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8 janvier 2019 2 08 /01 /janvier /2019 20:05

translated from : histoire de l'électricité, de l'ambre à l'électron.

 

 

Two kinds of electricity or one? We saw that until the end of the 19th century two systems coexisted.
 
The one initiated by Dufay of the two kinds of electricity: vitrous or positive, resinous or negative.
 
Franklin's: a single species of electricity charging the bodies more or less.
 
It is true that the choice is not necessary when one studies electricity in the static state.
 
Does the problem arise differently when one considers the circulation of this or these fluid (s), ie when one is 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, through Ampère and Maxwell, to discover the different answers that will be provided.
 

From charges to electrical currents.
 
The concept of electric current is already in germ in Franklin's letters to his correspondents. By defining electricity as a fluid that can accumulate on a body or be extracted from it, by designating by the term "conductor" the bodies capable of transmitting this fluid, the idea of a flow is necessarily introduced. The word "current" is also used by Franklin to describe the "effluve" that escapes from drivers. M.E Kinnersley, one of his correspondents, who has already had the opportunity to report the differents effects of  glass and sulfur, offers him a first fitting to  do this fluid circulating :

"If a globe of glass is placed at one end of the conductor, and a globe of sulfur to the other, the two globes being in good condition, and in an equal movement, we can not shoot any spark from the driver 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 the electric current. He connects by an archal wire (another name for brass, zinc alloy and copper), the two extremities of a battery of Leiden jars (we will soon talk about these first electric capacitors): "the archal wire was heated to red ". The interpretation of the phenomenon is very "modern":

 

"It can be inferred from this that, although the electric fire has no sensible heat when in a state of rest, it can by its violent movement and by the resistance which it experiences, produce heat in other bodies, when passing through them. A large quantity would pass through a big archal wire without producing any sensible heat, while the same quantity passing through a small wire, being restricted by a narrower passage, and its particles being tighter on each other, and experiencing greater resistance, it will warm up this little archal wire  until  being red and even it could melt."

 

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

The same point of view is expressed by the French 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 from its positive pole (the rubbed glass tray) and draws it to its negative pole (the leather cushions responsible for friction). The circulation of the fluid is clearly described:

 

"If the fluid is rarefied on one side and condensed on the other, it must form a stream from the body where it is condensed towards the one where it is rarefied".
 
For the proponents of the theory of the single fluid, the definition of the direction of circulation of the electric current owes nothing neither to chance nor to any convention. It is imposed by the chosen model: it is from "more" to "less".
 
The machines of Jean-Baptiste Le Roy are an attempt on the way of the electric generators, it will however be necessary to await the beginning of the XIXth century and the construction of the first electric battery by Volta so that the study of the electric currents and their effects became more important that static phenomenas. To follow this story to its tentative conclusion, let's begin our excursion to closer periods of our present.

 

From Volta's pile to Ampère's Bonhomme.

 

We will not detail here the observation published in 1791 by Luigi Galvani and which was to bring Volta to the discovery of the electric battery. We will come back to it. Let's just say, for the moment, that by assembling copper and zinc washers alternated and separated by cardboard washers impregnated with an acid solution, Volta realizes a generator capable of circulating an electric current in an outer conductor (metallic wire  or conductive solution).
 
This current is, for Volta, constituted of a unique fluid such as that described by Franklin. A fluid that flows, outside the "pile", from its positive pole to its negative pole. But the partisans of the two fluids do not disarm: the battery produces positive fluid at one of its poles 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 the chemists who seize the voltaic pile, and they do not take care of the quarrel. Extraordinary phenomena are emerging at the level of the electrodes connected to the poles of the cell when immersed in the multiple conductive solutions tested. The nature and the direction of circulation of the electric fluid are not their first concern. They are already sufficiently occupied by the study of the properties of the multitude of new bodies that electrolysis has just made them discover.
 

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

 

Oersted: the pile 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 rods of iron under the action of lightning is already reported in the works of Franklin as well as the movement of a magnetized needle on the occasion of the discharge of a bottle of Leiden. 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 one to whom luck will smile. Busy during the winter of 1819, showing his students the heat effect of the Volta pile, he observed the movement of a magnetic needle near the conductor through which the electric current flowed. A careful study shows him that the effect is maximum when the conductor wire is placed parallel to the magnetic needle. This 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 stack have been connected to the conductor.

 

We will come back to this experience, birth date of electromagnetism. For the moment let us see how it intervenes in the definition of "the" sense of electric current.
 
Interpreting this experiment we would say today that the direction of the deviation of the needle depends on the direction of the electric current. Oersted is adept of the model of the two fluids. The positive fluid and negative fluid currents, he thinks, move in opposite directions along the conductor. Heir to Cartesian theories, he describes them in the form of two "whirlwinds": The "negative electric matter describes a spiral on the right and acts on the North Pole" while "the positive electric matter has a movement in the opposite direction and has the property of generator on the South Pole ". When we reverse the poles of the generator to which the conductor is connected, we reverse the direction of each of the currents and therefore their effect on the compass.
 
Oersted easily succeeds in bringing his interpretation into his theoretical framework. The theory of the two fluids resists!

 

Ampere: the conventional sense.

 

We know that from the announcement, in France, of the observations done by Oersted, Ampère (1775-1836) began the series of experiments that will lead him to the development of the theory of "electromagnetism". Everyone knows the famous " ampère's man" placed on the wire so that the electric current enters through his feet. One would think that with Ampère the single current finally prevailed. Fault ! Ampère is a firm supporter of both fluids. He recalls it in his "Exposé des Nouvelles Découvertes on Electricity and Magnetism" published in Paris in 1822:
 
"We admit, according to 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 definite proportions, constitutes the natural state of matter. This theory provides a simple explanation of all the facts and, subject to the decisive test of calculation, gives results which are in accord with experience. "

 

On the other hand, he rejects the terms vitrious and resinous electricity, he prefers those of positive and negative, provided that these terms retain only the meaning of a convention:
 
"When we admited the existence of the two fluids, we should have said: they have the opposite properties of the positive and negative magnitudes of geometry with respect to each other: the choice is arbitrary, as we choose arbitrarily the side of the axis of a curve where its abscissae are positive, but then those on the other side must necessarily be considered as negative, and the choice once made, as it is with to the two electric current senses, we must not change it anymore".

 

Logically, the battery produces these two types of electricity:
 
"In the isolated pile, each electricity is manifested at one end of the apparatus, the positive electricity at the zinc end, and the negative electricity at the copper end." (Ampere respects here the polarities proposed by Volta and of which we will see that they were erroneous).
 
The conclusion is natural:
 
"Two currents are always established when the two ends of the pile are put to communicate."
 
The positive current of electricity starts from the positive pole and the negative electricity from the negative pole. As the magnetic phenomena are reversed when we change the sense of these two currents it is necessary, however, to identify these senses. This is the opportunity for Ampère to propose a convenient convention:

 

 
"It is sufficient 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, which is why, by employing from now on the expression "sense of the electric current" to designate the the direction in which the two electricities move, we will apply this expression to the positive electricity, implying that the negative electricity moves in the opposite direction ".
 

 

Here is finally this famous "conventional sense". In reality, what he describes is not the meaning of the current but that of currents. In choosing to call "the direction of the current" that of the circulation of the positive fluid, Ampère  found a vocabulary common to the "English" and "French" hypotheses. From then on, the famous "Ampère man" can serve as a tool for both models:

 

"To define the direction of the current relative to the needle, let us conceive of 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 : the austral pole of the magnetised needle is brought to the left of the observer so placed ".
 
The Ampere observer does receive the positive fluid from the feet but also receives the negative fluid through the head.

 

With the Ampère choice, it is the theory of the two currents that prevails in France and in most European countries, it is still classic in textbooks of the early twentieth century and requires teachers real educational prowess. It is indeed not convenient to expose how the two fluids can cross without neutralization.


 
The comeback of Franklin.

 

England has generally remained faithful to Franklin and to the unique fluid. Maxwell (1831-1879), for example, wants great caution with regard to the very notion of electric fluid:
 
"As long as we do not know whether positive or negative electricity, or if electricity itself is a substance, until we know whether the speed of electric current is several millions of leagues per second, or one hundredth of an inch. on time, or even if the electric current runs from positive to negative or in the opposite direction, we will have to avoid talking about electric fluid ". (Maxwell, elementary treatise of electricity - Paris - Gautier Villars - 1884).
 
Despite this caution, we must choose one of the models to interpret the electromagnetic phenomena, it is then the unique fluid and the model of Franklin who will have his preference:

 

"If there is a substance penetrating all the bodies, whose movement constitutes the electric current, the excess of this substance in a body, beyond a certain normal proportion, constitutes the observed charge of this body".
 
No ambiguity with the model of the "screw" (or the "corkscrew", as the French prefer it) proposed by Maxwell to describe the Oersted experiment: it advances, along the wire, in the direction of the current :
 
"Suppose a straight screw moves in the direction of the current, turning, at the same time, through a solid body, ie in the direction of 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 acknowledged that nothing so far has been able to separate the "dualist theory" of electricity from the "unitary theory":
 
"The fluids were mathematical fictions, intended only to provide a spatial support for the attractions and repulsions that occur between electrified bodies ... As long as we limit ourselves to questions that involve only the forces laws manifesting itself between electrified bodies and the simultaneous production of equal amounts of positive and negative electricity, the two theories must give the same result, and there is nothing that allows us to choose between the two ... Only when we wear our investigations on phenomena involving the physical properties of the fluid, which we are allowed to hope to make a choice between the two rival theories.

 

Thomson, at this period of his life, studies the "radiation" that crosses a tube emptied of its air and whose "cathodic" tubes equipped, not so long ago, the screens of receivers of television and computers .

 

At the moment when, in this radiation, he discovers the "corpuscle of electricity" that will later be called "electron", he thinks he can, in a certain way, observe the triumph of his national colors. Noting that the cathode rays are made up of "grains" of negative electricity of mass more than a thousand times smaller than that of the smallest atom, that of hydrogen, he can not doubt to have assured the victory of his camp. Remembering that Franklin considered that "the electric matter is 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 that has lost some of its corpuscles ".
 
It remains, indeed, this bad initial choice: the rubbed glass does not take electricity, it loses some!

 

 

Situation blocked.
 
Here we are at the moment the situation freezes. For a century and a half Franklin's conventions have permeated electrical science, Ampère has embedded this footprint by setting a conventional sense of current flow. The discovery of electrons, then protons, imposes a new interpretation of electrical conduction. 

Both positive and negative charges exist and it is true that in electrolysis two opposite charge currents cross each other in the electrolyte solution.
 
In metal conductors, on the other hand, only negative charges are mobile. The positive fluid remains immobilized in the fixed nuclei of the atoms. The electric current must now be considered, in a metal 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 provoke a revolution in electrical conventions? It must  note that we will accommodate with these electrons that move in the opposite direction of the "conventional" sense. This move is not spectacular. We can now answer Maxwell's question. The speed of the current of electrons in a continuous current is not several millions of leagues per second and if it is nevertheless greater than one hundredth of an inch per hour, it does not exceed a few centimeters an hour . This result speaks little to the imagination. This slow current of electrons goes badly with the observed power of electrical phenomena. This is perhaps why we prefer to continue reasoning about the mythical current of the early times of the electricity that rushes 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 we present to the beginner this contradiction in electrical science. What? More than a century has passed and the mistake is still not repaired?
 
In a certain way this "error" is beneficial: it breaks the linear discourse, it forces the interrogation and forces a return on the history of science. At least apprentice electricians will remember that scientific activity is a human activity, a living activity, and that sometimes there are scars of past mistakes.

 


 

 

 

 

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