5: Marat's Investigations of ELECTRICITY
THE ILLUSION OF REPULSION
RECHERCHES PHYSIQUES SUR L’ÉLECTRICITÉ
The third volume of Marat’s trilogy was devoted to an area of physics with a history much different from that of fire or light. Fire and light must certainly have been among the first natural phenomena to arouse the curiosity of early homo sapiens. Consciousness of the simplest electrical phenomena, on the other hand, seems to have arisen during classical antiquity with the recognition that amber, when rubbed, attracts tiny bits of other materials. The “amber effect” remained no more than a minor curiosity until the middle of the eighteenth century. Although it began to receive more systematic attention in the seventeenth century, it was only in the generation immediately preceding Marat’s that electricity was definitely recognized as a major force of nature.
Suspicions of its significance arose from obvious analogies between sparks and shocks, on the one hand, and bolts of lightning on the other, but the conjectures were not confirmed until the famous experiments of Benjamin Franklin and others in the 1750s. Only then did electricity move from the periphery to the center of physical investigations and attain the status of a full-fledged branch of physics. Franklin and other pioneers of electrical science were still living and contributing to debates over electrical theory at the time Marat was investigating the subject.
Marat’s Recherches physiques sur l’électricité followed the same general format as his two previous volumes: a narrative of experiments intended to illustrate and support a comprehensive theory. While the book reports 214 experiments, Marat believed his main contribution to electrical science would be theoretical, rather than simply augmenting the pool of empirical facts. He intended to bring theoretical clarity to the jumble of conflicting hypotheses.
The validity of Marat’s assessment of the chaotic state of electrical theory in the late 1770s may not be immediately apparent in retrospect. The electrical theory of Benjamin Franklin that was published in 1751 had gained wide support and influence, and since some of its key elements are still considered valid today, the extent of its acceptance by contemporaries tends to be overestimated. The very influential “Franklinist history” promoted by Joseph Priestley exaggerated the success of Franklin’s theory, especially with regard to continental physicists. In fact, Franklin’s single-fluid theory of electricity was confronted with some glaring anomalies that Franklin himself was well aware of. As important as Franklin’s contributions to electrical theory were, then, no serious scientist could have disagreed with Marat’s contention that in 1782 some very fundamental questions remained to be answered.
The ongoing debates over electrical theory were certain to stimulate Marat’s interest; he was invariably drawn to disputed issues. It was in his role as a controversialist that Marat the scientist most resembled Marat the revolutionary politician.
For centuries the mysterious hopping of tiny bits of matter toward amber or other materials supplied one of the key mysteries of “natural magic.” Like terrestrial gravity, and even more like the magnetic action of the lodestone, an occult force seemed to be creating action at a distance, confounding mechanical and materialist explanations of natural phenomena.
At the beginning of the seventeenth century, William Gilbert initiated the systematic examination of the amber effect. By the early eighteenth century the extent of electrical knowledge could be summarized in the following propositions:
• that small electrical charges could be created by rubbing glass, resin, or other materials;
• that these charges exhibited forces that could repel as well as attract;
• that sometimes they would attract, hold, and then repel the same small object;
• that the electric “virtue” could be transferred from one object to another by bringing them into contact with each other; and
• that one electrified object could temporarily electrify nearby objects without contact.
The ability to transfer electrification by contact gave rise to the idea that electricity, like heat, was a subtle fluid. This conclusion was difficult to resist after Stephen Gray demonstrated in 1729 that the electric virtue could be transmitted over long distances—hundreds of feet!—through silk cords. On the other hand, Gray also called attention to the mysterious temporary influence in the absence of contact, which suggested that perhaps electricity was not a fluid after all. This latter phenomenon, later called electrostatic induction, posed particularly difficult theoretical problems.
In 1733 Charles François Du Fay discovered the existence of two qualitatively different kinds of electricity, which he called “resinous” and “vitreous” after the kinds of materials they seemed to be associated with. The two electricities were distinguishable by their behavior: Two objects possessing resinous electricity would repel each other, as would two objects possessing the vitreous kind. But a vitreously charged object and a resinously charged object would always attract each other.
A major advance in electrical investigations occurred in the 1740s with the introduction of two important experimental tools: the electrostatic generator and the Leyden jar. The generator had been invented earlier, but only came into widespread use about 1743 when a number of German and Dutch experimenters began utilizing it.
Experimenters had long been able to produce small electrical charges by vigorously rubbing glass rods. This process was mechanized by attaching a crank to a glass globe or cylinder. By turning the crank with one hand while pressing the other hand against the spinning glass, an experimenter could generate a significant charge with minimal effort. Further innovations followed: A conducting wire was added, one end brushing continuously against the spinning glass and the other leading to a “prime conductor,” which was usually a long piece of metal such as a gun barrel.
A classic electrostatic generator. This machine, requiring two operators (note the hands on the spinning glass globe), is larger than the one used by Marat.
These electrical machines could produce big flashes and shocks, far more impressive than the little sparks and tingling sensations of ordinary static electricity. Touching the prime conductor of one of these machines could kill small animals and knock strong men to the floor. The power of electricity began to gain respect and to be compared with the effects of lightning.
In 1745, two physicists independently invented what became known as the Leyden jar. A German, E.J. von Kliest, and a Dutchman, Pieter van Musschenbroek, each discovered a surprising phenomenon when they attempted to transfer electricity from a generator’s prime conductor to a bottle of water. To accomplish this, they attached one end of a conducting wire to the prime conductor; the other end was hung into the water in a glass container.
When Musschenbroek tried this, he turned the crank for a while and then paused to examine the bottle of water. When he touched it he received a most unexpected result. In a letter to the Parisian Academy of Sciences, Musschenbroek reported what happened with a grave warning:
I am going to tell you about a new but terrible experiment which I advise you not to try for yourself. . . . I was making some investigations on the force of electricity. . . . Suddenly my right hand was struck so violently that all my body was affected as if it had been struck by lightning. . . . The arm and all the body are affected in a terrible way that I cannot describe: in a word, I thought that it was all up with me.
This device, the Leyden jar, seemed to offer further confirmation of the idea that the electric virtue was a sort of fluid. A fluid seemed to flow from the generator, along the conducting wire, and into the jar, where large quantities could remain stored.
The strangeness and newfound strength of electricity as revealed by the innovations in experimental apparatus stimulated efforts to understand it. A number of experimenters exchanged ideas and debated theories, but the most successful explanation was put forward by Benjamin Franklin in a series of letters between 1747 and 1754. Franklin’s theory first appeared in book form in 1751 and was published in French translation the following year. Although the physicists of Europe were surprised to see a major theoretical contribution coming from the American colonies, Franklin’s view of electricity rapidly gained adherents and attained a strong position among competing theories, such as that of the abbé Nollet. Nollet explained electrical attraction and repulsion by means of subtle effluvia streaming into and out of electrified objects. Franklin wrote in his autobiography that his own theory “was by degrees universally adopted . . . in preference to that of the Abbé.” While never achieving the dominant status of an unchallengeable paradigm, Franklin’s single-fluid theory became one to which all other hypotheses about electricity were referred and compared.
Franklin’s theory offered the simplicity of a combined explanation of both kinds of electricity, vitreous and resinous. It did away with Nollet’s cumbersome system of dual emanations. According to Franklin, unelectrified bodies all contain a certain natural amount of electric fluid. When the normal state of equilibrium is changed by a transfer of fluid from one object to another, the one that loses fluid is in a state of “negative” electrification; the one that gains fluid is in a state of “positive” electrification. The difference between vitreous and resinous electricity is not a matter of different fluids, then, but of a surplus or deficit of a single electric fluid.
The authority of Franklin’s theory was enhanced by his success in showing that lightning can fill a Leyden jar with electric fluid. Although his priority with regard to this discovery was challenged by others, Franklin justifiably retained the greatest share of fame for it.
In France, the abbé Nollet criticized Franklin’s theory and continued to oppose his own hypothetical emanations to it. Nollet’s ideas had a strong Cartesian flavor that helped win them a following in France. Nevertheless, some leading French physicists broke with Nollet and supported Franklin. Among them was J.B. Le Roy, the academician who was to lead the commissions investigating Marat’s experiments. Internal Academy politics played a large role in these developments. Franklin’s theory was promoted by Buffon and his faction as a weapon against another faction led by Réaumur, which included Nollet. Buffon had arranged for the translation and publication of Franklin’s book in order to embarrass Nollet.
The single fluid that Franklin postulated as the carrier of the electric virtue was a classic Newtonian elastic fluid, but with certain unique characteristics. An elastic fluid, such as ordinary atmospheric air, was conceived of as being made up of tiny particles that repel each other. Franklin was unable to offer a physical explanation for this repulsive power, but believed that some physical basis—an “aether” of some sort, perhaps—must exist. Franklin was apparently willing to accept attraction as a fundamental property of matter requiring no further explanation of its action at a distance, but not repulsion.
The power of the fluid’s particles to repel each other would account for its tendency to spread out and flow away from places where a surplus has accumulated. The unusual property of these electric particles, however, is that while they repel each other, they attract (and are attracted by) the particles of all ordinary matter. That would account for their tendency to flow into objects and remain there in a state of equilibrium.
The ordinary exchange of electricity by objects in contact was not difficult to conceive of in terms of the flow of Franklin’s single fluid. But how were the phenomena of electrostatic induction to be explained? How was it that simply bringing an electrified object very close to an unelectrified object could electrify the latter without touching it? This phenomenon was all the more puzzling for being only temporary; when the two objects are moved apart again, the one that was originally unelectrified returns to its neutral state. Without contact, through what channels could anything have flowed into and then back out of it?
One way of accounting for this was to postulate electric atmospheres surrounding electrified objects. Franklin did not originate this concept but his theory utilized it. When an object is electrified, according to this notion, the surplus particles of electric fluid tend to spread out beyond the confines of the object itself, forming a special atmosphere of electric particles. If that is the case, however, what ultimately confines the electric atmosphere to a specific, rather small volume around the object? Franklin suggested that the pressure of the surrounding air could perhaps account for it. This was an ad hoc explanation that was not fully satisfying even to Franklin. Some other theorists, including John Canton, sought to abandon the idea of electrical atmospheres altogether.
The greatest difficulty for the single-fluid theory was its inability to explain certain phenomena involving negatively electrified objects. First of all, how is it that a negatively electrified object can induce a temporary state of electrification in a nearby object? This presented a glaring problem. It was possible to conceive of a surplus of a single electric fluid supersaturating an object and spilling out of it to create an atmosphere. But how could a deficiency of that fluid create an atmosphere? Two different electricities that create two different kinds of atmospheres implies two distinct electric fluids.
There was an even more fundamental problem:
In terms of Franklin’s theory, there is no reason why two negatively charged bodies should repel one another, although . . . Franklin reluctantly came to admit that such a phenomenon always occurs.
How, indeed, could the deficiency of a fluid cause objects to mutually recede?
This question could not be avoided in discussions of the cause of electrical phenomena and it became a focal point for evaluating the merits of the whole Franklinian theory. . . . But Franklin had no contribution to make to the problem of why two negatively charged bodies repel each other.
The paradoxes of negative electricity were persistent anomalies for his system, not unlike that of diffraction for Newton’s color doctrine. In 1759, F.U.T. Aepinus published a modification of Franklin’s single-fluid theory that could “save the phenomena” (that is, account for the undeniable empirical facts) but at the cost of introducing a questionable theoretical innovation.
Aepinus’s proposed solution was a radical one involving the redefinition of the nature of matter. In Franklin’s theory the particles of electric fluid, it will be recalled, were endowed with the property of mutual repulsion but were supposedly attracted to the particles of ordinary matter. The particles of ordinary matter in the solid and liquid states, of course, were assumed to attract each other as well as those of the electric fluid. Aepinus’s theory required that the particles of ordinary matter not only attract each other, but also repel each other.
Aepinus, in other words, universalized repulsion as an inherent property of matter. According to this conception, the particles of ordinary matter and electric fluid coexist with respect to each other by the rules “like attracts unlike” and “like repels like.” In a neutral state of electrification, the attractive and repulsive forces are in equilibrium. When some electric fluid flows out of objects the balance of forces is disrupted, bringing the repulsive forces of the particles to the fore, thus causing two negatively electrified objects to repel each other.
Aepinus was well aware that many physicists would find his theory impossible to accept—that they would find “a contradiction in my assuming that a bodily matter is endowed at the same time with two opposite forces, namely, the repelling and the attracting.”
Particles of ordinary matter obviously attract each other; the fact that they aggregate in solid or liquid form could not be explained otherwise. The repulsive force postulated by Aepinus seemed by comparison to be no more than an ad hoc explanation designed to arbitrarily provide internal consistency to a theory of electricity. Nonetheless, as counterintuitive as his suggestion appeared, it did win adherents who preferred accepting some theoretical oddities to rejecting the single-fluid theory. The preference for a single-fluid theory as opposed to a dual-fluid alternative was based on a metaphysical desire to simplify the workings of nature to the greatest degree possible.
At about the same time as Aepinus published his electrical theory, a general theory of matter was proposed by the Jesuit Roger Boscovich that also utilized the concept of a combination of attractive and repulsive forces. Boscovich’s notion of matter as a dynamic unity of opposing forces strongly influenced Kant’s natural philosophy and through him the German school of thought known as Naturphilosophie.
In England and France, however, the trend of metaphysical thought was considerably more conservative. The idea of a particle that simultaneously attracted and repelled others like itself was generally considered to be a violation of formal logic. Such self-contradiction was considered intolerable and Aepinus’s electrical theory was not well received. More than a century later, in 1873, James Clerk Maxwell still found it necessary to argue against the objection “that if particles of matter repel one another, the theory must be in opposition to universal gravitation.”
A revised two-fluid theory of electricity proposed by Robert Symmer in 1759 met with more success. In spite of Franklin’s great prestige and authority, the two-fluid theory seemed to many to be more in accord with known empirical facts about electricity. The “height of the active controversy over whether there were two electric fluids or one” occurred “in the closing years of the eighteenth century and the opening years of the nineteenth.”
Charles Coulomb lent his great authority to the support of the two-fluid theory. Furthermore, the discovery of the process of electrolysis after 1807 strongly suggested the flow of two separate fluids in opposite directions: How else could one explain the simultaneous motion of sodium toward a cathode and chlorine toward an anode in an electrified solution of molten sodium chloride? The two-fluid theory became dominant and remained so well into the nineteenth century.
It is evident that the theoretical situation that Marat encountered in the field of electricity was far from settled, and was becoming even less so at the very time that he was performing his experiments.
By about the time of Franklin’s death [in 1790] the uncovering of many phenomena had introduced into electrical theory some new considerations and a deep range of complexity.
Marat’s Recherches Physiques sur l’Électricité
Marat was certainly not out of step with the science of his times in striving to develop a new synthesis of electrical theory. This was one of the central issues agitating physicists and Recherches physiques sur l’électricité (Research on the Physics of Electricity) was Marat’s contribution to the ongoing debate.
The volume, like the first in the series, appeared with the censor’s stamp of approval. The author is identified on the cover page, as he was in both previous volumes, as M. Marat, medical doctor and physician to the bodyguards of Monseigneur le comte d’Artois.
Marat’s Preliminary Discourse began with a brief survey of the history of electrical science. He described and praised the contributions of Gray, Du Fay, Nollet, and others, saying that while they had not been able to make a science of electricity, at least they had prepared the ground well. It is to “the celebrated Franklin,” however, that the new science owes its greatest debt:
The big picture that he opened our eyes to made better known a kind of matter that had for such a long time escaped observation. The subtle fluid, spread out everywhere throughout the surface of the earth, stays peacefully hidden in the depths of objects as long as it is proportionately distributed there. But after accumulating in the skies, if it brusquely regains its abandoned space, it often becomes one of nature’s most irresistible forces.
While Marat expressed appreciation for the empirical discoveries of his predecessors, he was generally scornful of previous attempts to create a synthesis of the known facts:
Most of the works published on this subject are hardly more than summaries of puerile observations, poorly done experiments, false inductions, random hypotheses, and contradictory opinions.
In support of this negative assessment, Marat cited Priestley’s History and Present State of Electricity. Although this work was itself a theoretical synthesis of sorts, Marat did not condemn it as such but utilized Priestley’s account of conflicting theories to illustrate the chaos in which the subject remained.
Marat’s stated purpose in undertaking the study of electricity was to create order where there had previously been only confusion. He began his exposition by outlining some of the general principles of his own theory of electricity. First of all, “electrical phenomena are dependent on the action of a fluid in motion.” While other physicists had confused this fluid with that of fire, of light, or of magnetism, “I have shown that it is of a unique nature.”
As for the essential properties of this fluid, previous theorists had believed them to be twofold; first, that its atoms are attracted by those of all other materials, and second, that its atoms mutually repel each other—“fecund principles from which all of its effects have always been made to flow.”
But in examining these principles carefully I have assured myself of the truth of the former and the falsity of the latter. Better than that, I have demonstrated that, far from the electrical globules repulsing each other, they reciprocally attract each other.
This is a key element of Marat’s system of natural philosophy. Like Franklin, he was willing to accept attraction as a basic force of nature requiring no further explanation, but could not accord the same privileged status to repulsion. He could see the same apparent repulsion that all experimenters had witnessed, but he insisted that it was only a superficial appearance. Unlike Franklin, Marat saw no need to ascribe apparent repulsion to a yet-to-be-discovered aether; he believed all such occurrences could ultimately be explained in terms of forces of attraction.
The prevailing concept of an elastic fluid, it will be recalled, depended upon its particles being endowed with a repulsive force. Marat, of course, could not allow that in his theory:
Elasticity is still counted among the properties of this fluid; I will prove that it is not at all elastic. From that it follows that attraction is the great principle that can shed light upon these phenomena. Thus attributed to a single cause, the theory of electricity becomes simpler, clearer, and brighter.
The extent to which Marat had adopted the general outlines of Franklin’s theory is apparent in his assertion that while other objects attract the electric fluid,
none can receive a surplus quantity without another losing a proportional amount. These latter are called less electrified or negatively electrified; the former are called more electrified or positively electrified.
A final major feature of Marat’s theory sought to account for how objects sometimes maintain their state of electrification without their electric fluid simply dissipating into the environment:
It has previously been believed that the fluid accumulated in objects is retained only by their attractive force; I will demonstrate that it is above all retained by the pressure of the ambient air.
Although Marat seems to suggest that this was an innovation on his part, it was reminiscent of Franklin’s speculations on the subject. Marat states it more positively, since it is more central to his system than it was to Franklin’s. Furthermore, there were differences in their conceptions of the role of the air. For Franklin the air served to restrict the extent of the electric atmosphere surrounding charged objects, and it did so simply by means of its nature as a nonconductor. For Marat, who denied the existence of electric atmospheres, the air kept the electric fluid within the objects themselves, and did so by means of its pressure as well as its nonconductivity.
This concept, it should be noted, was still defended decades later by such pillars of scientific orthodoxy as Biot and Poisson: “As for the vexed question of the agency that prevented the escape of fluids from the surface of conductors,” a historian of science writes, “Poisson was content to suppose with Biot that the ‘pressure’ of the air clamped electricity to its carrier.”
Marat described his experimental apparatus in great detail with the help of a set of excellent professionally drawn illustrations. His devices were based on the standard pieces of equipment used by his contemporaries in electrical research: the electrostatic generator, the Leyden jar, and so forth. In each case, however, he had introduced design innovations that he considered to be significant improvements, and most of the pieces had been given new names. Whether he exaggerated the importance of his innovations or not, his apparatus was certainly adequate, by contemporary standards, for the research purposes to which it was applied.
Marat’s electrical apparatus, from his Recherches physiques sur l’électricité (1782). Photo courtesy of Division of Rare and Manuscript Collections, Carl A. Kroch Library, Cornell University.
The first step was to make a clear distinction between the electric fluid and other subtle fluids, especially the magnetic fluid. Marat’s rather contemptuous attitude toward rival theorists began to be expressed more openly:
To distinguish different things that seem similar ought to be the primary concern of those who scrutinize nature, but the first operation of our modern philosophers has been to mix them all up.
Marat harshly criticized a “modern academician” for confusing electric fluid with fire and identified him in a footnote as J.B. Le Roy. Le Roy, it will be recalled, headed the commission that had given short shrift to Marat’s optical experiments.
Some physicists, Marat declared, had considered the electric fluid to be identical to the true principle of fire, and had likewise lumped electricity and magnetism together, leading to the bizarre conclusion that fire and magnetism share the same underlying cause. But “magnetism and fire have not a single phenomenon in common.”
As for electricity and magnetism, the only thing they have in common is a real attractive force and an apparent repulsive force. Their modes of action are entirely different. Touch a magnet; does one feel even the slightest tingle of a shock? “While magnetism indubitably depends upon a fluid, that fluid is certainly not homogeneous with the electric fluid.”
Marat had been able to render the electric fluid visible with his darkroom methods, but had not been able to do the same for the magnetic fluid. He did not believe it impossible in principle, however, and saw no reason to suppose that he would not be able to successfully do so in the future. Meanwhile, his attention was directed toward electricity, not magnetism.
Among a long list of differences in the modes of action of electricity and magnetism, an important one is that the most characteristic sign of magnetism is not exhibited by electrified objects: An electrified needle does not turn to align with the earth’s poles or with anything else. Marat concluded that “the electric fluid and the magnetic fluid therefore differ essentially, even though electricity can sometimes excite magnetism.”
Ocular demonstrations in the darkroom were designed to illustrate the differences between light and electricity:
The electric fluid does not manifest itself until it is put into action; then it appears in the dark in various luminous forms. It is not luminous itself, because it only appears with the help of the light that stimulates it.
Explaining the Appearance of Repulsion
As was seen earlier, Franklin shared with Marat the metaphysical stance recognizing attraction but not repulsion as a fundamental aspect of nature. Nonetheless, Franklin, together with most other physicists, conceived of the electric fluid as made up of particles that somehow repel each other. Marat was prepared to do battle against that opinion.
Franklin and others had found that the pointed extremities of objects displayed special electrical properties; surplus electric fluid seemed to be denser in pointed parts of objects than in flat or rounded areas. “Who could not see,” Marat asked rhetorically,
that if the electric globules mutually repelled each other, their repulsion would be most extreme at the tip of the point where they are instead found most condensed?
Marat then turned to the direct examination of electrical phenomena involving apparent repulsion. He replicated a standard experiment utilizing two light-weight cork balls suspended by conducting wires, hanging very close to each other but not touching. When the terminals of the wires were both brought into contact with the same terminal of a Leyden jar, the cork balls were electrified and moved apart from each other. Was this not a clear-cut case of repulsion?
Marat observed, however, that the apparent repulsion ceases before the Leyden jar is completely discharged; the balls return to their original positions. Furthermore, he noticed, even after the balls had ceased to repel each other, they still contained enough electricity to make them jump toward his finger when he brought it close.
Marat substituted small spheres made of thin glass and gold leaf for the cork balls and obtained the same results. He concluded:
If the repulsion of the electrified objects had come from a force essential to the electric globules, the cessation of the repulsion would be impossible as long as the Leyden jar continued to be charged, because a constant cause necessarily has a constant effect. It is therefore true that the electric globules do not repel each other.
Marat utilized an air pump (a glass cylinder that could be emptied of air) to repeat this experiment in a vacuum. The absence of air, he found, diminished the apparent repulsion. Since the balls moved apart less as the vacuum was stronger, he concluded that “their apparent repulsion therefore is caused by the ambient air,” which attracts the balls outward. Cranking the wheel of an electrostatic generator to supply the charge, Marat found that
if the vacuum is at a certain degree, no matter how vigorously you turn the wheel, the balls will not move apart at all for fifteen to twenty seconds.
Why would they move apart at all in a vacuum if the apparent repulsion is really caused by the attractive power of the ambient air? Marat concluded that the walls of the air pump’s vacuum chamber become charged and attract the balls outward, making it appear that they are repelling each other. This, it seemed to Marat, sufficiently explained the phenomenon, but he offered further arguments to support his denial of repulsion, including this observation: “The mutual repulsion of the electric globules is incompatible with the celerity of their movement in very small wires.”
The Pivoting Needle: Anticipating Oersted?
The investigation of attraction and repulsion led Marat to try some experiments that produced some surprises:
It is necessary that I deal with some unknown phenomena here that not only seem to invalidate my theory but also to overturn all received notions.
In experiment number 21 he balanced a needle on a pivot. Two inches from the needle, at the same level and parallel to it, he placed a conductor attached to an electrostatic generator. By turning the wheel of the generator, electricity could be made to flow into the conductor. Marat reported his findings:
When the wheel begins to turn with force the needle remains immobile, but when the needle is not parallel, as soon as the wheel begins to turn, the needle presents a point to the conductor. Then if you approach that point from the side with the tip of your finger, a brass wire, a glass tube, or a wax stick, it will move away. If you approach the other end of the needle with any of these objects, the needle tip will come toward it.
Marat was clearly taken aback by these results. “If I were one who loved marvels,” he said,
I would stop here to allow free rein to the imagination of my readers, for what phenomena of nature have ever offered a more curiosity-arousing contrast than that which I have just described in detail?
Marat would not stop there, of course; he would attempt to explain what had occurred. The circle through which the needle turned seemed to have been “halfway within the sphere of activity of a repulsive force and halfway within the sphere of an attractive force.”
Strangest of all, the repulsion appeared to occur between objects that according to all previous experience should have attracted each other: “The needle is electrified positively and the bodies that are presented to it are electrified in a negative manner.” In this case it seemed to him that it could not be the approaching object’s electricity that caused the needle tip to move away; this reinforced Marat’s conviction that the appearance of repulsion is purely illusory.
Why, then, did the needle move when the other objects approached it? Marat attributed it to the simple presence of the approaching object regardless of its state of electrification. The point of the needle had originally been motionless because it had been surrounded by a homogeneously attracting ambient milieu. When the object approached, it displaced that milieu on one side; the needle tip then swung toward the other side where the attracting milieu remained undiminished.
A reader familiar with the development of electrical science will at this point be wondering whether Marat had not almost stumbled upon one of the most important of discoveries: the later demonstration by Hans Christian Oersted of the underlying unity of electricity and magnetism. Oersted’s startling discovery occurred in 1820 when he found that a wire carrying an electrical current caused a magnetized needle to move. This sounds superficially similar to what Marat was doing forty years earlier.
Oersted, however, had the benefit of Alessandro Volta’s invention of the electric pile, or battery, as a source of electrical current. Even without the Voltaic pile Marat could have produced a current in a wire by turning the crank of his generator. Then, if his needle happened to have some residual magnetism (which is not at all unlikely) it might have moved as Oersted’s did.
But the “conductor” Marat used was probably not a wire carrying a current along toward some repository of charge (such as a Leyden jar). If it had been, it would not have exerted the force of an electrostatic charge, but would have produced an electromagnetic field. Such a field would not have caused the needle to turn to point toward the wire, but rather to rotate in a plane perpendicular to the wire.
More likely, then, Marat’s “conductor” was not a current-carrying wire, but rather a prime conductor; that is, a piece of metal receiving the electrostatic charge generated by turning the wheel. It would seem that the needle’s action could be explained, then, by a separation of charges on the needle induced by the conductor’s charge. If so, it is difficult to understand Marat’s surprise, because electrostatic induction was far from being an “unknown phenomenon” in the 1780s, and he certainly was familiar with it. Although his description of the experiment appears detailed and clear, there is not enough information to allow a definite determination of what caused the needle to behave as it did.
Marat, however, was looking for an anomaly that would raise doubts about electrical repulsion, and he believed he had found one. His curiosity and experimental imagination led him in potentially fruitful directions. His theoretical imagination, however, limited the value of his investigations by allowing him to satisfy himself with explanations of phenomena that he might otherwise have pursued further.
If such blame is to be assessed, however, some of it should be shared by those who ignored Marat’s work altogether. Once again, the extension of knowledge would have been better served had Marat’s experiments been replicated and his theoretical conclusions challenged rather than disregarded by the scientific elite.
Although Marat’s desire to completely eliminate the concept of repulsion from electricity may appear to be an extreme theoretical position, it was not unprecedented. In fact, it is likely that the source of Marat’s idea was a published letter of Ebeneezer Kinnersley, although he does not say so.
Kinnersley had challenged the “doctrine of repulsion in electrised bodies” in a letter that was included in the fourth (1769) edition of Franklin’s book along with a response from Franklin. In the letter, Kinnersley suggested that all of the electrical phenomena that seem to exhibit repulsion “may be well enough accounted for without it.” In particular, he thought the separation of charged cork balls in the classical experiment might be explained by the balls being attracted outward by a natural quantity of electricity in the surrounding air. In his reply, Franklin expressed an appreciation of Kinnersley’s position. He, too, he said, had once hoped to be able “to resolve all into attraction.” There were some cases of apparent repulsion, however, that he was unable to “so easily explain by attraction.” Franklin, then, remained skeptical with regard to Kinnersley’s position, which was apparently identical to Marat’s.
Explaining the Leyden Jar
If the jar is suspended in a room by a silk cord and if the air is dry, it cannot be charged, no matter how many times the wheel (of the electric generator) is turned. From this particular fact a general principle has been derived: “It is impossible to charge an isolated Leyden jar.”
To the contrary, Marat declared, an isolated jar could indeed be charged if only a needle a few inches long is attached to its external lining. This is a claim for which the abbé Bertholon heaped ridicule on Marat. It was not Bertholon, however, who Marat was challenging with his novel interpretation of the Leyden jar. His aim was higher; the target was Franklin:
The educated reader will already have perceived that here I am combating the principles of a great Master. It is not without regret that I find myself brought to this kind of discussion, but it goes to the heart of my subject. Moreover, I have too high an opinion of that wise man, a few of whose ideas I am challenging, to believe that he would object to not being followed blindly.
Apparently still smarting from the reception that greeted his attempt to revise Newton, Marat denied that he had “set out to attack great men” as his numerous adversaries claimed.
It is not true that Marat was uniformly hostile toward famous scientists, either predecessors or contemporaries. His challenges were to specific ideas of Newton, Franklin, and others. Like most other authors, he paid homage to their general contributions to science. There is no reason to suspect that his frequent praise of “the great Franklin,” for example, is anything other than sincere. His approval of Giambatista Beccaria and his work is evident, and although he criticized certain notions of Wilcke and Aepinus, he acknowledged their status as “two celebrated physicists.” His strongest polemics—and even here it would be an exaggeration to describe them as vilification—were directed toward Bertholon, as will be seen below. Marat’s scientific writings reveal that he could be testy on occasion, but there is really very little hint of the kind of polemics for which he would become known during the Revolution.
Continuing his analysis of the Leyden jar, Marat reported:
Finally, a modern author has shown that the water from a jar that has just been charged, if transferred to an empty bottle that is externally lined and not electrified, can give a shock. If the celebrated Franklin has found the opposite, it was because the water was too weakly electrified, or perhaps that the electric fluid it contained dissipated in the process of being transferred to the other bottle.
The “modern author” was identified as Nollet, so although Marat was generally supportive of Franklin against Nollet in theoretical matters, in this particular case he found himself in agreement with Nollet against Franklin.
The question of whether the water in a Leyden jar was a repository of electrical charge was important to interpreting how the apparatus functioned. It is evident that either Marat and Nollet, on the one hand, or Franklin, on the other, must have been in error on this critical issue. With benefit of hindsight it is obvious that Franklin was right. The point, however, is that at the time Marat was writing, even direct experimentation was unable to decisively resolve the disputed issue. The nature of electrical experimentation was such that even careful experimenters could obtain conflicting results.
Distinguishing Between Positive and Negative Charges
The existence of electric effects was recognized for millennia before Du Fay’s 1733 distinction between vitreous and resinous electricity, later designated “positive” and “negative” by Franklin. The long delay in detecting the two different kinds of charge was due to the great similarity in their behavior. They could only be distinctly differentiated by the fact that what one attracted the other repelled, and vice versa. Given a single, isolated charged object, however, how could an experimenter determine with certainty whether its charge was positive or negative? Or, given an isolated pair of objects of unknown charge that attract each other, how could it be determined which was positive and which negative? Marat listed a number of methods that had been suggested by various physicists:
The signs by which they have been accustomed to recognize positive electrification are: the attraction of freely movable light objects from a reasonable distance, the impression that the electric current makes on the sense of touch, the angles of inclination of the jets omitted by electrified bodies, and the combustion of volatile spirits.
These signs, Marat argued, are far from decisive. Attraction is ruled out because it is reciprocal and no final distinction can be made between what is attracting and what is attracted. As for the second point, a shock feels the same whether the current flows to or from the person receiving it. On the third and fourth points:
The inclination of the jets that electrified bodies emit escape the sharpest eye, and finally, the combustion of volatile spirits takes place whether one directs a spark toward them or draws a spark from them. It is therefore manifest that these signs are equivocal.
Marat noted in passing that the uncertainty of these signs was what led to “the absurd system of affluent and effluent matters,” a strong blast against Nollet’s Cartesian theory of electricity.
Another modern academician, Marat said, had attempted to put forward what he thought to be less equivocal signs. In a 1755 memoir Le Roy had suggested that the shape of the discharges as seen in a darkroom is different for positive and negative electricity: A positive discharge produces a “plume” while a negative one is evidenced by a “luminous point.” Marat did not dismiss Le Roy’s method out of hand but he did not consider it to be completely trustworthy.
Up to now physicists have not found definitive signs of the two kinds of electrification. Is it necessary, then, to infer that none exist? Not at all; there are infallible ones, as we shall see.
Marat’s suggested method was based upon acceptance of Franklin’s single-fluid theory:
To distinguish immediately between an object positively electrified and one negatively electrified, it suffices to perceive the direction of the fluid that is transported from the one to the other. But since the fluid has infinitely too much speed to be followed by the eye in such a limited interval, it is necessary to judge its direction by that of other corpuscles that are caught up in its motion. Nothing could be easier: By means of our method of observation in the darkroom, the electric fluid that emanates from a positively electrified object (equipped with a point) can always be seen to chase before it the igneous emanations of an incandescent object.
A negatively charged object, by contrast, does not agitate the igneous fluid. Marat said this was all plainly visible in the images projected onto the screen by his solar microscope.
Van Marum’s Ocular Demonstration
These endeavors of Marat can be better appreciated by comparing them with experiments performed at Leyden a few years later by Martin Van Marum, working with the most powerful electrostatic generator then in existence. In 1784 he put his huge machine to use to produce an ocular demonstration in support of Franklin’s single-fluid theory. The spark discharges produced by his machine gave the appearance of miniature bolts of lightning with multiple forked endings. The illustration published by Van Marum shows the forks all pointing away from the positive terminal and toward the negative one, indicating the direction of electric flow that would be predicted by Franklin’s theory.
VAN MARUM’S OCULAR DEMONSTRATION OF FRANKLIN’S SINGLE FLUID THEORY OF ELECTRICITY. The forked branches seemed to indicate motion of the electric fluid from left to right—from the positively charged small sphere to the negatively charged larger one.
In 1785 Van Marum went to Paris and described his experiments to leading members of the Academy of Sciences, including Lavoisier, Le Roy, Berthollet, and Monge. “They all agreed with me,” Van Marum later reported,
that, given the direction of the electrical discharge produced by the large machine, which had become so visible, it had been incontestably proven that, in conformity with Franklin’s theory, a single electrical fluid exits from the positive conductor and passes into the next conductor, which receives it.
Le Roy then took Van Marum to meet Franklin. According to Van Marum’s account of the meeting, Franklin declared:
This proves my theory of a single electrical fluid, and now, at last, the theory of two kinds of electrical fluids will have to be rejected.
Volta was particularly enthusiastic over Van Marum’s accomplishment:
All orthodox electricians owe you a great debt of gratitude, sir, for having dealt the deathblow to the heresy of the “dualists”; i.e., the new followers of Du Fay, Nollet, and Symmer.
Marat’s ocular demonstration was not as direct as Van Marum’s since it was dependent upon the visible motion of igneous fluid. As was seen earlier, however, Marat’s claim to have rendered the igneous fluid visible was accepted by a number of academicians, and none explicitly disputed it. If that premise were to be assumed valid, his ocular demonstration of the flow of electric fluid should have been as convincing as Van Marum’s. It also had the advantages of having been announced five years earlier, and of not requiring apparatus that only well endowed institutions could afford.
The fate of Marat’s demonstration, however, was quite the opposite of Van Marum’s. If any other prominent physicists had taken note of it, they did not reveal that they had. No commission of the Academy reviewed Marat’s electrical experiments as they had his earlier investigations of fire and light. Le Roy had apparently been alienated from Marat during the controversy over Newton’s theory of colors. Condorcet had drawn from “the Marat incident” the lesson that Marat was to be given the cold shoulder. Volta had also expressed antagonism toward Marat.
It would seem, then, that the difference between the receptions accorded Van Marum and Marat was not based on the scientific merit of their respective ocular demonstrations but on other, extrascientific grounds. In the context of contemporary electrical science, Marat’s work seems to have been as deserving of serious attention as Van Marum’s.
Two Kinds of Electricity
In fact, neither Marat nor Van Marum had resolved the dispute over whether there are one or two kinds of electric fluid. In spite of Van Marum’s and Volta’s triumphalism, it will be recalled from the discussion above that the two-fluid theory regained its hegemony and maintained it well into the nineteenth century. Marat, noting that “the question remains undecided in spite of the efforts of physicists,” turned to a discussion of “the system of so-called vitreous and resinous electricities.” Vitreous, he reminded his readers, is positive; resinous is negative.
Almost all eighteenth-century electrical experimentation utilized a spinning glass globe as generator. As a result, far more positive electricity was produced in the laboratory than negative. The properties of positive charge were therefore much more familiar to physicists than were those of negative charge. This most likely was a result of the fact that most “resinous” materials are far less durable than glass; continuous spinning and rubbing would cause them to crumble.
Whatever the cause, it was a fact that favored the survival of Franklin’s theory for which the most troubling anomalies were phenomena deriving from negative electrification. Marat, to his credit, recognized the asymmetry of experimental practice and proposed to rectify it:
All physicists being in agreement on the question of the electrification of glass, it is to the electrification of resin that we must confine ourselves.
Although Marat acknowledged the conventional use of the term “resinous” to denote negative electrification, he did not believe that substituting a resinous globe for the standard glass globe would really produce a negative charge.
Among several facts that demonstrate the contrary, let us limit ourselves to these. A conductor successively electrified by a glass globe and by a sulfur globe presents some phenomena that are more or less sharply defined, but absolutely identical.
His list of these identical phenomena culminates with their appearances in darkroom observations. In all cases, he concluded, the objects were positively electrified: “It is beyond doubt that glass and sulfur electrify the bodies they are in contact with in the same manner.” The hypothesis of two electricities is thus rejected as “false and absurd.”
The Speed of Electricity
Marat took issue with the generally accepted idea, attributed to Franklin, that electricity travels at the speed of light. He also disagreed with an unnamed “modern academician” who in 1746 concluded that electrical transmission was instantaneous.
Electricity, Marat stated, behaves like fluid in a pipe. If pressure is exerted at one end of the pipe, one particle of fluid bumps the next one, and so on all the way to the other end where a particle is simultaneously forced out. If the transmission seems instantaneous, it is not due to the actual transmission of any fluid from the beginning to the end of the pipe.
The only valid way to judge the speed of electricity, then, is by trying to observe it in motion through a free medium. Marat suggested what he admitted was an inexact method: estimating the speed of lightning jumping between clouds in a storm.
Among a thousand observations that I have made on this subject . . . I will limit myself to this one. On July 22, 1780, a violent storm presented me with the best possible opportunity.
He estimated the elapsed time of a flash of lightning leaping between two distant clouds at a third of a second and calculated its speed to be 19,200 toises (37,420,800 meters) per second. This figure, he cautioned, could not be interpreted as a constant rate of speed. The velocity of electric fluid cannot be constant because it must always be accelerating when in motion, for the same reason that objects accelerate when they fall to earth. It is a consequence of the inverse square law of attraction.
Marat’s application of the inverse square law to electric force provides the key to understanding his critique of the electric atmospheres postulated by Franklin and others, a term that Marat believed “should be proscribed from science.” Instead of explaining electrostatic induction by means of a cloud of electric fluid particles surrounding a charged object, Marat utilized a theoretical construct that he denoted as a charged object’s “sphere of activity.” Although at first glance the idea of an electrical sphere of activity may seem similar to that of an electrical atmosphere, the two concepts were not at all identical.
Marat’s treatment of the sphere of activity implied a complete acceptance of action at a distance. It depended upon a force of attraction no different in principle from gravity.
The sphere of activity of the electric fluid always extends to a certain distance from the objects in which it is found; often even around objects that it cannot penetrate.
The distance to which the sphere of activity extends “is always in ratio to its mass”; that is, the mass of the electric fluid itself. The energy of the fluid’s attractive force
is always deployed in direct ratio of the quantity of accumulated fluid, and always in inverse ratio of the square of the distance—the law common to all bodies that attract each other.
A modern reader might notice that this seems to be an expression of Coulomb’s law, if quantity of electric fluid is equated with magnitude of charge. Coulomb, however, formulated his law in 1785, three years after Marat’s book was published.
In fact, an inverse square relationship of electrical attraction analogous to the Newtonian gravitational formula had been suspected by many physicists well before Coulomb substantiated it. The hypothesis did not originate with Coulomb; his achievement was to demonstrate its validity by rigorous experimental means.
Priestley, for example, had explicitly stated the inverse square law of electrical force in his 1767 book, which Marat knew well. The basis of Priestley’s conclusion was complex. First of all, Newton, in his Principia, had mathematically demonstrated that the inverse square relationship implied that the gravitational force at any point inside a uniform sphere would be zero. Secondly, it had been experimentally shown that the force of electrical attraction at any point within a uniformly charged cylinder was zero. In spite of several logical difficulties, not least of which was extrapolating from spheres to cylinders, Priestley’s intuition led him to conclude that electrical attraction followed the general pattern of gravitational attraction.
As elegant as Priestley’s reasoning was, it did not establish the inverse square law of electrical force on a solid enough basis to satisfy skeptics. It remained to be demonstrated experimentally.
Perhaps Marat’s scientific abilities can most fairly be estimated by comparing his approach to this problem with that of his more successful contemporary, Coulomb. Here is Marat’s proposal:
In order to get an idea of this law, a little pith-ball can be used, suspended from the ceiling by a long flax thread, which can be approached at various distances by the knob of the terminal of the Leyden jar, charged to various levels. The more the jar is charged, the more the little ball will be promptly attracted, and the greater the distance over which it is attracted. This same method would suffice to demonstrate this law rigorously, if it were possible to determine in each trial the exact amount of fluid accumulated on the knob of the jar (or rather, on its internal surface).
Marat, then, suggested a method for a quantitative determination, but acknowledged its incompleteness by doubting the possibility of measuring the key independent variable (charge, or amount of electric fluid). Furthermore, even if the independent variable could have been measured precisely, Marat’s method could not have determined the quantitative value of the dependent variable (force of attraction) with accuracy.
Coulomb, by contrast, was able to “demonstrate this law rigorously,” not only in principle but in fact. His new apparatus, the torsion balance, provided a supersensitive scale for the measurement of forces that allowed him to give an impressively precise quantitative demonstration of the inverse square law of electrical force.
In comparison with Coulomb’s torsion balance, Marat’s suggestion for a means of measuring the force of attraction between charged objects seems astonishingly rudimentary:
The force of their attraction can only be estimated by the size of the spark that they throw. The difficulty, or rather the impossibility, of determining with precision the size of each spark therefore renders all the results uncertain.
But Marat was not as unsophisticated as this notion of spark measurement makes him seem. If anything, his view of the problem was oversophisticated: He considered it necessary to distinguish between forces acting between charged objects and forces exerted by charges (or electric fluid) per se. If spark sizes could be gauged, that would give a direct measure of electrical force, but since that is impossible an indirect method would have to suffice. The indirect measure he suggested was the determination of the amount of mechanical force that charged objects exert on each other, which was the method Coulomb was to utilize. Whereas Marat believed this could only be done with “embarrassingly imprecise” results, Coulomb achieved a degree of precision with his torsion balance that was high enough to satisfy most of his contemporaries.
Such a comparison runs the risk of imposing today’s criteria on eighteenth-century events. It is worth noting, however, that the two physicists were thinking about the same problem at almost the same time, that Marat concluded that it could not be solved in practice, and that Coulomb did devise a convincing solution.
The Uses of Electricity
The last quarter of Marat’s book on electricity turns from theoretical to practical matters. What are “the uses to which the electric fluid is destined to be put”?
It seems that electricity, so full of marvels, has finally exhausted the admiration that it had at first excited. It is still being cultivated by a large number of devotees, but the public, less attuned than in the old days to the singularity of the phenomena it presents, today complains that for all of the discoveries made in this branch of science, almost nothing has been produced for the good of society.
Marat discussed the applications of electricity to chemistry and seemed to conclude that there is not much to be hoped for in that regard, based on what had been accomplished up to that point. He added a caveat, however:
Let us not jump to a conclusion too hastily. To find out what promise this field holds, let us wait until some enlightened mind has made it an object of research.
As for medical applications of electricity, Marat believed the potential was great, but the surface had hardly been scratched with regard to specific knowledge. He had utilized therapeutic electricity in his medical practice, and it would form the subject of one of his prize-winning academic essays. It would also be the primary field of dispute in his bitter rivalry with Father Bertholon.
A number of investigators had tested the effects of electric fluid on vegetation. The work of Jallabert, Nollet, and others had “proven that electric fluid favors vegetation.” Marat intended to go farther, “to prove that [electricity] is the principle of vegetation, conjointly with heat.”
No one that I know, except M. de la Cépède, has yet advanced a similar assertion. But since the electric fluid is for that author a composite of water and heat, and even an integral part of plant life, it can readily be seen that our assertions have nothing in common but the similarity of the sounds that serve to express them.
Marat’s ideas about this subject were apparently influenced by experiments showing that electricity could stimulate capillary action.
Congealed earth cannot produce anything; saps that have become solid cease to circulate. But when heat renders them liquid, it is the electric fluid that opens their passage in the channels of plants. It is that which begins and accomplishes the work of germination.
“Such assertions should be backed up by unequivocal facts,” he declared. “Now it so happens that there are decisive facts.” As evidence, Marat reports a botanical experiment that he performed in December 1780. It was a controlled experiment utilizing two terrariums, each of which contained three pots. Seeds generated from the same plant source were planted in all six pots. The temperature was kept constant at slightly above freezing throughout the course of the experiment. One of the terrariums was electrified by keeping it charged like a Leyden jar for 17 of every 24 hours over a fifteen-day period. The control terrarium was not electrified.
Vegetation appeared in the electrified terrarium after seven days; by the fifteenth day there were little plants comparable to those grown in a considerably warmer environment. Meanwhile, no vegetation at all appeared in the control terrarium. “It is manifest that the electric fluid is one of the principal agents of the fertility of the earth.”
Marat’s discussion of the general utility of electrical science ended on a hopeful, if ambiguous, note. He insisted that it would certainly become useful, and cited the lightning rod and medical applications as harbingers of its promise, but no other specific uses were suggested.
The Lightning Rod
The sixth and final section of Recherches physiques sur l’électricité—nearly a hundred pages long—was devoted entirely to consideration of “lightning and the means of defense against its blows.” Marat began with high praise for Franklin’s lightning rod. After describing the device and the theory behind it, he commented: “If I may be permitted to say so, the consequences may be true, but the premises do not seem to me to have been demonstrated.” Marat, in other words, believed the lightning rod to have been a practical success that could have been even more useful had the underlying phenomena been better understood.
Without doubt, conductors raised above an edifice are an excellent means of preserving them. But is it really true that they peacefully extract from storm clouds the fluid of which they are formed?
Franklin designed his device not to attract bolts of lightning in order to channel them harmlessly to earth, but to attract a slow, silent leakage of electric fluid from the sky to the ground. This notion was based on his laboratory observations of the power of pointed conductors to draw off electrical charge from considerable distances. Marat, however, considered it an unwarranted inductive leap to extrapolate from the laboratory to the great outdoors. It could not be assumed that the same phenomena that operate at distances of a few feet continue to operate all the way up to the clouds.
Despite the lightning rod’s proven utility, he said, its record was far from spotless.
How many examples there are of buildings struck by lightning in spite of being armed with conductors! And how many examples of conductors themselves struck!
Marat cited eleven specific case histories, stating the places and dates they occurred, and concluded: “It is well demonstrated that the sphere of attraction of conductors does not extend to the clouds.”
Nonetheless, Marat considered the electrical grounding of high objects to be a wise policy. “These devices are often useful; that is incontestable. But are there cases when they may be dangerous? Now, that is a good question!” It is possible, he warned, that lightning rods sometimes expose rather than protect a place.
As a prelude to his own general proposal for protection against lightning, Marat discussed two controversial issues. First, while it is not true that lightning rods “expose neighboring houses by preserving the one to which it is attached,” as some had argued, a single lightning rod could not be expected to protect several neighboring houses.
The second issue, one that had been hotly debated for years, was over whether the ends of lightning rods should be pointed or blunt. Franklin’s original design called for points, and he and his partisans vigorously defended that choice. Benjamin Wilson had made a name for himself by arguing in favor of blunt-tip or knobbed rods. Marat, citing the Journal de Physique’s summary of the literature on this debate as of 1779, concluded that “the results have been in favor of points.” Most of the previous studies, however, had been based on the idea that the conductors could silently siphon electric fluid from the clouds, a premise Marat did not accept. His decision in favor of points was based on the belief that their sphere of activity is less limited.
Taking all of his theoretical conclusions into consideration, Marat suggested some design improvements for individual lightning rods as well as a comprehensive method for the protection of entire towns. Lightning rod efficacy could be optimized, he said, by constructing them with multiple points aimed in all directions, and by equipping them with very thick ground wires sunk deep into the earth, to the depth of underground water.
A town’s protection could be maximized by placing rods on the houses at both ends of every street, however long any given street may be, and connecting all of their ground wires into a single grid. The rods should ideally rise to at least 40 feet, should be made of wood coated with lead, and should end in multiple copper points.
Putting such a plan into effect, of course, would be a social project. Should it be forced upon the populace against its will? This is the only place in all of Marat’s physics trilogy where social theory is considered.
The superstitious nature of the people, he said, would be an obstacle to implementing the plan. The idea that ringing the church bells can ward off thunderstorms, for example, was very strong in the countryside in spite of the fact that bell-ringers were frequently killed by lightning.
To abolish such a harmful practice, the voice of persuasion is surely best if the common people are able to heed the voice of reason. The more widespread the superstition, the more important it is to destroy it. Will it be necessary, then, to use force? Any violence in this regard would amount to real tyranny. How, then, should this be handled? Let the bells be rung, but put a lightning rod on the spire of the steeple and run the groundwire down through the priest’s home in order to shield it from the worried, superstitious types who might try to cut it. This is the only appropriate way of stopping so many useful subjects from foolishly seeking their own deaths.
All in all, there is not much in this prescription that gives any hint of the future development of Marat’s political outlook. The radicalism he had earlier expressed in Chains of Slavery would only reappear after the hidden tensions in French society erupted into open conflict.
Bertholon and the “Earthquake Rod”
Marat concluded his work on electricity with a lengthy, sarcastic polemic against a pet project of his rival, the abbé Bertholon. Citing an article by Bertholon in the August 1779 Journal de Physique, Marat jeered:
In a work full of grandiose claims but little substance, a surefire method for preventing the terrible effects of volcanic eruptions and earthquakes is announced. Sublime project! If only it were as solid as it is flashy.
Bertholon had extended the concept of lightning rods by inventing earthquake rods and volcano rods. Although this scheme is devoid of all foundation, Marat stated, he would take the trouble to refute it because Bertholon’s celebrity might blind his readers to its falsity.
He quoted Bertholon:
Earthquakes are nothing more than subterranean thunder, and since it has been demonstrated that thunder is in fact an effect of electricity, one cannot help but recognize that the cause of earthquakes is nothing other than the matter of electricity.
This was not an uncommon notion; Lacépède, for example, held similar views about the origins of earthquakes. Bertholon had set up a small model of an area of terrain and by applying electricity had made it “quake” and had caused miniature volcanoes to erupt. Marat declared that these “childish games” in no way added support to the false hypothesis that earthquakes and volcanoes are electrical in origin. In fact,
all available evidence points to the conclusion that volcanic eruptions and earthquakes are caused only by the explosion of a prodigious quantity of combustible materials.
Bertholon’s earthquake-and-volcano rods were pointed conductors designed to be sunk deep into the ground like upside-down lightning rods. Insofar as they succeed in attracting electric fluid, Marat warned, they would be hazardous rather than beneficial.
It is on that note that the volume concludes. The sudden ending, with no summary or conclusion, is uncharacteristic of Marat and suggests that perhaps he was in a hurry to get it to the printer.
• • •
Marat’s experiments on fire had received a positive review from a commission of the Academy. His second endeavor, the optical investigations, met with a much less favorable response. The final episode of his trilogy of physical researches, the electrical experiments, attracted almost no notice: They were ignored. Taken together, however, the three works constitute an impressive body of work that demonstrates Marat’s seriousness and legitimacy as a scientist of the late eighteenth century. Charges that he was a pseudoscientist are untrue.
 J. L. Heilbron, “Franklin, Haller, and Franklinist History” (Isis LXVIII, no. 244 [December 1977], 539–49).
 William Gilbert, De Magnete (1600). For a concise account of the early history of electrical science, see Duane Roller and Duane H. D. Roller, “The Development of the Concept of Electric Charge” (Harvard Case Studies in Experimental Science, vol. II).
 The electrical machines were “capable of generating about 10,000 volt when first employed in the 1740s.” By 1785 an immense machine constructed at Leyden and used by Martin van Marum “probably gave over 100,000 volt” (J. L. Heilbron, Elements of Early Modern Physics, 74). Van Marum’s electrostatic generator can be seen today at the Teyler Museum in Haarlem, the Netherlands.
 Letter from Musschenbroek published by Nollet in 1746 in the Mémoires of the Academy of Sciences. Quoted by Roller and Roller, “Development of the Concept of Electric Charge,” 594.
 Jean-Antoine Nollet (1700–1770) was a French clergyman and physicist.
 Benjamin Franklin, Autobiography and Other Pieces, 148.
 Georges Louis Leclerc de Buffon (1707–88), author of the encyclopedic Histoire naturelle and longtime director of the Royal Botanical Gardens, and René Antoine de Réaumur (1683–1757), author of major works on such diverse subjects as entomology and metallurgy, were two of eighteenth-century France’s most influential scientists.
 John Canton (1718–72) was an English pioneer of electrical investigations and a friend of Franklin’s.
 Cohen, Franklin and Newton, 518.
 Ibid., 531, 534.
 Aepinus, Tentamen theoriae electricitatis et magnetismi (St. Petersburg: Typis Academiae Scientiarum, 1759).
 Ibid., 39; quoted in Cohen, Franklin and Newton, 541.
 R. G. Boscovich, Philosophiae naturalis theoria redacta ad unicam legem virium in natural existentium (1758).
 James Clerk Maxwell, A Treatise on Electricity and Magnetism (Oxford, 1873), quoted in Cohen, Franklin and Newton, 560.
 Cohen, Franklin and Newton, 373.
 Ibid., 515.
 Marat, Recherches physiques sur l’électricité, 10.
 Ibid., 12.
 Ibid., 13.
 Ibid., 14; emphasis in original.
 Heilbron, Elements of Early Modern Physics, 290. On Biot and Poisson, see note 31 in chapter 4 on this website.
 Marat, Recherches physiques sur l’électricité, 27.
 Ibid., 28.
 Ibid., 31. Marat cannot be counted among those natural philosophers who formed the advance guard of the Naturphilosophie school. He was not seeking the underlying unity of diverse phenomena; his view of the relationship between electricity and magnetism was every bit as orthodox as Coulomb’s.
 Ibid., 32.
 Ibid., 36.
 Ibid., 40–41.
 Ibid., 43.
 Ibid., 42.
 Ibid., 44.
 Ibid., 48.
 Ibid., 49.
 Ibid., 51.
 Ibid., 50.
 This assumes that the needle is free to rotate in a plane perpendicular to the wire.
 Franklin, New Experiments and Observations on Electricity (1760). Quoted in Cohen, Franklin and Newton, 531.
 Ibid., 532.
 Marat, Recherches physiques sur l’électricité, 116.
 Ibid., 122.
 Ibid. By way of illustration, Marat cited an experiment reported in the Journal de Physique, 1775, of a M. de Lor.
 See the letter from Bertholon to Antoine Buissart, 15 December 1782, in Revue historique de la Révolution française, vol. III (1912), 294–7. Bertholon edited a journal called La Nature considérée sous ses différens aspects, ou Journal d’histoire naturelle (Paris: Perisse; the first volume appeared in 1787). Bertholon was identified on the title page as professor of experimental physics, member of sixteen academies, “etcetera.”
 Marat, Recherches physiques sur l’électricité, 126–7.
 Ibid., 84.
 Ibid., 101. Johan Carl Wilcke, a professor of physics in Stockholm, translated Franklin’s work on electricity into German in 1758. Marat called Beccaria a “highly distinguished physicist” and said of his meterological research: “Never has there been a more assiduous observer of more different states of the atmosphere.” Ibid., 378.
 Ibid., 128–9.
 Ibid., 145.
 Ibid., 146.
 Ibid., 150.
 Martin Van Marum (1750–1837). On the power of Van Marum’s electrical machine, see note 3 above.
 Martin Van Marum, Sur la théorie de Franklin, suivant lequel les phénomènes électriques sont expliqués par un seul fluide (1819).
 Ibid., 9.
 Ibid., 10–11.
 Ibid., 12. Van Marum was quoting from a letter Volta had written him, dated 8 March 1786. Volta’s obituary of the two-fluid theory was premature.
 See Brissot, Mémoires, vol. I, 201.
 Marat, Recherches physiques sur l’électricité, 157.
 Ibid., 159.
 Ibid., 160.
 Ibid., 161.
 Ibid., 162.
 Ibid., 225–6.
 Ibid., 257–8.
 Ibid., 269.
 Ibid., 272.
 Joseph Priestley, History and Present State of Electricity (1767).
 Marat, Recherches physiques sur l’électricité, 273.
 Ibid., 275–6.
 Ibid., 354.
 Ibid., 355.
 Ibid., 357.
 Ibid., 358; emphasis added. Jean Jallabert (1712–1768) was a Swiss physicist and politician.
 Ibid.; emphasis in original.
 Ibid., 359.
 Ibid., 360.
 Ibid., 364.
 Ibid., 408.
 Ibid., 408–9.
 Ibid., 414.
 Ibid., 415.
 Ibid., 416–7.
 Ibid., 418.
 Ibid., 419.
 Ibid., 427.
 Ibid. Bertholon’s article had appeared in the Journal de Physique, August 1779.
 Marat, Recherches physiques sur l’électricité, 437–8.
 Lacépède, Essai sur l’électricité naturelle et artificielle, vol. I, 4–5.
 Marat, Recherches physiques sur l’électricité, 447.