet to increase its conductivity. Holding the kite by the dry silk ribbon so as not to be electrified, as expected a bolt of lightning from a passing cloud struck the wire detector and was transmitted through the cord to the key where it was then collected into a Leyden jar as proof of its electrical nature.

In another variation of the experiment, attaching a pointed metal rod to the peak of his roof he hung from it a long wire descending from the side of the house to a metal frame holding two iron bells with metal clappers. As before, when lightning struck the electrical discharge it was transmitted to the rod down the wire to the iron bells which produced a clanging sound. Owing to these experiments, lightning rods were installed on the tops of buildings and church spires to deflect lightning from striking them and causing a fire. Although others had thought of the possibility of such experiments and protective devises Franklin was unique in actualizing them.

In conclusion, as stated by Duane and Duane H. D. Roller in the book cited previously:

By 1757 the public demands on Franklin's time had become so great that he ceased completely the experimentation that had already earned him the reputation of the foremost electrical scientist of his day.

By this time he had received the Copley Gold Medal, which is the highest distinction that the Royal Society can bestow, and had also been elected a Fellow of the Society. In 1773, the French Academy of Sciences made him a “foreign associate,” an unusual honor and one that was not to be accorded to another American scientist until a century later. (p. 607)

The growing confidence in the progress of science due to acquired scientific explanations confirmable by experimental evidence and expressed in the language of mathematics, as proposed by Galileo and Newton, having been reinforced by the electrical investigations, especially Franklin's quantification of electrical phenomena, there followed an attempt to ascertain whether one could discover electrical laws comparable to Newton's universal laws of gravitation. Based on the analogy with Newton's law that gravity is a function of mass, distance, and gravitational forces, perhaps electricity can be measured in terms of mass, distance, and
electrical
forces.

In fact the Swiss physicist Daniel Bernoulli invented an electrometer that directly measured the strength of “the electric force between two charged metal disks when they were at known distances apart,” the measurements indicating that “the force varies inversely as the square of the distance between the plates,” conforming to Newton's gravitational law (p. 610). In another experiment Joseph Priestley, the identifier of oxygen, also confirmed that the strength of the electric force agreed with Newton's inverse square law. Then French physicist Charles Coulomb devised an “electrical torsion balance” that proved so accurate that he could measure “with the greatest exactitude the electrical force exerted by a body, however slightly the body is charged” (p. 617), confirming that the strength of the
repulsive
force between two equally electrified bodies varies inversely with the square of the distance.

Again, according to Duane and Duane H. D. Roller, since Newton's law applies to the
attractive
force between two objects this also had to be confirmed, as Coulomb succeeded in doing with an “electric raised torsion pendulum” that he also invented (p. 620). The question then was whether the other portion of Newton's law also applied, that the gravitational force was proportional to the product of the masses (density per volume). Though the concepts of “electric fire” or “electric fluid” would not seem amenable to such a confirmation, Franklin's hypothesis of bodies being composed of two kinds of particles (a kind of matter) that exert opposite forces, negative for electric and positive for natural matter, suggested that there was the possibility of a determination if one substituted electric mass” for “gravitational mass.”

Believing it was possible, Coulomb declared that the “electrical force between two electrified objects is proportional to the inverse square law of the distance between them and to the product P of their electrical masses, or
f µ P/
d
²
” (p. 621), again conforming to Newton's law of gravitation and that became known as “Coulomb's law.” As Duane and Duane H. D. Roller state:

With this quantification of electrical science, it becomes possible to bring to bear upon its further study the entire weight of mathematical techniques. Eighteenth-century mathematics had to a very large degree developed along lines applicable to Newtonian mechanics, and with the formulation of electrical science in quantitative terms so analogous to mechanics, electricity became thoroughly amenable to mathematical treatment, with striking results in the nineteenth century. (pp. 621–22)

Turning next to the investigation of light, throughout history fire, sun, and sunlight have been of intense interest. The sun was deified as
Helios
and
Sol
respectively by the ancient Greeks and the Romans. The Pythagoreans placed fire in the center of the cosmos calling it the “Hearth of the World” and the “Throne of Zeus.” Plato in the
Republic
declared that “of all the divinities of the skies the sun is the most glorious because it not only . . . gives to the objects of vision their power of being seen, but also their nourishment and existence.”
⁴⁵
It was partly due to its exalted position that Copernicus and Kepler ceded to the sun its central place in the universe, though little was known then about the nature of light and its transmission.

By the time of Newton the two dominant theories of the transmission of light were Descartes's view that light as seen was the physiological effect on our senses of the “
instantaneous
pression” of the contiguous motionless particles comprising the fluid vortices of the universe while the other was the wave theory of light held by Robert Hooke, Christiaan Huygens, and others. Yet for reasons presented in our previous discussion of the
Opticks
, Newton rejected both theories based on his prismatic experiments and corpuscular theory of light. So just as Newton's Queries in the
Opticks
stimulated research into the theories of an ethereal medium, gravity, particles, magnetism, and electricity, in the eighteenth century, his Queries from 21 to 31 discussing the properties and transmission of light, especially that it consists of rays composed of corpuscles, encouraged investigations into optics and light.

Though his theory had gained ascendance by the early eighteenth century, it was challenged by Thomas Young in a paper entitled “Outlines of Experiments and Inquiries Respecting Sound and Light” published in the Royal Society's
Philosophical Transactions
in 1800. Drawing an analogy with the transmission of sound, Young rejected the particle theory of light in favor of the wave theory that depicted light, like sound, as the undulations of an underlying stationary medium.

Young presented several objections to the corpuscular theory before providing the main evidence in favor of the wave theory. The first was that if light were composed of material particles they would be attracted by gravitational forces so that their velocities would vary with the strength of the gravitational force of the emitting body, yet light seems to travel with an invariant velocity; however, this objection does not apply to waves which, if propagated in an aetherial medium, are not affected by gravity. Second, Young believed that Newton's explanation that the light and dark rings of light, known as “Newton's rings,” are caused by the partial reflection and transmission of the light particles when directed through two lenses separated by a film of air, described as “Fits of easy Reflection and easy Transmission,” could more reasonably be explained by the refraction of alternating light and dark waves.