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21 septembre 2020 1 21 /09 /septembre /2020 06:37

A first lesson about electricity is the occasion of a classic staging in the experimental tradition of physics teachers: A rod of ebonite is rubbed, a ball of elder hanging on his silk or nylon thread is attracted then strongly repulsed. Then begins a series of manipulations based on wool cloth, cat skin, glass rod or rule of synthetic material, supposed to reveal a fundamental property of matter: the existence of two kinds of electricity.


Progressing in the course we quickly arrive at the notion of electric current. This is where the "problem" appears. As soon as we have defined its conventional direction of circulation, from the positive pole of the generator to its negative pole in the external circuit, we must add that the electric fluid is, in reality, made up of negative electrons moving in reverse !
An explanation is needed. The busy teacher will evoke an old mistake.  However, a brief return on the history of electricity would suffice to reveal, instead of hasty decisions, the obstinate search for a physical reality. Dufay is one of the first links in this chain.


Dufay (1698-1739) and the electric repulsion:


Charles-François de Cisternay Dufay is from a family of high military nobility. He himself entered the regiment of Picardy, at the age of fourteen, as a lieutenant. He took part in the short war in Spain and retained his military position until 1723, when he joined the Academy of Sciences as a chemistry assistant.
How can a 25-year-old jump from being a soldier to becoming a member of a prestigious science academy? To understand it, we need to say a few words about Dufay, the father.


This soldier had been educated by the Jesuits at Louis-le-Grand. He keeps, of it, a culture that he continues to enrich during his military campaigns. "The muses," he said, "heal the wounds of Mars." In 1695, the loss of a leg ends his military career. He returned to Paris where he devoted himself to educating his children and enriching a fabulous library. Charles-François will be able to cultivate his taste for science in the same time that his father teaches him the profession of arms.
At Dufay's we meet powerful characters. Like the Cardinal de Rohan who supports the young Charles-François when he applied for the post of chemistry assistant at the Academy, in 1723. Réaumur accepted this candidacy.


Dufay will make a point of honor to deserve this distinction. His early works are marked by unbridled curiosity. It goes from the study of phosphorescence to that of the heat released by the "extinction" of the "quick" lime. From the solubility of glass to geometry. From optics to magnetism. His energy earned him the title of Intendant of the King's Garden in 1732. It was not long after this promotion that he heard of Gray's work. He finally holds "his" subject. Electricity will give him the opportunity to implement a method whose rigor will be equivalent to that of Lavoisier, in the field of chemistry, half a century later.


Beautiful discoveries will be at the rendezvous. They will be the subject of a series of memoirs published in the History of the Academy of Sciences from April 1733.
The first of these memories is presented as a "History of Electricity". This text remains, even read in hindsight of nearly three centuries, an honest document. Before reporting on his personal contribution, Dufay chose to "put under the eyes of the reader, the state where this part of physics is currently". He wishes, he says, to give back to each one his merit and to preserve, for him, only that of his own discoveries. Above all, he wants to free himself from the obligation of having to quote, at every moment, the name of one or another of his predecessors. His project, in fact, is ambitious: he proposes to lay the first stones of a real theory of electricity. Most of the authors who preceded him, he said, "reported their experiences in the order in which they were made." His plan is different: he wants to classify their experiences in order "to unravel, if possible, some of the laws and causes of electricity."


A discourse of the method:


The second memory announces its method in the form of six questions.

It's about knowing:

Which bodies can become electric by friction and if electricity is a quality common to all matter.

If all the bodies can receive the electric virtue by contact or by approach of an electrified body.

Which bodies can stop or facilitate the transmission of this virtue and which are most strongly attracted to electrified bodies.

What is the relationship between the attraction virtue and repulsive virtue and whether these two virtues are related to each other or independent.

If the "force" of electricity can be modified by vacuum, pressure, temperature ...

What is the relation between electric virtue and the faculty of producing light, properties which are common to all electric bodies.

A beautiful program which will be carried out with remarkable rigor.

The first three questions concern the problem of the electrification of bodies and electrical conduction. We have already seen how Dufay interposed between Gray and Franklin to establish the first laws. The fourth question poses, for the first time, the problem of repulsion.


Repulsion joins attraction.


Since William Gilbert, and even since antiquity, electricity has been synonymous with attraction. Dufay is no exception to the rule and, in the introduction to his first memoir, he defines electricity as "a property common to several materials and which consists in attracting light bodies of all kinds placed at a certain distance from the electrified body. by rubbing a cloth, a sheet of paper, a piece of cloth or simply by hand ".

However, he was disturbed by one of the observations made by Otto de Guericke: that of the sulfur globe which repels the down that it first attracted. He admits that he never managed to reproduce it. On the other hand it meets success with a similar experience proposed by Hauksbee. It involves rubbing a glass tube held horizontally and dropping a piece of gold leaf on its surface. The result is spectacular:

"As soon as it has touched the tube, it is pushed up perpendicular to the distance of eight to ten inches, it remains almost motionless at this place, and, if we approach the tube by raising it, it also rises , so that it always remains in the same distance and that it is impossible to make it touch the tube : one can lead it where one wants so, because it will always avoid the tube " .

Even if the prowess achieved by the "electricity fairy" has quenched our thirst for the marvelous for a long time, the experience, even today, is worth trying. For this it is important to have the right glass tube. That of Dufay is of the type used by Gray and which has become a standard. It has a length close to one meter and a diameter of three centimeters. It is made in a lead glass. Gray and Dufay say nothing about how it was rubbed, perhaps simply by the very dry hand of the experimenter as recommended by several authors.

Having tried the experiment, I can attest to the importance of choosing the glass tube. A simple test tube will not work, much less the glass rod of an agitator (although this is how, since the 19th century, the experiment is described in the physics textbooks). Their diameters are insufficient. I have personally had success with the 50cm long neck of a pyrex glass flask extracted from chemical equipment. Dried well and rubbed using the first bag of "plastic" recovered, it gives spectacular results. Finding a gold leaf is not too difficult if you know a marble worker or a bookbinder. We can simply use a down or a few cotton fibers. For my part, I would recommend the plumes of a thistle picked dry at the end of the summer.

This experience shows that electrical repulsion is much more spectacular than attraction. The piece of gold leaf, the down or the thistle plume, which you will have released, will rush on the rubbed tube to be violently pushed back to thirty, forty, fifty centimeters, or even more. No one can be insensitive to the strangeness of such a "levitation".

Dufay gives these facts an immediate interpretation: "when we drop the sheet on the tube, it strongly attracts this sheet which is not electric, but as soon as it touched the tube, or that it has only approached, it is made electric itself and, consequently, it is repelled from it, and always stays away from it ".

But let's approach the finger or another conductive object of the sheet : it comes to stick on it to fall again on the tube and rise again.

Another simple explanation, Dufay tells us: "As soon as the leaf has touched this body, it transmits all its electricity to it, and consequently, being stripped of it, it falls on the tube by which it is attracted, just as it was before it touched it; it acquires a new electric vortex " and is therefore repelled. This explains the strange behavior, sometimes observed, of gold leaves dancing a saraband between the glass tube and a close object.

A simple remark: Dufay speaks of an electric "whirlwind". The theory of "vortices" is borrowed here from Descartes. For this each celestial body is surrounded by a whirlwind of subtle matter. These touching vortices keep the stars at a distance from each other and draw the whole into the clockwork movement that everyone can observe even if the cogs remain invisible. In the same way, the "electric" vortices surrounding two electrified bodies will separate them from each other.

Dufay's law.

Dufay then reviews previous observations and in particular those of Hauksbee concerning cotton threads tied inside a rubbed glass globe and which "extend in the sun from the center to the circumference." All these facts lead him to a first law of repulsion:


"It remains for constant, that the bodies becoming electric by communication, are driven out by those which made them electric".

Using this mechanism of "attraction - contact - repulsion" (A.C.R), Dufay elegantly explains a host of observations. However, the phenomenon needs to be explored further. In particular, the following question must be answered:


Will two bodies charged with electricity from two different sources also repel each other?


In seeking to verify this, Dufay makes electricity take a new leap forward: "this examination", he says, "has led me to another truth that I would never have suspected, and of which I believe no one 'still had a clue ".


The moment is important enough that we let him speak:

"Having lifted a gold leaf in the air by means of the (glass) tube, I brought a piece of copal gum (exotic tree resin of the legume family) rubbed and made electric, the leaf was applied to it on the spot, and remained there, I admit that I expected a completely opposite effect, because according to my reasoning, the copal which was electric had to push back the sheet which was also; I repeated the experiment several times, believing that I did not present to the leaf the place which had been rubbed, and that thus it only went there as it would have done to my finger, or to any other body, but having taken my precautions on this, to leave me no doubt, I was convinced that the copal attracted the gold leaf, although it was repelled by the tube: the same thing happened when the gold leaf approached of a piece of amber or Spanish wax (vegetable wax extracted from certain species of palm trees) rubbed.

Will two bodies charged with electricity from two different sources also repel each other?


In seeking to verify this, Dufay made a new leap into theelectricity science: "this examination", he said, "led me to another truth that I would never have suspected and of which, I believe, no one 'still had no idea'.

After several other attempts which did not satisfy me at all, I presented to the gold leaf repelled by the tube, a rock crystal ball, rubbed and made electric, it pushed back this leaf in the same way, so that I could not doubt that glass and rock crystal do precisely the opposite of copal gum, amber and Spanish wax, so that the leaf repelled by some, because of the electricity it had contracted, was attracted to others: it made me think that there were maybe two different kinds of electricity."


In a first time such a bold hypothesis frightens its author. If two electricities really exist, how have they not yet been pointed out! Many checks must be done. Dufay rubs all the materials at his disposal : we have to accept the facts, the phenomenon is general.


"There are therefore constantly two electricities of a different nature, namely that of transparent and solid bodies such as glass, crystal, etc. and that of bituminous or resinous bodies, such as amber, copal gum, Spanish wax. , etc.


Both repel bodies that have contracted electricity of the same nature as theirs, and instead attract those whose electricity is of a different nature from theirs. "

What more can be said ? The law of electrical attraction and repulsion is entirely in these two sentences. If we look for it in a contemporary textbook we find it practically in the same terms. It remains to name these two different electricities :

"Here then are two well demonstrated electricities, and I cannot dispense with giving them different names to avoid the confusion of terms, or the embarrassment of defining at any moment the one I would like to speak about: I will therefore call one vitreous electricity, and the other resinous electricity, not that I think that only bodies of the nature of glass are endowed with one, and resinous matters with the other, because I already have strong evidence to the contrary, but it is because glass and copal are the two materials which gave me the opportunity to discover these two species of electricity. "

Vitreous electricity, resinous electricity ... these two terms at least have the merit of proposing convenient standards. The end of Dufay's text is the beginning of a classification. In the register of bodies that present resinous electricity we find amber, Spanish wax, copal gum, silk, paper. Vitrous electricity appears on glass and also crystal, wool, feather ... but let Dufay present his finest example :

"Nothing has a more noticeable effect than the hair on the back of a living cat. We know it gets very electric when you run your hand over it; if you then get close a rubbed piece of amber the hair is strongly attracted to it, and we see them rising towards amber in very large quantities; if, on the contrary, we  get close to it a glass tube, the hair is pushed back and lying on the body of the animal ”.

Thus begins the long tradition of cat skins in the laboratories of our high schools.


After the fundamental discoveries by Stephen Gray of conduction and electrification by influence, the discovery of the two species of electricity opens up promising avenues. The conclusion of the dissertation expresses the hope of rapid progress.


"What should we not expect from such a vast field which opens to physics? And how many singular experiences can it not provide us which will perhaps reveal to us new properties of matter?"

When he writes these lines, Dufay is thirty five years old. His untimely death five years later left him little time to trace his path further. Above all, he missed the time to defend a theory that was too bold for most of his contemporaries. Her direct disciple, Abbé Nollet, barely younger than him, is the first to reject her.

Abbé Nollet and Dufay's theory.

In his "Essay on the Electricity of Bodies", he engages in a vigorous critique of the theory of two electricities:

"Question: Are there two kinds of electricity in nature that are essentially different from each other?

Answer: The late M. Dufay seduced by strong appearances and embarrassed by facts that it was hardly possible to relate to the same principle thirty years ago, that is to say in a time when we still did not really know things which have since manifested themselves, M. Dufay, I say, concluded with the affirmation on the question in question. Now, many reasons drawn from experience lead me to strongly oppose the opposite opinion; and I am not the only one of those who have examined and followed electrical phenomena, which abandons the distinction between the two resinous and glazed electricities ".

For his part, he proposes the theory of a single electrical matter which leaves and joins electrified bodies in a simultaneous double movement.

"The electric matter springs from the electrified body in the form of rays which are divergent between them and this is what I call effluent matter; such matter comes, in my opinion, from all sides to the electrified body, that is to say from the atmospheric air is from other surrounding bodies and this is what I call affluent matter; these two currents, which have opposite movements, both take place together. ".


Confused theory and without any real explanatory significance, but Abbé Nollet has become the most famous "electrifying physicist" in European society and his opinions have the force of law. For many years it will be an obstacle, alas effective, to the dissemination of the theory of two electricities.

We will not leave Dufay without a regret. Discoveries of equivalent significance generally do not remain anonymous. Coulomb, Volta, Galvani, Ampère, Laplace ... always live in the electrical vocabulary through a law, sometimes a unit. Who still knows Dufay?

Already in 1893, Henri Becquerel, who had chosen to praise it on the occasion of the centenary of the Museum of Natural History, had to note this oversight:

"Among the statues and busts which adorn our galleries, among the names engraved on our monuments, I searched in vain for the figure or even the name of the man who did the most good and the most honor in the old Jardin des Plantes, the name of Buffon's predecessor. What am I saying, I looked up to his memory, and neither in all the museum, nor in Paris itself, I could find a portrait of Charles -François de Cisternay du Fay, steward of the Jardin Royal des Plantes ".


Is it really too late to perpetuate the memory of this talented physicist?


Nothing prevents us from pointing out in our lessons and in our textbooks that the law of attraction and of electrical repulsion is "Dufay's law".


Dufay forgotten, it will take a long series of contradictory observations and interpretations for the theory of "two electricities" to come back to us. The second link in this chain is, again, Benjamin Franklin.

Benjamin Franklin (1706-1790): a new vocabulary for a unique fluid.


Unlike his predecessor, fame has not forgotten Franklin, the "inventor" of the lightning rod, that we can now get to know better.

In the field of physics he describes himself as an amateur. Born in Boston in 1706, he is self-taught. His father is a modest candle maker and it is with his printer brother that he can indulge his passion for reading. He ran into electricity around the age of forty. He was then in Philadelphia where he participated in the activities of the cultured circles of the city. These received from England an "electrical box" containing a glass tube with an explanatory note on the use that can be made "to carry out", with it, certain electrical experiments. "The author of this mailing is Peter Collinson , Fellow of the Royal Society, the English Academy of Sciences. He is a Quaker merchant of London with trade relations with the American colonies and who aims to encourage Americans in the study of scientific subjects. he did not fail to include an explanatory note with his shipment: a account of the spectacular experiments carried out in Germany by Bose and his successors. This caused vigorous shaking in the "All-Philadelphia" for several months.

Franklin made a more scientific use of this material, which he reported from March 1747 in the form of several letters to his English correspondent Mr. Collinson, member of the Royal Society.


We have already mentioned his proposition that will serve as the basis for all its subsequent interpretations: electricity is a fluid that permeates all bodies. The friction causes a certain amount of it to pass from one body to another.


This new way of perceiving electricity is perfectly illustrated by the second letter he addressed to Pierre Collinson. Three characters are staged there: A, B and C.


A is isolated on a wax cake, he rubs a glass tube which he hands to B himself isolated. B brings his hand to the tube and receives a spark. At this moment, the character C, who remains on the ground, in contact with the earth, extends his fingers towards A and B and receives an electric shock from each. Franklin offers a seductive interpretation:


"We assume that the electric fire is a common element, of which each of the above three persons has an equal portion before the beginning of the operation with the tube: the person A who is on a wax cake, and who is rubbing the tube, gathers the electric fire of his body in the glass, and its communication with the common store (the earth) being intercepted by the wax, his body does not first recover what it lacks; B, which is likewise on the wax, extending the knuckle of his finger near the tube, receives the fire which the glass had picked up from A; and his communication with the common store also being intercepted, he retains the surplus quantity which has been communicated to him. A and B appear electrified to C, which is on the floor; because this one having only the average quantity of electric fire, receives a spark from B, which has more, and it gives some to A which has less ...


From there some new terms introduced themselves among us. We say that B (or any other body under the same circumstances) is positively electrified and A negatively; or rather B is electrified plus and A is electrified minus, and every day in our experiences we electrify bodies plus or minus as we see fit."


For the first time, the notion of positive and negative charges is therefore expressed. However, we understand that Franklin ignores Dufay's interpretation in terms of two kinds of electricity. For him, the electric fluid is unique, a positively charged body carries an additional quantity of it, a negatively charged body has lost some. "Plus" and "minus" are therefore not a new convention to designate two different electricities but have the real meaning of gain and loss.


This model, opposed to that of Dufay, can easily convince. However, it has serious shortcomings. How can we say, for granted, that the man who rubs the glass tube is passing electricity from his body to the tube? Was it harder to imagine this same man pulling electricity from the rubbed tube? Franklin proposes a strange hypothesis: he imagines that the "rubbing thing" loses part of its fluid in favor of the "rubbed thing". But who rubs and who is rubbed in this operation?


We regret, by the way, that Franklin did not first rub sulfur. He would have, for the same reason, attributed a positive charge to it which, as we shall see later, would have simplified the task of the professors of the following centuries.


The publication of these first letters earned him a critical letter on this subject. One of his correspondents pointed out to him the different behavior of sulfur and glass and suggested the existence of two electricities. Franklin maintains his original interpretation. At most, he has to admit that a body can not only gain electricity when rubbed, but also lose it. Persevering in his first intuition, however, he decrees that it is indeed the glass that charges "plus" while the sulfur charges "minus".


A second warning is more severe. No one will be surprised if we say that Franklin's favorite subject has been thunder. He imagines the process as follows: the land is the reserve, the "store" of electricity. As it evaporates to form clouds, water pulls a certain amount of fluid from the terrest globe, which is then returned to it in the form of lightning. However, after the discovery of the lightning rod, Franklin was able to collect and analyze the electricity carried by the clouds. He then notices that they are generally charged "minus". So water would have to give up electricity to the ground and, in the phenomenon of thunder, it would be "the earth hitting the clouds and not the clouds hitting the earth". Ultimately, doubt sets in:


A second warning is more severe. No one will "Amateurs of this branch of physics will not find it bad that I recommend them to repeat with care and as exact observers the experiments which I have reported in this writing and in the preceding ones on positive and negative electricity, and all those of the same kind that they will imagine, in order to be sure if the electricity communicated by the glass globe is really positive ... "


It will take almost a century and a half to answer this question. This answer, alas, will be negative.


This does not prevent the theory of the unique fluid from taking hold. It has, in fact, a highly developed deductive power and will be the source of rapid progress in experimentation. Even today, Franklin's scheme remains the basis of most of our reasoning.

Between Dufay and Franklin: Robert Symmer's silk stockings.


Robert Symmer (1707 - 1763) is Scottish. After a career in finance he devoted himself to science. In 1759 he published in the Philosophical Transactions of the Royal Society of London, the account of experiences which, despite their strange character, earned him lasting fame.




It begins with a trivial observation: sparks erupt in the evening when he takes off his stockings. Many of his friends tell him they made the same observation but, he says, "he has never heard of anyone who has viewed the phenomenon in a philosophical way." It is indeed an idea that does not spring to mind and yet it is what he sets out to do. So he decides to wear two pairs of stacked stockings every day, one in virgin silk and the other in combed wool. Good initiative because then the phenomenon is reinforced and especially the two pairs of stockings, when separated, show a furious tendency to attract each other. He can even measure this attraction by ballasting one of the pairs with marked masses of significant weight.


There comes a day when a death in his family brings him to mourning. He does not give up his experience, however, and puts on a pair of black silk stockings over his usual natural silk stockings. That evening, when it comes time to undress, the effect is extraordinary! Never have stockings been attracted so fiercely!




When the period of mourning comes to an end, and more classic stockings resume their place in the external position on Symmer's leg, the phenomena regain their more moderate course. Here are therefore two materials of choice for an experiment on electrical attractions: natural silk and black silk to which the dye has brought new properties. To describe these observations Symmer first uses Franklin's vocabulary but, unable to decide which of the two socks loses or gains electricity, he refuses an arbitrary choice and turns, after reading Dufay, towards the idea of ​​two different electric fluids:


"It is my opinion, that there are two electric fluids (or emanations of two distinct electric powers) essentially different from each other; that electricity does not consist of effluence and affluence of these fluids, but in the accumulation of one or the other in electrified bodies; or, in other words, it consists in the possession of a large quantity of one or the other power. it is possible to keep a balance in a body, on the other hand if one or the other power dominates, the body is electrified in one or the other way ".


To designate these electricities Symmer keeps the terms "positive" and "negative" which associate a mathematical neutrality with the electric neutrality of matter. Knowing that it is arbitrary, he will also keep the Franklin convention and call the electricity which appears in excess on rubbed glass positive and negative that which accumulates on sulfur. So this is Dufay's theory dressed in Franklin's vocabulary. It is still the model of our "modern" manuals.




Several authors would like an armistice in the quarrel. This is the case of the Swedish T. Bergman who proposed in 1765, shortly after Symmer's death, a "compound neutral fluid". Made up of equal amounts of negative fluid and positive fluid, it does not manifest in the normal state of equilibrium. Certain operations, such as friction, break it down into two opposite fluids. This theory will make followers after the discovery of the electric battery.


Dufay, despite the rigor of his method, was quickly forgotten. On the other hand, we still find the name of Symmer in textbooks from the beginning of the 20th century.




The XIXth century thus saw two different models coexist, that of the single fluid rather taught in England and that of the two fluids mainly used in continental Europe. The reasons for choosing one or the other are often more philosophical than practical. An attitude that Charles-Augustin Coulomb (1736-1806) illustrates quite well, when he had just established the mathematical law of attraction and repulsion from a distance in 1788.




To understand this difficulty in choosing, it must be admitted that, of course, the single fluid model offers serious advantages but that it also raises several difficulties that it would be too easy to ignore. Among them, that of the repulsion between two negatively charged bodies.


The repulsion between two bodies carrying plus electricity does not pose a problem for Franklin and his followers: this additional electricity forms, they believe, an "atmosphere" which surrounds each charged body. These atmospheres, by their simple elastic mechanical action, explain in a simple way the repulsion between two positively charged bodies.




The problem is different with two bodies having "lost" electricity. No atmosphere surrounds them. So where does the repulsion come from? This phenomenon, which they fail to explain satisfactorily, will be the source of permanent torment for Franklin and his followers.


One of them, Franz Aepinus (1724-1802), professor in Berlin and then in St. Petersburg, abandons the hypothesis of electric "atmospheres" and adopts a "Newtonian" view of electric action. This would be done remotely, without any mechanical support.




The "ordinary" matter would have the power to attract the electric fluid until it "gorges" itself like a sponge and thus acquire a state of electric neutrality. On the other hand, particles of electrical matter repel each other. Two bodies loaded with excess electricity must therefore repel each other.




But why would two bodies having lost electricity repel each other? Quite simply because ordinary matter, deprived of electricity, itself has the property of repulsion. Thus repulsion would appear between two bodies charged with too much electricity but also between two bodies that have lost electric fluid.


"Mr. Aepinius assumed in the theory of electricity, that there was only one electric fluid whose parts repelled each other and were attracted to body parts with the same force as they repelled each other. ... It is easy to feel that the supposition of M. Aepinius gives, as to the calculations, the same results as that of the two fluids ... I prefer that of the two fluids which has already been proposed by several physicists, because it seems contradictory to me to admit at the same time in the parts of the bodies an attractive force in inverse ratio to the square of the distances demonstrated by universal gravity and a repulsive force in the same inverse ratio of the square of the distances ". (Of the two natures of electricity - History of the Royal Academy of Sciences - year 1788, page 671).


It remains true, however, that the choice does not arise when studying static electricity. Does the problem arise differently when we consider the circulation of this, or these, fluid (s), that is to say when we are interested in the electric "current"?




The question will be asked very quickly and we will allow ourselves to travel the time that will take us from Dufay to J.J. Thomson, via Ampère and Maxwell, to discover the different answers that will be given to him.




But this is another story.


A development of this article can be found in a book published in September 2009 by Vuibert: "A history of electricity, from amber to electron"



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