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/ d2” (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 “Outli
nes 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.
As additional support he found that when monochromatic light is projected on a screen that has a circular opening the diameter of which is larger than the wavelength of the light passing through, it produces a circular image on the screen behind it. But if the diameter of the opening is about equal to the wave length of the light then a series of alternating light and dark bands indicative of the interference of waves appear on the posterior screen. He found that the latter effect is produced also when two holes very small and close together are cut in the screen and a beam of monochromatic light strikes the screen midway between the two points. In an essay titled “On the Theory of Light and Colors,” published in 1802, he described these bands as being “constructive (in phase) and destructive (out of phase) interference.”46
Despite the nearly incontrovertible evidence, as an indication of how strong Newton’s influence was at the time and how difficult it is for even some scientists to question or reject their theories, Henry Brougham described Young’s paper as “destitute of every species of merit . . .” (p. 19). However, Young’s conclusions did resonate in the thinking of a gifted French engineer, Augustine Jean Fresnel, who rejected the corpuscular explanation of diffraction declaring that it had been refuted experimentally. When, in defense of the corpuscular theory, the supporters maintained that the diffraction patterns in Young’s experiment were produced by the edges of the circular holes deflecting the particles passing through, Fresnel tested the explanation by altering the shape and the mass of the holes and finding that it had no effect at all, the diffraction pattern depending only on the relative sizes of the apertures and the wave lengths of the monochromatic waves.
He then supplemented Young’s experiments by attributing mathematical dimensions to the properties of the diffracted waves. As we now know, the properties of particles and waves are the converse of each other: particles having a discrete location in space with various shapes and sizes, possess mass and momentum along with the energy of motion, and interact by deflection with a loss of energy. In contrast, waves are defined by their lengths, frequencies, amplitudes, and intensities, are diffused in space as wave trains, and interact to reinforce if in phase or destruct if out of phase. As described by Peter Achinstein:
[Fresnel’s] account is much more sophisticated than Young’s, not only because it is quantitative, but also because in determining the resulting vibration . . . Fresnel derives mathematical expressions for the amplitude of the vibration at any point behind the diffractor, and for the light intensity at that point. From these he infers the positions and intensities of the diffraction bands—inferences that were confirmed experimenally. (p. 21)
Fresnel was awarded a prize when he sent his results in a “Memoir on the Diffraction of Light” to the Paris Academy in 1819.
As optical investigations continued further evidence was discovered to support the wave theory. The initial inability of the wave theory to explain the polarization of light emerging from Iceland spar due to the assumption that light waves were transmitted longitudinally, running lengthwise like sound percussions, was surmounted when they were discovered to be produced by transversal vibrations (up and down) perpendicular to their direction of movement. Because of being transversal when they are reflected through Iceland crystal the latter’s internal structure separates the vibration into perpendicular directions, thus the emerging light is polarized at right angles to each other.
Fresnel was even able to rebut the main optical evidence that had convinced Newton of the superiority of the corpuscular theory: the sharp outline of shadows cast by large objects when deflected by light. Fresnel argued that one can explain the sharp outline as due to the large object’s obstruction of certain waves at the edge of their propagation, but if one reduces the size of the object to the magnitude of the light wave then the light bends around the object as sound waves do. Finally another crucial test could be made based on the change of the velocity of light when passing through a lesser to a denser medium.
Newton had predicted that on the corpuscular theory the greater gravitational attraction of the denser medium would cause an acceleration of the light particles while on the wave theory the diffraction of the light would cause a retardation of the velocity. In a series of ingenious experiments now cited as experimentum crucis (critical experiments) begun in 1850, by French physicist Jean Léon Foucault confirmed that water or glass impedes the velocity of light in accordance with the wave theory. Then, based on these results, another French physicist named Hippolyte Louis Fizeau determined the velocity of light to be 300,000 kilometers per second, or 186,281 miles per second.
Illustrating how the correct paradigm of scientific inquiry leads to the determination of the relative truth of hypotheses, along with opening up new vistas of discovery, by the middle of the nineteenth century, Huygens’s wave theory of light had superseded Newton’s corpuscular theory, although radically new interpretations were yet to come, including the discovery that light was a form of electromagnetism, and the twentieth-century discovery that it can exhibit either wave or particle properties depending on the experimental conditions.
As for the discovery of electromagnetism, since ancient times electricity and magnetism were considered separate phenomena. But then in the winter of 1819–1820 Hans Christian Oersted (1777–1851), professor of natural philosophy in Copenhagen, during a course of lectures wondered if an electric current might have an effect on a magnetic needle. To test the supposition he placed an electrified wire at a right angle to the north south axis of a compass to no effect. Deciding to align the electrified wire parallel to the N-S axis he was surprised to find it produced a pronounced deflection of the needle, showing a relation between electricity and magnetism.
This was supported by Michael Faraday (1791–1867), a bookbinder’s journeyman who apprenticed at age thirteen and therefore had little formal education but became an outstanding scientist, again showing the more common backgrounds of these later scientists. Attracted to science, he began attending the lectures by Sir Humphrey Davy at the Royal Institution in London becoming a member in 1823 and then a fellow of the Royal Society the following year. In 1833 he attained the position of Fullerian Professor of Chemistry at the Royal Institution. By then his reputation was such that he was offered knighthood and the presidency of the Royal Society but declined both. He is especially noted for his discovery of electromagnetic induction.
It had long been known that when iron filings were spread on a sheet of paper and a magnet placed underneath, the filings became aligned in a curved pattern around the magnet that, according to Sir Edmund Whittaker, “suggested to Faraday the idea of lines of magnetic force; or curves whose direction at every point coincides with the direction of the magnetic intensity at that point. . . .”47 He then discovered that a moving magnet brought near an electric circuit induc
ed a current, just as Oersted had found that an electric current changed the magnetic direction of the compass needle. As Whittaker continues:
Faraday found that a current is induced in a circuit either when the strength of an adjacent current is altered, or when a magnet is brought near to the circuit, or when the circuit itself is moved about in presence of another current or a magnet. He saw from the first that in all cases the induction depends on the relative motion of the circuit and the lines of magnetic force in its vicinity. The precise nature of this dependence was the subject of long-continued further experiments. (p. 172)
Faraday’s realization that a magnetic field can induce an electric current combined with Hans Christian Oersted’s complementary discovery led to the conception of an independently existing electromagnetism either as a field or a current due to the interactions, replacing the previous conception that they were fluids. Reinforcing again the importance of mathematics in modern science, James Clerk Maxwell (1831–1879), based on these experimental discoveries, formulated intriguing equations describing the structure of electromagnetic fields and how they change in time due to the interactions. It was Maxwell’s equations that implied that the velocity of the propagation of the waves of the electric field is identical to that of light indicating that light too is a form of electromagnetism.
This was confirmed towards the end of the nineteenth century when Heinrich Hertz (1857–1894) experimentally proved the existence of electromagnetic waves having the same velocity as that of light. Because electromagnetism involves the interaction of contiguous fields rather than forces emanated by discrete physical bodies in space as in Newtonian science, the conceptual framework of electromagnetism represents the beginning of the third scientific revolution that transformed our conception of reality. In their book The Evolution of Physics, Albert Einstein and Leopold Infeld declared that the “theoretical discovery of an electromagnetic wave spreading with the speed of light is one of the greatest achievements in the history of science.”48 Indeed, it was these developments that made possible the later introduction of radar, electric power, telegraphy, radio, television, the internet, and so forth. As Carl Sagan states in The Demon-Haunted World: Science as a Candle in the Dark, this “has done more to shape our civilization than any ten recent presidents and prime ministers” (p. 390).
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