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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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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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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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19 septembre 2018 3 19 /09 /septembre /2018 15:08

In 1731, in the "Philosophical Transactions", the publication of the "Royal Society", appeared a text that was to make a giant leap forward for the young electrical science. Its author, Stephen Gray, is not a character in sight. Considered an "amateur", he had to suffer the contempt of scientists in place. He will rise, however, at the level of his compatriot Gilbert in the esteem of European "electricians".

 

Stephen Gray ( 1670-1736).

 

Stephen Gray is the son of a Canterbury dyer and is a dyer himself. He made serious studies that led him to focus more specifically on astronomy. As such, he is invited to participate in the work of the Royal Astronomer John Flamsteed at Greenwich, the author of the first modern catalog of the celestial world giving the exact position of nearly 3000 stars. In 1707 he was again called to Cambridge, also for astronomical work.

 

This experience is disappointing. His relations with academics are difficult. He notes with bitterness that his communications are refused for publication, which does not prevent them from being regularly looted. He returned to his Canterbury business in 1708. Too tired to continue his business, he applied for admission to a retirement home known as "the Charterhouse". This institution, located in a former convent of Chartreux, was created to be both a day school for poor children and a pension for the elderly. His boarders were usually distinguished men with serious references. Gray had to wait eight years before being admitted, in 1719, on the recommendation of the Prince of Wales.

 

Freed from his financial worries, he intended to occupy this retreat to cultivate his interest in the various branches of science. He had, in particular, provided himself with various glass tubes and small equipment useful for electric demonstrations.

 

Already in, 1708, he had sent a memoir to the Royal Society concerning "new experiments on light and electricity". He was amazed at how easily he could reproduce Guericke's experiments using a simple glass tube. The "expulsive" virtue, in particular, manifested itself spectaculary. An  feather close of the tube was first attrected and then pushed back. It could stay a long time "hovering" above the tube and even go up and down at the rate of friction.

 

It seemed to him, however, that "expulsive" virtue, far from being a new property of sulfur or the earth, as Guericke had estimated, was, more simply, as well as attraction, a property of electric virtue.

 

Another observation merited attention : if the feather, once pushed back, reached a body outside the tube, it was attracted by this body . It then fell back on the tube to be repelled again. The carousel could last from 10 to 15 round trips before stopping. These observations led Gray to suppose that the pen, placed near the rubbed tube, must itself acquire an electric virtue.

 

Such facts should have attracted the attention of his contemporaries, but Hauksbee, to whom he addresses his memoir, does not consider it useful to publish it. Fortunately, they will continue to obsess Gray and allow him a brilliant revenge.

 

Late and fabulous discoveries.


 
In February 1729, having already been at Charterhouse for 10 years, he began experimenting with the electrification of metals. Having found that it was impossible to electrify them by friction, he proposes to achieve this by placing them, as he has already done with a feather, in the "electric vapour" surrounding a glass tube rubbed.

See : 

IV. A letter from Mr. Stephen Gray to Dr. Mortimer, Secr. R. S. Containing a farther account of his experiments concerning electricityPhil. Trans. 1731

I. Two letters from Mr. Stephen Gray, F. R. S. to C. Mortimer, M. D. Secr. R. S. con­taining farther accounts of his experiments concerning electricityPhil. Trans. 1731

 

Before starting, he decides to test his tube. The latter, which he describes with precision, is a lead glass tube three feet five inches (1 meter) long and one inch and 1/5 (3 centimeters) in diameter. This tube is closed at each end by a cork, so that dust does not enter. Gray has, indeed, noticed that this one harms the effectiveness of the tube.

 

The caps are usually removed when the tube is used. Yet this time, Gray wants to test the effectiveness of the clogged tube. He rubs the extremity of a tube clogged by its plugs and finds that it works just as well.
 
Suddenly, chance gives him a fabulous gift.
 
Gray says:

"As I held a down feather over the upper end of the tube, I saw that it wanted to go to the cork, and that it was attracted and repulsed by him, just as by the tube, when it had been excited by friction. I therefore held the down near the flat surface of the cork, which attracted and repulsed it several times in a row, to my great surprise, whence I concluded that the excited tube had certainly communicated to the cork an attraction virtue."

 

The following experiences have a "surrealist" side:


 
"Having on me a ball of ivory, about an inch and a third in diameter, pierced from side to side, I fastened it on a piece of fir wood, about four inches long, and I made to enter the other end of the piece of wood into one of the corks. Rubbing the tube, I saw that the ball attracted and repulsed the feather with more force than the cork had done; attractions and repulsions repeating themselves a very large number of times right away. "

 

Stems of wood of 8, then 24 inches, driven into the cork, are tried with the same success. How far can we reach? After several tries, Gray makes a combination of reeds and fir rods totaling more than 18 feet long, which corresponds to the length of his room. The result is convincing, the attraction is as strong as that obtained with shorter stems.

 

Then comes the turn of a hemp rope three feet in length. Attached to the tube, it is ballasted by the ivory ball that attracts the copper sheets with just as much ease.

 

A rope is a convenient fastener. It will soon be ballasted by a ball of lead, a piece of gold, a piece of tin, a shovel, a silver vase, a kettle of copper sometimes empty and sometimes full of water, hot or cold. All these metal bodies attract the copper sheets to the height of several inches when the glass tube is rubbed. Metals, which can not acquire electric "virtue" by simple friction, can therefore receive it from a rubbed glass tube to which they communicate. In the same way pebbles, bricks, a magnet, tiles, chalk, vegetables.

 

Gray knows that a royal road has just opened before him, he engages there enthusiastically. A question naturally comes to his mind: how far can he transmit electrical virtue?


 
A first answer was given to him in May 1729 at his friend John Godfrey's home in Norton-Court, Kent. A stem 32 feet long is made from hollow canes and fir stems, all finished by the usual ivory ball: the electric virtue is transmitted at this distance. A string 26 feet long, hung in the air, from a balcony also works. Similarly, a 34-foot rope suspended from an 18-foot stem, a total of 52 feet.

 

The successes are spectacular, but the first failure occurs!


 
Wanting to transmit the electric virtue horizontally by means of a string, Gray supports it by ropes fixed to the beams of the room where the experiment is practiced. The result is negative.

 

 

 

Gray is not particularly surprised. The fixing ropes, he thinks, transmit an essential part of the electrical virtue to the beams and there is only a tiny part left that can reach the ball. He will have to imagine another device.
 

The opportunity is given to him on July 2, 1729. He is then at his friend Granvil Wheler. In order to stretch the string, silk threads are fixed between the side walls of a long gallery. Why silk? It is the thread that combines the best resistance with the greatest finesse. But Gray, alerted by his first failure, is persuaded "that such a thread, expected its small size, could make the experiment succeed, since it would divert less the electrical virtue of the line of communication" constituted by the string.

 

The hypothesis seams to be true. Electric virtue can thus be worn up to a distance of 147 feet. The gallery becomes too short, one passes in a barn where the distance of 293 feet (nearly 100 meters) is easily reached. At this moment, an incident disrupts this race to the record and brings a new course for the observations.


 
One can easily imagine the agitation that could accompany such an experiment. One of the silk threads does not resist. Very opportunely, Gray is equipped with a brass wire (alloy of copper and zinc) having the required fineness while being more solid. He replaces the defective silk crossbar with this brass wire. But with this system, Gray must observes his failure: " What ever the vivacity of the rubbing  to the cylinder, the ball did not produce any movement, and did not excite any attraction."

 

The obviousness imposed then  on both observers:


 
"We were convinced that we owed the success of our previous experiences to the silk threads, not because they were small, as I had first imagined, but because they were silk"


 
Thus the string and the brass have a behavior different from the silk. With this new data, Gray and Wheler take back their experiences. They know now that silk threads, even of a respectable diameter, will perfectly isolate the string they will bear. After passing from the gallery to the barn, the experimenters go to the garden and reach a distance of 650 feet, more than 200 meters.

 

Engaged in this race for the record, Gray discovers a new effect of the "electric virtue": it can be transmitted without contact! Meticulous, he notes that this revelation was made to him on August 5, 1729. That day he had suspended a lead weight of 14 pounds on a rope of Crin. Under the mass of lead, copper sheets were arranged. He approaches the glass tube and, suddenly:


 
"The pipe having been rubbed and held near the rope, but without touching it, the weight attracted and repelled the leaves several times in succession to the height of three inches, if not four. "

 

From then the experiments take a new course. We can transmit the electric virtue without having to be encumbered with a cork, a stick or a string. The simple approach of the rubbed tube will suffice. The place is left free to the imagination. His most spectacular demonstration will inspire generations of electricians. Let him speak:
 


"On the 8th of April, 1730, I did the following experience of an 8 to 9 year old boy, who weighed all dressed 47 pounds 10 ounces. I hung it horizontally on two ropes of horsehair, (similar to those on which the linen is dried) 13 feet long.


 
These strings suspended from the ceiling, each with two hooks, are presented as two loops close to each other.

 

 

 

"On these two cords the child was laid face down, one of the cords passing under his breast, the other under his thighs. The copper sheets were placed on a small pedestal, round, one foot in diameter, covered with white paper, and supported by a stem one foot high.
 

 

As soon as the tube had been rubbed, and presented to the little boy's feet, but without touching them, his face attracted the copper sheets with great force, until the raise to the height of 8 and sometimes 10 inches. "
 

 

A human can therefore, without damage, receive and transmit the electric virtue!

 

Gray has just inaugurated the experimental staging most often repeated in the "physics salons" European. If we were to keep only an image of the 18th century electricity works, it would be that of a damsel richly dressed and lying on a plateau held in the ceiling by silk cords. A young abbot moves near to his feet a glass tube rubbed while young people present to her, on a silver tray, gold leaves she attracts at a distance.
 

Gray is not short of imagination. He even manages to electrify the soap bubbles Dufay: first ranking. by means of a pipe.


 
After a last experiment "to see how far the electrical virtue could be carried in a straight line, without the tube touching the string", the record is reached. It is 886 feet, almost 300m!

 

 

 

Dufay: first ranking.

 

Gray is enthusiastic but untidy. The account of his experiences, however, holds the attention of Charles-François de Cisternay Dufay (1698-1739), a young French physicist who, at age 35, is already a member of the Paris Academy of Sciences.

 

Using a rigorous method, he first takes up the problem of the electrification of bodies: does the faculty of attraction at a distance exist in all bodies?

 

The question is not new. Gilbert, the first, had approached it. Dufay, of course, takes up the impressive list of bodies already tested by Gray and his predecessors: amber, resins, precious stones, glasses of all kinds, sulfur, wool, silk, feathers, hair. He added bodies as diverse as marble, granite, sandstone, slate, ivory, bone, tortoiseshell, and animal hair.

 

These bodies do not always react to a simple friction. Some have to be heated, sometimes even to burn your fingers. All, however, especially if one has them thoroughly dry, can be electrified by friction.

 

All? Not exactly. There remains a category that resists: that of metals: "whatever pain I have given myself," he says, "and in any way that I took it, I could not succeed to make them electric; I heated them , rubbed, filed, beaten without noticing sensible electricity.

 

It follows from these observations a first conclusion:

 

"With the exception of metals and bodies which their fluidity or their softness makes it impossible to be rubbed, all the others which are in the nature are endowed with a property which has been thought for a long time peculiar to the amber and which, until now, had been recognized only in a small number of subjects. "

 

As Gilbert had already pointed out, electricity is more than a magic virtue confined to amber and precious stones. It is a general property of matter worthy of a systematic study.

 

There are therefore two classes of bodies: Dufay proposes to designate under the name of "electrical bodies", those which, like glass, can be electrified by friction. Those who, like metals, can not be, will constitute the class of "non-electric" bodies.

 


"Electrical" and "non-electric" bodies, what differences?

 

First, the problem of attraction. Are these two types of bodies, the "electrics" and the "non-electrics", different in the way they are attracted?

 

Dufay moves his glass tube rubbed near to amber powder, shellac, crushed glass, wood sawdust, crushed brick, these bodies being "as much as possible, of the same volume and same weight compared to each other ". He finds that bodies "that are not electric by themselves" such as metals, wood or even brick are more strongly attracted than those that are electrics, such as amber, glass, wax.

 

In our current experiments, cotton fragments or pieces of paper will be suitable as they are light and "conductive" (as we now call "non-electric" bodies). The ideal body of the 18th century experimentalists to show attractions and repulsions will be the gold leaf both very conductive, very light and offering a large surface to the electric influence.

 

Franklin: the vocabulary.


 
Before following Dufay on the path of new discoveries, let us pause for a moment on the concept of electric conductor and insulator. If it is clearly analyzed by Dufay, it is necessary to wait for Franklin (1706-1790) so that the vocabulary agrees with the idea.


 
We will then detail Franklin's contributions to electrical science. Suffice it for the moment to know that, from his contact with electricity, in 1747, he creates a real break.


 
Electricity, he says, is not created by friction on "electric bodies". Nor is it a "virtue" proper to these bodies alone. It is a fluid that permeates all bodies and is able to pass from one body to another.


 
This intuition naturally leads him to dress the old categories in a new vocabulary:


 
What is the difference between an electric body and a non-electric body The terms electric by itself and non-electric were first used to distinguish the bodies, in the false assumption that the only bodies called electric by themselves The same contained in their substance the electric matter which could be excited by the movement, that it came from and was drawn from it, and communicated to those who were called non-electric, which was supposed to be devoid of this material. I now suspect that it (the electrical matter) is spread fairly evenly throughout the earth's matter.


 
That being so, the terms "electric by itself" and "non-electric" could be abandoned as improper; and since the whole difference is that some bodies conduct the electric material and the others do not conduct it, we could substitute for them the terms "conductor" and "non-conductor".


 
One can not perfect science without perfecting language, had later asserted Lavoisier in the introduction to his elementary treatise on chemistry (1789). "Whatever may be the facts, no matter what the ideas they might have produced, they would still transmit only false impressions, if we did not have exact expressions to render them," he added.


 
Franklin, who will regularly attend his laboratory during his stay in Paris, will have preceded him in this way. The facts have given birth, in his mind, the idea that electricity is a "fluid" that permeates all bodies. The facts, the idea, require a precise vocabulary: the bodies do not share into "electrics" or "non-electrics", but in "conductors" and "non-conductors" (we say today insulators).


 
Let us stop here on what might seem like a paradox: the first electric conductor known , a string of hemp, is rather considered, today, as an insulator. To understand it, it must be remembered that, if the quantities of electricity used in the electrostatic phenomena are minute, the corresponding voltages are themselves thousands or tens of thousands of volts. Under the effect of such tensions even hemp becomes conductive. Therefore, it is recommended not to play with a kite near a high-voltage line, or to touch an electric cable dropped to the ground by means of a wooden rod. Because in this case the high voltage would be accompanied by high currents and electrocution would be at the rendezvous.


 
The concepts of electrical fluid, conductor and insulator are born. The idea, of course, had also already sprung up in several English authors, but Franklin is the one who will have taken the step with the most boldness. Those who, on the old continent, will know how to adopt his views will only have to congratulate themselves on it.

 

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Repost0
23 août 2018 4 23 /08 /août /2018 07:57

Gérard Borvon

Emanation, fluid, particle, wave ... what is the identity of this elusive but very present thing whose quest dates back to twenty-five centuries and whose reality escapes us as soon as we think we have identified it?

 

In the course of this story - that of a succession of generally discrete phenomena which, under the watchful eye of observers, led to spectacular applications - we will meet dozens of scientists, inventors and researchers whose names we are already familiar: from Ampère to Watt and Thales from Miletus to Pierre and Marie Curie, it is also Volta and Hertz, Ohm and Joule, Franklin and Bell, Galvani and Siemens or Edison and Marconi who, among others, come to populate this adventure.

We will see amber lead to the lightning rod, contractions of a thigh of frog lead to the battery, the action of a current on a compass announce: the phone, the airwaves and electric motors, or the light filling a vacuum tube to produce a cathodic radiation. Of course, X-rays and radioactivity are also part of it.

From happy discoveries to dramatic experiences, electricity remains a natural force that has not ceased to inspire research and raise passions.

_________________________________________________________________

Table of Contents.

When was electricity born?
The Amber.
An attractive material.
The long sleep of the succin.

 

William Gilbert, the first electrician.
The birth of electricity.
Electricity is a general property of matter.

 

The first electric machines.
Otto de Guericke (1602-1686).
Francis Hauksbee (? - 1713).
Tube or globe?
Georg Matthias Bose (1710-1761).
Abbé Nollet (1700-1770).
The tray machines.

 

Gray, Dufay, Franklin and the electrical conduction.
Stephen Gray (1666-1736).
Late and fabulous discoveries.
Dufay: first ranking.
Electrical and non-electric bodies, what difference?
Benjamin Franklin: the vocabulary.

 

From Dufay to Ampere: from the two kinds of electricity to the both directions of electric current.
Dufay (1698-1739) and the electric repulsion.
A speech of the method.
Repulsion joins the attraction.
The law of Dufay.
Benjamin Franklin (1706-1790): a new vocabulary for a unique fluid.
Between Dufay and Franklin: Robert Symmer's silk stockings.
From loads to electrical currents.
From the Volta pile to the Ampère man.
Oersted: the pile and the compass.
Ampere and the conventional current direction.
The return of Franklin.
A situation blocked.

 

History of electricity. From the two kinds of electricity to both directions of electric current.

 

The Leiden bottle: the hidden power of electricity.
Terrible news from Leiden
This first electric capacitor, how does it work?
A miraculous bottle.

 

To the conquest of the celestial fire: the lightning rod.
The long history of thunder.
A thunderclap in the Parisian sky.

 

Coulomb and the time of the measure.
The law of Coulomb

 

From Galvani to Volta: the discovery of the electric battery.
Galvani and the frogs.
Volta and the battery.

 

Electricity and chemistry.
Humphry Davy (1778-1829).
A race for new elements.
 
The other magic stone: the magnet.
Chinese heritage.
Pierre de Maricourt (thirteenth century).
William Gilbert.
Coulomb and the measure.
 
Oersted, Ampere and the birth of electromagnetism,
or when the amber finds the magnet again.

Hans Christian Oersted (1777-1851).
Ampere (1775-1836).
An ingenious montage.
Earth is an electromagnet.
From mobile frame to solenoid.
From the solenoid to the right magnet.

 

Faraday and the fields.

Michael Faraday (1791-1867).
From the engine to the generator.
Lines of force and fields.
The law of Faraday.
Maxwell (1831-1879), putting the fields into equations.


Maxwell and the electromagnetic waves: at the rendezvous of light and electricity.
The luminous ether
Electromagnetic ether and the nature of light.
Establish the equations of propagation of an electromagnetic disturbance.
Build a coherent system of electrical units.


Hertz and the reality of electromagnetic waves.
At the conquest of high tensions: the Ruhmkorff coil.
Towards the discovery of the hertzian waves.
Does the ether exist? The experience of Michelson and Morley.
Branly, Marconi and the beginning of the radio.


The time of the engineers: the International Electricity Exhibition of 1881.
The era of electric generators.
The international exhibition of electricity in Paris.
The electric light.
The new generators.
The driving force of electricity.
After the exhibition of 1881.
The dark side of the electric force.
What future for electricity?
 
Electrical units, or when electricians give birth to a universal language.
The decimal metric system.
Birth of electric units.
Before 1881: different national systems.
1881: first international congress of electricians and first international system.
A success noticed.
The 1881 congress suites: the joule, the watt ...
Mechanics overwhelmed.
To the MKSA system.

A strange light: the cathodic radiation.
William Crookes and the radiant matter.
 
Röntgen and the X-rays.
Röntgen and the discovery
The epic of X-rays
X-rays, the latest fashion.
The other side of the medal
A memorial to the victims of radiation.
 
New radiation: the radioactive radiation.
Henri Becquerel: the discovery of radioactive radiation.
Marie Curie and the first hypotheses.
Polonium.
Radium.

 

Life and death of the electron.
Thomson and the discovery of the electron.
The electron and the atom, from Thomson to Rutherford.
Planck, Einstein and the birth of the photon.
The atom of Bohr.
Louis de Broglie and the wave nature of the electron.
When uncertainty becomes a principle.
And electricity, the electron, the electric charge in all this?
 
History to follow.
No science without his story.
This is just the beginning, the story continues.
 
Bibliography. Index of names; Index of subjects. The dates of the electricity.

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3 juin 2018 7 03 /06 /juin /2018 18:01

The Nature is generous. By endowing sulfur and glass with the property of attraction, it has allowed everyone to seize the electrical phenomenon. The simplest stick of sulfur or the most banal glass tubes already give beautiful effects. But these materials lend themselves especially to the manufacture of "machines" which will complete the "cabinets of curiosities", obligatory attraction of any noble or bourgeois home that respects itself, from the second half of the 17th century.

 

Otto de Guericke (1602-1686)

 

Among the builders, a name emerges, that of Otto de Guericke. He is the descendant of a family of notables from the free city of Magdeburg. His father and grandfather served as mayor, helping to make it a prosperous and populous city. He studied first at the University of Leipzig and then joined Leiden to complete his studies in languages as well as in the art of fortifications and war machines.

 

In 1626, he returned to Magdeburg where his knowledge quickly became useful because, in 1631, the Protestant city was besieged by the armies of the German Emperor in conflict with Sweden whose city is allied.

 

On May 20, at dawn, the troops of Catholic mercenaries of warlord Tilly, composed of Spaniards, Italians, French, Poles and Germans enter the city. The population resists heroically but fails to repel the attackers. Then begins what has been remembered as the "massacre of Magdeburg": in four days, twenty thousand civilians have been killed by the sword or burned alive in the fire of their house.

 

Once peace is restored, Otto de Guericke helps raise the city from its ruins and becomes mayor. In this position, he represented Magdeburg at the peace congress which, in 1648, ended this "thirty-year war". Good negotiator, he gets for his city, the recognition of his old privileges. This mission leads him to sit on the Imperial Diet. It was at one of these meetings, in Regensburg, in 1654, that he chose to reveal the capabilities of the vacuum pump he had recently developed.

 

The so-called "Magdeburg hemispheres" experiment is well known. It follows Torricelli's experiments (1608-1647) on atmospheric pressure.

 

In 1643, to respond to the problem posed by the Florence fountain-makers who had difficulty pumping water into their wells beyond 32 feet (about 10 meters), Toricelli had spilled a tube full of mercury on a tank containing the same liquid. He could see that the mercury was falling down the tube to stabilize at a height of 28 inches (76cm) above the free surface. He thus demonstrated the existence of the atmospheric pressure but also that of the emptiness which, according to his adversaries, Nature had "horror".

 

The subject fascinates Otto Guericke who undertakes successfully, the development of a pump capable of evacuating  air from a container full of it. After trying to empty a barrel that did not resist the experiment, Guericke had a copper sphere made up of two contiguous hemispheres and equipped with a tap. In front of a large audience, he is emptying into this imposing sphere of a diameter of 1.19 meters. Twenty-four horses hitched to the hemispheres are unable to break the adhesion between the two parts.

 

This experience radically inaugurates the practice of "science show" whose popularity will also be decisive in the advancement of electrical science.

 

The experience of the "Hemispheres of Magdeburg" is a landmark in the history of mechanics. Guericke's place in that of electricity is more modest. His contribution in this area was, moreover, ignored by most of his contemporaries. Yet, nearly a century later, several physicists, and in particular the Frenchman Dufay, note that one would have gained to consider his experiments with more attention.

 

Guericke, in fact, is not realy interested in electricity. He meets it only through the questions he asks himself about the functioning of the Universe and first of all about that of the earth. Among the "virtues" he attributes to our globe, two seem to him fundamental. First a "conservative" virtue: the earth attracts all the materials that are necessary for its formation, water, rocks ... Then an "expulsive" virtue: it repels everything that can destroy it. Fire, for example, whose flame rises to the sky.

 

Guericke offers of it a spectacular demonstration. Take, he says, a glass balloon the size of a "child's head", fill it with finely ground sulfur, heat up to the fusion of the sulfur, let cool, break the glass and collect the sulfur globe . Equip the globe with a handle and place it on a wooden support. Rub this ball vigorously with a very dry hand.

 

 

The ball will then manifest many of the earthly virtues. "Conservative" virtue first, attracting light objects to her.

 

More amazing is the observation of the "expulsive" virtue ! The globe sometimes repels what it first attracted. A feather, for example, after touching the globe is repulsed. So suspended in the air, it can be walked around the room. Better: whatever the movement of the globe it seems to always present the same face. Exactly like the moon opposite the earth.

 

Guericke, who has read Gilbert, can not doubt for a moment that the attraction virtue of the earth is simply electrical in nature. As for repulsive virtue, no one before him seems to have noticed it. He attributes to it a different cause and imagines it only proper to the constituent elements of the earth and among these to sulfur. It passes, thus, beside a truth which will remain long obscure until the French Dufay shows that the electricity also has a "repulsive virtue"!

 

Guericke's experiments contain other rich intuitions. To prove that the air is not the vehicle of the attraction, it shows that this virtue can be transmitted by means of a linen thread, more than a meter long, stretched from the surface of the globe. This first observation of the electrical "conduction" will also remain without a future. It will be up to the Englishman Gray to rediscover it almost a century later.

 

Even if its title of glory remains the famous experiment of the hemispheres and if its theoretical contribution in the field of electricity remained limited, the talent of observer and experimenter of Guericke, recognized by his successors, deserves the place which him is reserved in the Pantheon of electricians.

 

Hauksbee ( ?- 1713)

 

 

Electricity and vacuum works together in the machines devised by Francis Hauksbee.
 
The first years of his life are not well known. Self-taught, he is noticed by Newton. In December 1703, the famous physicist, author of the law of universal gravitation, became president of the Royal Society of London, the largest English Scientific Academy. He hires Hauksbee as his lead experimenter. Until 1705, it animates the sessions of the Academy. In particular by classic vacuum experiments inspired by Guericke.

 

From this date he moves towards the study of "mercurial" or "barometric" phosphorescence. Since 1675, a fortuitous observation intrigues physicists. When a barometric tube arranged in the conditions of the Toricelli experiment is jostled in the darkness, a phosphorescent glow appears in the emptiness released at the upper part of the tube. When Hauksbee tackles the problem, it is generally accepted that this glow comes from an emanation of mercury. For his part he chooses to use method and study the respective roles of emptiness, glass and mercury.

 

The vacuum ? Hauksbee partially fills a balloon with mercury in which he creates vacuum. The whole remains dark as long as the liquid remains motionless. It is therefore clear that the vacuum is not sufficient but that, on the other hand, the friction caused by the movement is essential.
 
Friction on mercury or on glass? From November 1705 Hauksbee uses, to answer this question, a montage which ignores mercury. It is a sphere of glass provided with two diametrically opposed copper pieces serving as its axis. This sphere can be put in rapid motion by placing it on a machine inspired by a carpenter's wheel. But its essential property is to have been conceived so that one can realize the emptiness. Hauksbee took the precaution of keeping a valve in one of the parts of the shaft that can be connected to a vacuum pump.

The Hauksbee electric machine. A tap allows to empty it

(Louis Figuier, Les Merveilles de la Science)

The sphere, emptied of its air, is set in motion and rubbed by the hand of the experimenter. Suddenly, in the darkness, the sphere fills with a strong diffuse glow. A wall ten feet away is illuminated. A book held near the globe can be read. When a finger approaches the sphere, the light is concentrated in filaments that seem attracted by this finger. The light gradually decreases when, little by little, the air is allowed to enter the tube.
 

 

Even when the atmospheric pressure is reached, we can still catch some light from the globe. It is external this time, and present themselves in the new form of sparks. Hauksbee still hesitates but for Newton opinion, the light does not come from emptiness, nor from mercury but from glass!

 

We now know that if it is the glass that is electrified, the light comes from the air. In the "empty" globe, there is still residual gas and it is "ionized" under the effect of the electric field created by the friction of the glass. It becomes, by this fact, bright, like neon in a tube of lighting. Naturally this interpretation was impossible to those who had neither the knowledge of the nature of the air, nor, still less, of the existence and constitution of the atoms.
 
This "electrical phosphorescence" will continue to obsess generations of physicists. His study will lead to cathode-ray tubes, which for some time still equip our televisions and computers screens. The discovery of X-rays, that of electrons, that of radioactivity, will also be at the end of this adventure that we will discuss later.

 

For the moment, Hauksbee's spectacular and frightening demonstrations in the darkness of a cabinet are becoming the star experiences of physics shows.

 

Tube or globe?

 

One thing is certain: for those who saw glass as a secondary material and with few electrical effects, and who continued to prefer amber, sulfur or wax, Hauksbee opposed them a convincing denial.

 

Glass is essential, but in what form? Hauksbee himself for his classical demonstrations renounces his spheres and uses only a tube of flint-glass, the flint-glass used for optics and of which the English are the specialists. With a tube one meter long and three centimeters in diameter, it attracts thin sheets of copper several tens of centimeters apart. These sheets of copper, or better of gold, more sensitive than pieces of string or paper, will become the classic material of electrical laboratories. To put them in motion, a glass tube is more than enough.
 

 

The globe, mounted on a tower, will be forgotten for thirty years until, around 1733, a German physicist, Bose, takes up the idea.


 
Bose (1710-1761)

 

Georg Matthias Bose, born in Leipzig, is interested in new physics and mathematics while pursuing his medical studies. In 1738 he was appointed to a chair of "natural philosophy" at the University of Wittenberg. From this position, he establishes a close relationship with all that Europe counts as well-known people, both scientists and men of letters, religion and politics. The magic aspect of electricity seduces him. When his readings lead him to meet the electrical experiments of Gray and Dufay (two persons of prime importance that we will talk about again), and in particular those on conductors and insulators; when, moreover, he finds the description of Hauksbee's globe, he knows that he has found both his vocation and his public.

 

It first completes the Hauksbee device with an assembly that will become the standard for all European laboratories. An iron tube, sometimes in the form of a rifle barrel, hangs horizontally from two cords of silk. He grazes, without touching it, the rubbed glass globe. This "first conductor" will then be used to distribute the "electrical fluid" through various chains or conductors to the surrounding experimental devices.

 

Bose then organizes "electric parties" that are not limited to its student audience. Imagine a meal where you have invited all the prominent notables in your city. The legs of the table have been isolated by wax patties as well as the chair that you have reserved for yourself. From the electric machine you have operated and concealed, a connecting wire is brought near your hand. At the moment your guests want to grab their fork, you just have to do the contact with the table so that an electric shock comes to make them jump on their chair. At dessert you will set a liquor cup on fire simply by the approach of one of your fingers from where only the closest spectators will have seen a spark escape. Your guests will then be ready to follow you in the cabinet of curiosities where you will transport them in a universe at once wonderful and terrifying.

 

Wonderful! Wafers of thick wax are placed on the floor. Each participant climbs on one of them and reaches out to his neighbors, forming a chain whose first link firmly holds the rifle barrel suspended above the globe of the machine. When the globe is set in motion, the person at the other end of the chain reaches out over gold leaves placed on a plate. Each one then sees the leaves rise from a light flight, as attracted by a magic will, towards the open hand of the experimenter. Let's put out the candles that light up this closed-shuttered salon and reach for the driver of the machine, we will see sparkling sparks. In the form of apotheosis we can propose the demonstration of the "electric beatification". The loveliest person in the assembly is invited to climb on a cake of wax and to seize the driver. When the machine is vigorously activated, its hair unfolds in a halo which illuminates, in the darkness, a thousand gleams of holiness.

 

Terrifying ! The man who has the courage to run a few drops of his blood sees them glitter like fire beads in the dark as he grabs the electric conductor. Tense fingers of a person connected to the machine can kill the poor flies to which the spark will be directed. Could we not make more serious victims tomorrow? Such manipulations would certainly have condamned their authors to be burned in the times, still close, of the Inquisition!
 

 

Terrifying and traitor! As beautiful as the young person haloed by the contact of the machine be, it will not be prudent to approach his lips for a kiss. The "Electrified Venus" will defend its virtue by a vigorous electric shock.

 

(Louis Figuier, Les merveilles de la science)

 

L’abbé Nollet (1700-1770)

 

 

The news of these wonders reaches France and in particular to the Abbé Nollet who is then one of the most prominent European electricians. He said he could not sleep until he himself had built and perfected a machine.

 

The globe, one foot in diameter, used by Nollet, is thick glass. The wheel which drives it by means of a belt passing by a pulley fixed on its axis, must be at least four feet in diameter and be provided with a crank which allows two men to activate it. Nollet prefers to rub the globe by hand but many European physicists have chosen to add a leather cushion.

 

 

The plate machines.

 

This voluminous machine will fit most physics cabinets until the Englishman Ramsden (1735-1800) builds the first plate machine in 1768. The plate machine is perfected quickly and will become really effective when the first machines appear. " with electrical influence ", ie requiring no friction. The famous machine invented by the English Wimshurst in 1883, still equips the laboratories of our high schools.

 

The Van Marum machine built in 1784 is still a notable attraction at the Netherlands pavilion of the Paris International Electrical Exhibition in 1881.

 

See : 

History of the electricity, from amber to electron. Gérard Borvon

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30 mars 2018 5 30 /03 /mars /2018 14:53

Gérard Borvon

Twenty centuries separate us from Thales, the first to have cited the attractive properties of amber and of the natural magnet.
 
The Greek science which had taken refuge in the Egypt of Alexandria found its heirs among the Arab scholars. Europe awakens from the "Middle Ages", this long succession of centuries traditionally, and often unjustly, described as those of the deepest obscurantism.

 

It was not without danger, in the heart of the 13th century, to be interested too closely in the attraction properties of amber or magnet. The Franciscan Roger Bacon (1214-1294), considered one of the first medieval experimenters, made of it the painful experience. His practices having been denounced and condemned, he had to suffer many years of imprisonment. Some of his colleagues did not have the same luck, their career and their writings ended on the pyres of the Inquisition.
 
So we reached the heart of the "Renaissance". A new freedom reigns in the arts and letters. One can again be interested in the attractive phenomena without being suspected of trade with the devil. William Gilbert (1544-1603), physician to Queen Elizabeth of England, decided to work about it. Under the title "De Magnete" (about the magnet), he published the results of the in-depth study of magnetism to which he devoted himself. He also studies the attraction of rubbed yellow amber. On this occasion he forges the word "electric".

 

Bust of William Gilbert in the library of Trinity College in Dublin.


 
How could he have imagined that the rigor of his experimental conduct and the perceptiveness of his conclusions would not only open a royal road to a new branch of knowledge, but also revolutionize human civilization as a whole.
 
Gilbert followed, in Cambridge, the classical studies of a medical student. Mathematics and astronomy, dialectics, philosophy, Aristotelian physics, metaphysics and ethics occupy his first four years. Medical studies by themselves consist mostly of readings of Galen and his commentators. The Greek physician who had structured in Rome, in the second century of our era, the theory of the four "moods" (blood, phlegm, yellow bile and black bile) and the four "temperaments" (sanguine, phlegmatic, choleric and melancholic) , is still the only authority recognized by the Royal College of Physicians. Everything happens as if no observation, no new technique, had come to enrich the art of healing for more than ten centuries. Gilbert refuses a knowledge thus frozen. He completes his studies in a self-taught way. Even before getting his doctorate, in 1569, he began to study the properties of the magnet.

 

The new doctor creates a cabinet in London in the middle of the year 1570. He quickly gets a clientele in the aristocracy and intellectual circles of the capital and becomes an influential member of the College of Physicians. In parallel, he continues his private studies with many "troubles, insomnias and expenses". Two books are from this work. The most remarkable, "De Magnete", appears in 1600. Happy New Year for its author! At the same time, he was chosen as the Queen's appointed physician and promoted to the presidency of the Royal College of Medicine.

 

Birth of electricity:
 
As its title suggests, De Magnete is essentially devoted to the magnet, that is to say to the ore that we now call "magnetic oxide" and which is naturally magnetized. The book is a good synthesis of the knowledge of the moment. He substantiates the idea that the earth is, itself, a huge magnet. We will talk about it again.
 
For the moment, the aspect of the work that deserves our immediate interest lies elsewhere. It is contained in the long chapter devoted to amber. In doing so, Gilbert aims at an objective : he wishes to establish, in a sure and definitive way, the difference between the attraction of amber and that of the magnet.

 

This work was of first urgency. Tradition regularly confused these two types of action. Thales, the first, had been quoted as "communicating life to inanimate things" by using both amber and magnet. However, the differences could only imposed themselves on those who decided to rely on observation rather than just texts inherited from the ancients.
 
Gilbert was not the first to insist on these differences. In the middle of the 16th century the Italian Girolamo Cardano had already established a first list of them. Cardano himself was a physician ; the natural magnet powder, like that of amber, was probably one of the remedies he proposed to his patients.

 

Cardano found five different behaviors to amber and magnet. We will retain three of them:
 
1) Amber attracts all kinds of bodies. The magnet only attracts the iron.
 
2) The action of amber is caused by heat and friction, that of the magnet is permanent.
 
3) The magnet only attracts towards its poles. the amber to any rubbed part.
 
Gilbert resumes these propositions on his account. He adds two observations:
 
1) A wet surface or a humid atmosphere removes the effect of amber. Which is not the case for the magnet.
 
2) the attractive property of amber, unlike that of the magnet, belongs to a wide variety of substances.
 
With regard to the history of electricity, it is naturally this last observation which is the most remarkable.

 

Electricity is a general property of matter.
 
Gilbert already knew, after reading the Greek authors, that amber was not the only body with attraction properties. The diamond, other adornment of men and gods, was itself endowed with it. The question naturally arises : can we still enlarge the list of the bodies presenting the property which it designates by the term "electric"? This word, coined by Gilbert in reference to amber, will have, as we know, a beautiful career.
 
Guided by an intuition still influenced by tradition, Gilbert begins his investigations with gems and precious stones.

 

To help in his research he uses an instrument inspired by his study of magnetism : a metal needle of about ten centimeters mounted on a pivot. Any metal is suitable. Copper, for example, or even money. Iron would also be appropriate, but it is better to dismiss it if one wishes to avoid any confusion with magnetism : a magnet has no action on a copper or silver needle. This "versorium", as Gilbert calls it, is a very sensitive detector. It allows you to highlight attractions that would remain hidden if you were trying to attract only bits of string or paper placed on a table. Gilbert thus establishes a list of at least 23 "electric" bodies.

 

The most humble are often found to be the most active. Two in particular stand out : sulfur and glass. What's more banal than these two materials ? Yet they are much more effective than a ball of amber of good size. They are so so that it is astonishing to note that it took twenty centuries before one became aware of it.
 
We measure the obstacle erected on the path of knowledge by the mythical valorization of amber. As if the very idea of searching attraction property in ordinary materials might have seemed sacrilegious. After Gilbert, glass and sulfur will become experimental materials of choice.

 

But let's not forget that the goal was to highlight the different natures of the magnetic attraction and electric attraction. He was thus reached beyond all hope. On the one hand, there is a property that is found only in the "magnet stone" or, temporarily, in the steel put in contact with a magnet. On the other hand it is already possible to draw up a list of more than twenty bodies which, rubbed, can manifest the attraction property of amber.
 
With hindsight, this distinction could appear as an obstacle on the road that, two centuries later, will lead to the fusion of the two disciplines and the birth of electromagnetism. In fact, this temporary separation must be considered as an essential first step. By letting both knowledge develop in parallel, we have allowed each to flourish. It was the slow journey that led from amber to the "Leyden Bottle" and then to the "Voltaic generator" which produced the electric currents that will soon power the electromagnets.

William Gilbert M.D. demonstrating his experiments before queen Elizabeth (painting by A. Auckland Hunt).

 

If Gilbert is to be credited with having substantiated the distinction between "electric" and "magnetic", we owe him above all to have been able to "trivialize" the attraction property of amber and to have erased his "magic" character. To have, at the same time, opened the way of a new discipline and to have baptized it.
 
Gilbert died three years after the publication of his book on magnetism. He will not have had time to write the one who would come to complete it and that could have been entitled: "About electric bodies".

 

See : 

History of the electricity, from amber to electron. Gérard Borvon

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30 mars 2018 5 30 /03 /mars /2018 11:17

Gérard Borvon

Thales (625-547 BC), Greek of the city of Miletus, at once physicist, astronomer and geometer, is traditionally designated as the first electrician. It is by Aristotle and Hippias that we learn that he "communicated life" to inanimate things by means of the yellow amber, referred to under the Greek term "Elektron", transcribed by the Latin electrum, which is the origin of the word electricity.
 
Communicating life to inanimate beings ... from his birth electricity is surrounded by mystery.

 

Amber.

A quick glance at a contemporary dictionary tells us that amber is a "hard and brittle resin, whose color varies from pale yellow to red and from which is made of necklaces, articles for smokers, etc. ...". The photograph that accompanies this text shows us an insect prisoner of a blond stone with transparency of crystal.

 

Amber, mythical material of ancient Greece, has still, today, an important place in the crafts of the southern Mediterranean. He alternates on necklaces and bracelets with coral and filigree silver beads. One could believe this mineral, like the rose of sands, matured in the sun of the desert.
 

Yet the amber comes from the cold.For millennia, the inhabitants of the Baltic coast have been collecting this precious gift of the sea, deposited on the sand after every storm. Is its origin marine or terrestrial? From antiquity to the end of the 18th century, long controversies followed before it was admitted that amber is a fossilized resin.

 

Forty to fifty million years ago, in a period geologists refer to as the Eocene, a tropical climate prevailed over Europe and Scandinavia. The resin-producing pines, the source of amber, grew among date palms, redwoods, cedars, cypresses, and most of the hardwoods that we still find in our region : oaks, beeches, chestnuts. Clouds of mosquitoes, flies, wasps filled the air with their buzzing. Ants, beetles, scorpions swarmed under the moss. All this little people came to get sticked in the still fresh resin. In the spring, magnolias and rhododendrons were blooming over juniper rugs and even tea trees growing where the soil was not flooded. Water, indeed, was everywhere present. It is it who protected the resin of an oxidation which would have destroyed it. This water fed rivers that concentrated amber at their mouths, creating rich deposits.

 

Then the climate cooled. The glaciers that covered Northern Europe transported and deposited these sedimentary earths. The amber is still there today. When, by chance, the deposits line the current seas, erosion releases the blocks. The density of amber being very little higher than that of sea water, currents and storms bring it easily on beaches where it is convenient to fish it.

 

An attractive material
 
Sweet, warm to the touch, mysterious jewellery case of strange insects, endowed with the extraordinary gift of attraction at a distance, this stone has certainly provoked in our oldest ancestors, the fascination which is still ours.
 
A piece of 30,000-year-old perforated amber, probably a talisman, is considered the first object of this material associated with man. Bears, wild horses, wild boars, elk were there shaped by the men who lived in northern Europe 7000 years before our era. Neolithic farmers who inhabited the same regions three thousand years later were buried with necklaces and amulets of amber. During the next two millennia, amber spreads gradually throughout Europe, to the Mediterranean. By the same routes circulate copper and tin which will make flourish the civilizations of the Bronze Age.

At that time, real trade routes crisscross Europe.

From Jutland, they take the road to the Elbe or the Rhine and the Rhone. From the eastern Baltic, they descend the Oder and Vistula to reach the Mediterranean through the Black Sea. A sea route also exists that descends from the North Sea across the Channel and bypasses Spain to reach the Mediterranean.

 

The tombs under Tumulus of the princes and princesses of the Bronze Age excavated in the south of England and on the shores of the Armorican coasts have transmitted to us fabulous treasures. Amber is associated with gold to exalt the power of their owners.
 
In Greece, the amber of the Baltic arrives around 1600-1500 before J-C. The tombs of this period found in Mycenae contain hundreds of pearls that seem to have been imported already cut. Shortly after, this same amber is found in Egypt in the royal tombs. This trade seems to have been the specialty of the Phoenicians. It was not until the 4th century BC that Pytheas, Greek from the colony of Marseilles, gives us the story of his journey to the Baltic seas where he would have ballasted his ship with amber blocks.

 

The tears of the Heliades.
 
In Greek mythology, amber is of a divine nature. These are the rays of Helios, god of the sun, petrified when the solar star sinks into the floods. These are the tears of the Heliads, mortal nymphs, who cry every night for the death of their brother Phaeton.
 
Phaeton, son of Helios, had obtained permission to drive the chariot of the sun. Alas, he did not know how to master the winged horses of the team. He approached too near the earth. Mountains began to burn, fires devastated the forests, drought spread to vast areas that became deserts. Zeus, in his anger, threw his thunderbolt on Phaeton and made it sink in the floods of the Eridan River (often associated with the Po, one of the paths of entry of amber but also designating the seas bordered by the Celts and germans countries). Rushed up to the banks of the great river, the Heliades, sisters of Phaeton, remained inconsolable. The gods, out of compassion, turned them into poplars so that they could eternally accompany with their tears, the disappearance of the setting sun. Their tears, petrified in golden pearls, become the finest adornment of Greek women.

 

Rubens. Fall of Phaeton.

 

The names of the amber.
 
"Ellektron", that is the name that comes from the Greeks. To describe amber, the Latin gave us the term succin (succinum), derived from sucus (juice, sap). The word "amber", meanwhile, could come from a series of unfortunate translations. The Arabs used the term "Haur roumi" (poplar Roman) to designe the tree whose sap they considered as the source of succin. This word turned into "avrum" by the Latin translators of Arab authors would have been confused with "ambrum" which meant ambergris, "anbar" in Arabic. Ambergris, an odoriferous concretion forming in the intestines of sperm whales and used in perfumery, has nothing in common with yellow amber. Only the name confuses them in French as in Spanish (ambar) or in English (amber).
 
German uses the word "Bernstein" which refers to a "burning stone". The northern populations encountered by Pytheas were, in fact, reputed to use amber as a fuel. Slavs use the word gentar or jantar meaning amulet. The word "goularz" Breton Armorican could evoke the light (goulou) and would, in this case, close to the Greek myth.
 
Each language expresses, thus, one of the aspects of the myth of amber: that of a stone of sun or light, that of a stone which attracts, that of a stone which protects, that of a stone who heals. The amber has left no people indifferent.
 
But what do the Greek authors tell us apart from the myth? Few things really. They know, at best, that amber attracts but do not always indicate that it must first be rubbed.
 
The phenomenon therefore remains very superficially studied. Nothing evokes the beginning of a practice or a reflection which is related to a "scientific" behavior. Unlike chemistry, which can claim a tradition dating back to the very origins of human civilizations, electrical science has no real prehistory.

 

The long sleep of amber.
 
Improved transportation, combined with the wealth of deposits, amber is gradually losing its market value. Inevitably, its "magic" character is diminished. It is however prolonged in the form of the medicinal properties attributed to it.
 
Amber pearl necklaces are particularly popular among healers. Eighteenth-century academic literature commonly refers to collars worn to cure migraines, eye or throat diseases. An archeology book published at the beginning of the 20th century describes these talisman necklaces worn by some Breton families in Morbihan. The author imagines them issued from tumulus, these "fairy rocks" or "dragon caves" so often visited by their ancestors.
 
A piece of amber is, still today, given to chew to children from the shores of the Baltic to relieve toothaches. Our century seems to be one where old myths are reactivated. The amber has returned to the center of a trade which is adorned with the virtues of esotericism. One can buy on the internet collars which will make run to the babies of risks of accident that the simple wisdom should lead to avoid.

 

 

From amber to succin.
 
More academically, amber, under the name of succin is the basis of a host of remedies prepared by apothecaries until the end of the 18th century and perhaps beyond. It can be used in powder form but also in solution. Witness this recipe brought back from Copenhagen in 1673 by Thomas Bartholin, correspondent of the Paris Academy of Sciences: "to burn to ashes, the blood and the hare skin in a new vessel, the laundrying of these ashes hot dissolves the succin that one there throws". A remedy prepared with such refinements necessarily had to be effective.

 

If one believes the list of evils that it is supposed to cure, the succin would indeed be a true panacea. Such a universality can only, however, alert a critical mind. Scrupulous doctors question themselves. For example, Dr. J. Fothergill of the " College of Physicians of London", who considers in an article published in 1744 that only a "prejudice" has maintained its use in medicine and advocates for a sanitation enterprise in the medical science: "If Skilled and experienced people wanted to devote their free time to inform us of the inefficacy of methods and remedies similar to this one, Medicine would be enclosed in narrower limits ".

 

Even if we find, still today, "succinic" acid in the list of our pharmaceutical products, the succin certainly has a limited therapeutic interest. Fortunately, however, his prolonged presence in pharmacies and doctors' offices will have had the merit of saving him from oblivion.
 
It is therefore a physician, William Gilbert who, in the 17th century, will study the attractive properties of amber with the new look of nascent science and may, better than Thales, claim the title of "first electrician".
 
See:

History of the electricity, from amber to electron. Gérard Borvon

 

 

 

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17 janvier 2018 3 17 /01 /janvier /2018 08:35

A class of the lycée de l’Elorn, in Landerneau, Brittany, France, has chosen to discover that ancient, rich and varied industry of seaweed, while dealing with different parts of its curriculum. We present the result of that work in the following pages


Northern Finistère, in Brittany, is not really welknown for its chemical industry. Yet, since the 17th century, that is to say when chemistry started to develop, a chemical industry was carried out, non stop, around seaweed.

In the past

The industry of « soda » (sodium carbonate) first developed. This product is extracted from ashes of dried seaweed. It is necessary to make glass and soap. That activity came to an end at the end of the 18th century when new ways were discovered.

It resumed in 1829 after Bernard Courtois, the chemist, had discovered in 1812 a new an useful product in seaweed ashes : iode. It is mainly used in photo-making and medecine. Its production in Brittany stopped in 1952, because of the competition of iodine, extracted from nitrates in Chili.

Today

Today, the extraction of alginates contained in big laminaria has taken over. In 1883, Edward Stanford isolated the algine of seaweed, later Axel Kefting, a Norvegian, extracted algine acid. Its production on a large scale started in 1930. Brittany produces about 2000 tons in its factories in Lannilis and Landerneau. Alginates are thickening and stabilying agents, that are used both in the pharmaceutical industry and food industry, or in that of paper, colouring or moulding products.

The use of seaweed in food, pharmacy or cosmetics is less known., though worthy of interest. Many laboratories in Finistere work in that field for « top quality » products, often meant for export.

The seconde C of the lycée de l’Elorn, in Landerneau, has chosen to discover that ancient, rich and varied industry of seaweed, while dealing with different parts of its curriculum. We present the result of that work in the following pages.


Our work on the seaweed industry

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The story of the seaweed industry, that of soda and iodine, is made lively thanks to the museum of seaweed gatherers in Plouguerneau, which supplied us with the ash from ovens, operated for shows during the summer, so as to analyse it.

The « Centre for the Study and Promotion of Seaweed (C.E.V.A) » in Pleubian looks for the properties of seaweed and implements new uses. We contacted them for the food applications (the making of a « flan »)

Today, many factories work on seaweed. It’s the case for DANISCO and TECHNATURE, which agreed to help us.

DANISCO deals with laminar collected in North-Finistere, it’s one of the largest European producers of alginates. We visited the factory. It supplied us with refined alginate of sodium for our experiments.

TECHNATURE packages alginates and other seaweed extracts, to make casting products, cosmetics, or food products. Its customers are in the U.S.A, as well as in Japan, Spain, or France. The company allowed us to test its products and to prepare new ones, following its advice (face creams).

Our school syllabus is well adapted to a study of seaweed. In a first part, the study of ionic compounds can be made on the seaweed ash. In a second part, the study of organic molecules can be made on alginates. The appliances are varied and entertaining.

We have divided the form into four groups, each responsible for a part of the work and for the links with one of the companies concerned.

- Seaweed ashes. Analysis, extraction of iodine.(in connection with the museum of the seaweed gatherers)

- Extraction of alginates. (in connection with Danisco company)

- The use of alginates for castings . (with Technature).

- The making of a new face cream.(with Technature)

- The making of a flan (a pudding) (with C.E.V.A Pleubian)

- Translations into English ( documentation and reports).

- A video report on our project ( and the making of a poster).


Seaweed in the past

Treating the « soda loaves »


The burning of seaweed

Each year, the museum of seaweed gatherers, in Plouguerneau, on the Northern coast of Finistère organises the burning of seaweed in its old furnaces so as to get ashes with a large amount of soda. We went on the spot, to extract a « soda loaf », in a compact shape. The hot cinders seem to be melting, and are cast in the cells of the furnace, while they are cooling.

The mechanical processing of the ashes :

We first roughly broke the « soda loaf » with a hammer. We, then, crushed the ashes in a mortar with a pestel. Then, we sifted them, to obtain a thin powder.

The washing of the ashes

We left to boil 20g of the ashes in 100 cm3 of water for about 5 min. We filtered it. A solid deposit of about 9g was left (weighed after drying). The solution contains soluble substances, mainly carbonate and iodur ions.


 

 


 

The search for carbonate ions

The carbonate ions, CO32- , represent the main active principle of soda and gives it its basic character.(in the present the word « soude » ,in French, refers to sodium hydroxide).

Experimental file

 

measure of the pH using pH paper and pHmeter : The solution has a pH=11, so that, its basic character is obvious.

Characterisation of the CO32- ions :

(first method) : action of the calcium chlorur. You get a precipitate of insoluble calcium carbonate according to the reaction :

Ca2++ CO32- -> CaCO3

(second method) : action of the concentrate chlohydric acid. You can notice an important emission of carbon dioxide, according to the reaction :

CO32-+ 2H+-> CO2 + H2O

The extraction of iodine

We extracted iodine from the solution, through the action of Hydrogen Peroxide H2O2 in acid surrounding.

experimental File

- Acidification of the solution using concentrated hydrochloric acid : The first result of the acidification of the solution is to let out carbon dioxide coming from carbonate ions.

- Iodine is let out using hydrogen peroxide : The hydrogen peroxide oxidises iodide ions, iodine appears and turns the solution brown. One can also see a light precipitate of iodine.

- Getting the gassy iodine to appear by heating the solution : a light heating lets out purple vapours of iodine.

 

Measuring the iodine : this experiment is part of the curriculum of the 1ere S form, so we asked them to measure the iodine in the solution. The iodine is measured with the thiosulphate of sodium. They found 1,29g of iodine in 100g of ash.


Seaweed Today


A visit to two factories processing alginates

In the Landerneau area, two firms process seaweed for theit alginates. The Danisco firm has specialized in extracting alginic acid from raw seaweed. The Technature firm uses alginates to elaborate finished goods.

Danisco :

Mr Pasquier, the manager, conducted our guided tour of the factory. Every year the plant (9000 m2 of workshops and laboratories) processes 6000 tons of dried seaweed to produce 3000 tons of alginates.

The alginates supply numerous industries all over the world. Used as binders and thickeners, they can be found in inks, creams, glues, rubbers, toothpastes. As gelling agents they come in useful to make jams, custards, impression powders. These products ar marketed under the brand name SOBALG.

The Danisco firm provided us with a smal quantity of purified alginic acid so that we could study its properties. The danisco manager also explained to us a great length how they extract the alginates from the seaweed.

We conducted that experiment in our scholl laboratory.

Technature :

We were welcomed by the manager, Mr Le Fur, and the commercial manager, Mr Winckler (today manager of Lessonia). The firm packages the alginates for its different uses : casts, cosmetics, foodstuffs.

The firm has clients all over the world (Euope, the USA, Japan...). The breton products ar renowned for their quality and their purity.

The firm gave us some casting alginates so that we could make a cast.

They also offered us to elaborate a new "beauty mask". We will give more details about these two experiments in the following pages.

How to create a beauty mask

Technature entrusted us with the creation of a beauty mask. It is a new product the company wishes to launch. It’s a product made with tropical fruits, based on casting alginate.

The formula of the « tropical fruit » mask.

Product usedQuantityproperties
Bioprunte (alginate of sodium, sulfate of calcium, salt of phosphorus, neutral charge of diatomees earth.)30gWhen in close contact with the skin, it creates a film. The mask sets into action active agents, and also has a mechanical effect ( it eliminates the dead cells of the skin).
Pinaple Pouder Retour ligne manuel
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Papaye powder

0,15 g

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0,15 g

The cells of the skin are constantly replaced (every one to two months). With age, the process slows down, and the dead cells accumulate, which cause the skin to thicken. The dead cells are retained by a ciment of proteins ; it has to be hydrolysed to eliminate the dead cells.Retour ligne automatique
Papaye contains papaïn, an enzym, which acts on the hydrolysis of proteins. Pinaple contains bromeline which plays the same role.
yellow pigment n°5Retour ligne automatique
yellow pigment n°6

0,03g

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0,03g

Naturel pigments are used to obtain a pleasant colour of fruit.
Flavours : fruit de soleil, papaye0,015gThey are natural extracts from fruit, with very concentrated effects.

Our work

First, we tested an alginate mask, with no additive, so as to watch the « casting » effect of that product. Retour ligne automatique
We then tried several formulas, by varying the colours and flavours.Retour ligne automatique
At last, we tested the resulting cast.

How to operate

Dose : 30g of powder for 100g of water

Dilution of the product : Pour the water quickly on the powder. Mix briskly until you get a smooth paste.Retour ligne automatique
Important : water must be at 20°C

How to apply it : Apply it immediately over the face, avoid the eyes. It sets after about six minutes.

It takes about 15 mn to use

Résult

your skin is finer

your complexion Retour ligne automatique
brighter



 

Agar-Agar and the formation of colloids

Agar-Agar is a Malaysian word. That product used in Malaysia, was also often used in Japon and the Far East. Agar-Agar comes from various seaweed, in particular from the gelidum species. Those seaweed, after frequent washings, are dried and boiled. The colloid we get is then dehydrated and turned into powder.

Agar-agar has a stong gelling power. If you add two gramms into a quarter of a litre of water, and boil it for five minutes, you get a hard gel, if tou leave it to cool.

At the biology laboratory, Agar-Agar is used to prepare nutrient supports for plants.Retour ligne automatique
At the chemistry laboratory, it can be used to prepare conducting electrolytic bridges in the study of batteries.

We prepared Agar-Agar colloid, coloured with helianthine. Agar-agar is also used to prepare pudding, but for that we used a seaweed from Brittany, Pioka, which contains carrageenans.

Agar-Agar : an excellent gelling agent extracted from red algae


« Pioka » and carrageenans

Pioka is the Breton name of a seawweed that is also called sea « lichen ». It is collected at every low tide, its high price attracts seasonal pickers. Its scientific name is chondrus crispus. The active principle extracted from it is made up of carrageenans. It has a real gelling power in milk. In the traditionnal way, it is used by people along the Northern coast of Brittany to make puddings named « flans ».

The préparation of seaweeds.Retour ligne automatique
After the gathering of seaweeds, they are spread on the dunes, and dried by often turning them. They can be also washed with fresh water to clear them of salt at various remains. At the end of treatment, the seaweeds are white and dry, and can then be preserved.

Just before use. Retour ligne automatique
One can improve the rising process with several soakings ans rinsings. The seaweeds must completely get rid of their « sea » smell.Retour ligne automatique
Seaweeds today, in food

A recipe of pioka pudding

We have prepared the recipe of this dessert. It was given to us by an elderly person from the Brignogan area in North-Finistere. She herself had seen her parents make it.

N.B : carrageenans of pioka easily give a gel with milk, it gives no gel with water. For that, on should use the agar-agar we also tested (it is also used for puddings).

Our recipe

Take a handful of dried seaweeds per quarter of a litre of milk. Rinse them. Make them boil for five minutes stirring them. Filter the hot milk with a strainer or a skimming ladle. Make it boil again for five minutes with the flavour choose, either chocolate or vanilla ( for exemple, three sponfils of Nesquick per quarter of litre of milk). Pour into bowls. Leave it cool and place it into a fridge.


Conclusion

When we started working on this project, we were not aware chemistry had been concerned with seaweed for so long.

We now, know, that here, people make products that are used all over the world.

Our impression is that the chemists who do that work really enjoy it, they extract from nature the best it can offer. The issue will be to increase the stock of seaweed and no doubt to plan its culture.

As far as our school project is concerned, it developed without our knowing it. The theorical study, the search for information, the experiments at the laboratory, the visit of factories, the elaboration of a new product, the test of an old recipe...all that was part of our project.

By writing this project, we intend to keep track of our work.

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24 novembre 2013 7 24 /11 /novembre /2013 07:30

The first International Exposition of Electricity in Paris ran from August 15, 1881 through to November 15, 1881 at the Palais de l’Industrie on the Champs-Elysees.

 

It served to display the advances in electrical technology since the small electrical display at the 1878 Universal Exposition. Exhibitors came from the United Kingdom, United States, Germany, Italy and Holland, as well as from France.


From Wikipedia, the free encyclopedia

 

 

This show was a great stir. The public could admire the dynamo of Zénobe Gramme, the incandescent light bulbs of Thomas Edison, the Théâtrophone, the electric tramway of Werner von Siemens, the telephone of Alexander Graham Bell, an electrical distribution network by Marcel Deprez, and an electric car by Gustave Trouvé.

 

The first International Congress of Electricians.

 

As part of the exhibition, the first International Congress of Electricians, which met in the halls of the Palais du Trocadero, presented numerous scientific and technical papers, including definitions of the standard practical units volt, ohm and ampere.[1]

 

George Berger was the Commissioner General. Aside from the provision of the building by the French government, the exhibition was privately financed. Organizers would donate profits to scientific works in the public interest.

 

Adolphe Cochery, Minister of Posts and Telegraphs of the time, had initially suggested that an international exposition should be held.[2]

 

Among the exhibits were :

 

Apparatus for production and transmission of electricity, natural and artificial magnets, and compasses, devices used in the study of electricity, many applications of electricity (sound, heat, light, electroplating, electrochemistry, signage, power,industrial applications, agricultural and domestic), lightning, old instruments in connection with electricity.

 


The Edison dynamo.


Electric lighting was one of key developments on display at the exposition, with up to 2500 electric lamps in use. Comparative testing of Edison, Swan, Maxim, and Lane-Fox incandescent lamps were conducted by William Crookes to establish the most efficient form of lamp.[3]


A : Edison lamp.

B : Maxim lamp.

C : Swan lamp


Using the described Théâtrophone apparatus, visitors could hear the live opera two kilometres away.



The Théâtrophone


References

 

* CNAM (ed.). "Exposition internationale d’Électricité". Retrieved Aug 16, 2008.

 

* Gérard Borvon, Histoire de l’électricité, de l’ambre à l’électron, Vuibert, 2009, ISBN 978-2-7117-2492-5

 

* (1) a b K. G. Beauchamp, Exhibiting electricity IET, 1997 ISBN 0-85296-895-7, pp.160-165

 

* (2) Walker, George (1881). Consular reports, Issues 4-8,United States. Dept. of State, 1881. United States Dept of State. p. 253.

 

* (3) Proceedings of the Institution of Electrical Engineers, Volume 11, page 230


External links

 

* Gérard Borvon (2009-09-12). "Histoire de l’électricité. L’exposition Internationale d’électricité de 1881, à Paris".

 

* 1881 : first international congress of electricians, first international electrical units system.

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