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20 octobre 2020 2 20 /10 /octobre /2020 12:54

Two kinds of electricity or just one? We have seen that until the end of the 19th century two systems coexisted.


That initiated by Dufay of the two types of electricity: vitrious or positive, resinous or negative.


Franklin's one : a single kind of electricity charging bodies more or less.


It is true that there is no obvious choice 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.



From charges to electric currents.




The concept of electric current is already germinating in Franklin's letters to his correspondents. By defining electricity as a fluid which can accumulate on a body or be extracted from it, by designating by the term "conductor" the bodies capable of transmitting this fluid, we necessarily introduce the idea of ​​a flow. The word "current" is also used by Franklin to describe the "vapors" which escape from the conductors and ME Kinnersley, one of his correspondents, who has already had the opportunity to point out to him the different effects of glass and sulfur, offers him a first assembly suitable for circulating this fluid:


"If a glass globe is placed at one end of the conductor, and a sulfur globe at the other, the two globes also being in good condition, and in equal movement, no spark can be drawn from the conductor , because one of the globes attracts (the electric fluid) of the conductor as fast as the other provides it! "


The same Kinnersley observes the calorific effect of electric current. he connects with an archal wire (another name for brass, an alloy of zinc and copper), the two armatures of a battery of Leyden bottles (we will speak soon about these first electric capacitors): "the archal wire was heated to red ". The interpretation of the phenomenon is very "modern":


"It may be inferred from this that, although electric fire has no sensible heat when it is in a state of rest, it can by its violent movement and by the resistance which it experiences, produce heat in other bodies, passing through them provided they are small enough. A large quantity would pass through the coarse wire of archal without producing sensible heat therein, while the same quantity passing through a small one, being restricted to one narrower passage, and its particles closer to each other, and experiencing greater resistance, it will heat this little archal thread until it reddens and even melts it. "



As for wondering about the direction of circulation of this current of electric fluid, the question is never asked by the partisans of the single fluid as the answer is obvious: it necessarily circulates through the conductor of the body which contains it "in more "to the one that contains" less ".



The same point of view is expressed by the Frenchman Jean-Baptiste Le Roy (1720 - 1800) who prefers to speak of electricity "by condensation" and electricity "by rarefaction". He describes his electric machine as an "electric pump" which pushes it away from its positive pole (the rubbed glass plate) and pulls it towards its negative pole (the leather cushions responsible for the friction). Fluid circulation is clearly described:



"If the fluid is rarefied on one side and condensed on the other, there must be a current flowing from the body where it is condensed to where it becomes rarefied."



For the proponents of the theory of the single fluid, the definition of the direction of circulation of the electric current therefore owes nothing neither to chance nor to any convention. It is imposed by the model chosen: it goes from “more” to “less”.



The machines of Jean-Baptiste Le Roy are an attempt on the path of electric generators, it will however be necessary to wait for the beginning of the XIXth century and the construction of the first electric battery by Volta for the study of electric currents and their effects to replace those of the static phenomena. To follow this story to its provisional conclusion, let's begin our excursion to periods closer to our present.



We will not detail here the observation published in 1791 by Luigi Galvani which was to lead Volta to the discovery of the electric generator. We will come back to that. Let's just say, for the moment, that by assembling alternating copper and zinc washers separated by cardboard washers impregnated with an acid solution, Volta realizes a generator capable of circulating an electric current in an external conductor (wire metallic or conductive solution).




This current is, for Volta, made up of a unique fluid such as that described by Franklin. A fluid that circulates, outside the "battery", from its positive pole to its negative pole. But proponents of both fluids do not disarm: the battery produces positive fluid at one pole and negative fluid at the other, they say. Two currents in the opposite direction, one of positive fluid, the other of negative fluid, therefore circulate in the conductor which connects the two poles.


It is first of all the chemists who happily seize the voltaic pile and they do not settle the dispute. Extraordinary phenomena emerge at the level of the electrodes connected to the poles of the generator and immersed in the multiple conductive solutions tested. The nature and direction of circulation of the electric fluid are not their main concern. They are already sufficiently busy studying the properties of the multitude of new bodies into which electrolysis has just introduced them.


It was not until 1820 that Oersted brought back the interest of physicists in the currents passing through metallic conductors, highlighting their magnetic and mechanical effects.




Oersted: the electric courant and the compass.




Despite the opposition established by Gilbert, the hypothesis of the common nature of electricity and magnetism has not been totally abandoned. The magnetization of iron rods under the action of lightning is already noted in the works of Franklin, as is the movement of a magnetized needle during the discharge of a Leyden jar. Unfortunately, this research was doomed to failure until its authors had a continuous source of electricity.




Hans Christian Oersted (1777-1851), professor of physics at the University of Copenhagen, is the lucky one. Busy during the winter of 1819, showing his students the calorific effect of the Volta battery, he observed the movement of a magnetic needle located near the conductor through which the electric current passes. Careful study shows him that the effect is greatest when the wire is placed parallel to the magnetic needle. This one then tends to a position of equilibrium perpendicular to the wire. The direction of this movement depends on the order in which the poles of the battery were connected to the conductor.


Expérience de Oesrsted.

Voir la vidéo sur le site Ampère/CNRS


We will come back to this experience, the birth certificate of electromagnetism. For the moment let us be satisfied with seeing how it intervenes in the definition "of" the direction of the electric current.


Interpreting this experience we would say, today, that the direction of the needle deflection depends on the direction of the electric current. Oersted is a follower of the two-fluid model. The currents of positive fluid and negative fluid, he thinks, move in opposite directions along the conductor. Heir to Cartesian theories, he describes them in the form of two "vortices": "negative electrical matter describes a right spiral and acts on the north pole" while "positive electrical matter has a movement in the opposite direction and has the property to act on the South Pole ". When we reverse the poles of the battery to which the conductive wire is connected, we reverse the direction of each of the currents and therefore of their effect on the compass.


Oersted easily manages to fit his interpretation into his theoretical framework. The theory of two fluids resists!


Ampere: the conventional sense of electric courant.


We know that as soon as the observations made by Oersted were announced in France, Ampère (1775-1836) began the series of experiments which would lead him to the development of the theory of "electromagnetism". Everyone knows the famous "guy" placed on the conductive wire so that the electric current enters him by the feet. You would think that with Ampere the single current got the better of it. Mistake ! Ampère is a firm supporter of both fluids. He recalls this in his "Exposé des Nouvelles Découvertes sur l'Electricité et le Magnétisme" published in Paris in 1822:


"We admit, in accordance with the doctrine adopted in France and by many foreign physicists, the existence of two electric fluids, capable of neutralizing each other, and whose combination, in determined proportions, constitutes the state naturalness of bodies. This theory provides a simple explanation of all the facts and, subjected to the decisive test of calculation, it gives results which agree with experience ".


On the other hand, he rejects the terms vitreous and resinous electricity, he prefers those of positive and negative on the condition that these terms retain only the meaning of a convention:


"When we admit the existence of two fluids, we should have said: they present with respect to each other the opposite properties of the positive and negative quantities of geometry; the choice is arbitrary, as we choose arbitrarily the side of the axis of a curve where its abscissas are positive; but then those of the other side must necessarily be considered negative; and the choice once made we don't have to change it anymore ".



Logically, the electric generator produces these two types of electricity:



"In the volta electric generator, each electricity manifests at one end of the device, positive electricity at the zinc end, and negative electricity at the copper end." (Ampère respects here the polarities proposed by Volta and which we will see were wrong).


The conclusion is natural:


"Two currents are always established when the two ends of the stack are made to communicate."



The current of positive electricity goes from the positive blade and that of negative electricity from the negative blade. As the magnetic phenomena are reversed when we change the direction of these two currents it is necessary, however, to clearly identify these directions. This is the opportunity for Ampère to come up with a convenient convention:



"It suffices to designate the direction of the transport of one of the electric principles, to indicate, at the same time, the direction of the transport of the other; this is why, henceforth, by using the expression of electric current to designate the direction in which the two electricities move, we will apply this expression to positive electricity, implying that negative electricity moves in the opposite direction ".


So here is finally this famous "conventional sense". In reality, what he describes is not the direction of the current but that of the two currents. By choosing to call the positive fluid flow “sense of current”, Ampère was able to find a common vocabulary for the “English” and “French” hypotheses. Thus, the famous "Ampere man" can be used as a tool for both models:


"To define the direction of the current with respect to the needle, let us design an observer placed in the current so that the direction from his feet to his head is that of the current, and his face is turned towards the needle. ; it can be seen that, in all the experiments reported above, the south pole of the magnetic needle is brought to the left of the observer thus placed ".


The "Ampere Watcher" receives positive fluid through the feet, but also receives negative fluid through the head.


"Bonhomme d’Ampère" nageant dans le courant
(Louis Figuier, Les Merveilles de la Science)


voir aussi :


Au sujet du sens du courant électrique, du bonhomme d’Ampère et du tire-bouchon de Maxwell.


With Ampère, it is the theory of the two currents which is essential in France and in most of the countries of Europe, it is still classic in the textbooks of the beginning of the XXth century and requires from the teachers real pedagogical prowess. It is indeed not easy to explain how the two fluids can cross without neutralizing each other.


Franklin's Return.




England has generally remained loyal to Franklin and the One Fluid. Maxwell (1831-1879), for example, wished great caution vis-à-vis the very notion of electric fluid:


"As long as we do not know whether electricity positive or negative, or whether electricity itself is a substance, until we know whether the speed of electric current is several million leagues per second or a hundredth of an inch per hour, or even if the electric current goes from positive to negative or in the opposite direction we should avoid talking about electric fluid ". (Maxwell, elementary treatise of electricity - Paris - Gautier Villars - 1884).


Despite this caution, it is necessary to choose one of the models to interpret the electromagnetic phenomena, it is then the single fluid and the Franklin model that he preferred:


"If there is a substance penetrating all bodies, the movement of which constitutes the electric current, the excess of this substance in a body, beyond a certain normal proportion, constitutes the observed charge of that body."


No ambiguity with the model of the "screw" (or the "corkscrew", as the French prefer it) proposed by Maxwell to describe Oersted's experiment: it advances, along the wire, in the direction of the one current :


"Suppose that a straight screw advances in the direction of the current, while rotating as through a solid body, ie clockwise, the North Pole of the magnet will always tend to rotate around the current in the direction of rotation of the screw, and the south pole in the opposite direction ".


We can finish this brief history with J.-J. Thomson (1856-1940). In 1897, he too recognized that nothing, so far, had been able to separate the "dualistic theory" of electricity from the "unitary theory":


"The fluids were mathematical fictions, intended only to provide a spatial support for the attractions and repulsions which manifest themselves between electrified bodies ... As long as we confine ourselves to questions which only involve the law of forces manifesting between electrified bodies the two theories must give the same result, and there is nothing that can allow us to choose between the two ... Only when we wear our investigations on phenomena involving the physical properties of the fluid, that we are allowed to hope to be able to make a choice between the two rival theories ". (JJ.Thomson. Electricité et Matière. Paris: Gautier Villars - translation-1922)


Thomson, at this period of his life, studies the "radiation" which passes through a tube emptied of its air and whose "cathode ray" tubes fitted, not so long ago, the screens of our television receivers and computers.


The moment he discovers the "corpuscle of electricity" which will later be called "electron" in this radiance, he thinks his national colors are going to triumph . Seeing that cathode rays are made up of "grains" of negative electricity with a mass more than a thousand times less than that of the smallest atom, that of hydrogen, he cannot doubt that he has secured the victory of his camp. Recalling that Franklin soon considered "Electrical matter to be composed of extremely subtle particles," he writes:




"These results lead us to a conception of electricity that bears a striking resemblance to Franklin's 'unitary theory'.


The triumph, however, is not total:


"Instead of considering, as this author did, the electric fluid as being positive electricity, we consider it as negative electricity ... A positively charged body is a body which has lost part of its corpuscles ".




There remains, in fact, this poor initial choice: the rubbed glass does not take a charge of electricity, it loses it!




Situation blocked.




Here we are when the situation freezes. For a century and a half Franklin's conventions have permeated electrical science, Ampère has entrenched this imprint by establishing a conventional direction of current flow. The discovery of electrons, then protons, imposes a new interpretation of electrical conduction. Both positive and negative charges do exist, and it is true that in electrolysis two oppositely charged currents cross in the electrolyte solution.


In metallic conductors, on the other hand, only negative charges are mobile. The positive fluid remains immobilized in the fixed nuclei of atoms. The electric current should now be considered, in a metallic circuit, as a current of electrons moving from the negative pole of the generator to its positive pole.


Is this discovery a sufficient event to cause a revolution in electrical conventions? It should be noted that we will put up with these electrons which move in the opposite direction of the "conventional" direction. This move is not spectacular. We can now answer Maxwell's question. The speed of the current of electrons in a direct current is not several million leagues per second and if it is still greater than a hundredth of an inch per hour, it does not exceed a few centimeters per hour. . This result speaks little to the imagination. This slow current of electrons does not match the observed power of electrical phenomena. This is perhaps why we prefer to continue to reason on the mythical current of the first times of electricity which rushed from the positive pole where it was concentrated towards the negative pole where it had been rarefied.




There remains a certain astonishment and sometimes irritation when one presents this contradiction in electrical science to the beginner. What? Over a century has passed and the mistake is still not corrected?


In a way, this "mistake" is beneficial: it breaks linear discourse, it forces us to question and forces us to return to the history of science. At least apprentice electricians will remember that scientific activity is a human activity, a living activity, and that we sometimes encounter in it the scars of past mistakes.


On peut trouver un développement de cet article dans ouvrage paru en septembre 2009 chez Vuibert : "Une histoire de l’électricité, de l’ambre à l’électron"



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