Brief Historical Review about Magnetism: From the Ancient Greeks up the Beginning of the XXth Century

Magnetism is omnipresent in multiple natural and constructed phenomena we interact with daily. However, few students knew about the different explanations advanced relative to this mysterious magnetic force imperceptible by our senses before the advent of the discovery of the atomic and molecular aspects of matter. Unfortunately, the concepts associated with the study of magnetism presented in many textbooks do not consider the conceptual difficulties encountered by scientists and the false theories developed and abandoned following new experiments and theories. Towards the end of the 18th century, with the developments of classical physics, among others in mechanics, electrostatic, electricity, and measuring instruments, scientists pierced the secret of the magnet stone and the compass used in navigation for several centuries BC. On the other hand, the progress made in studying matter at the atomic scale at the beginning of the 20th century made it possible to explore the phenomenon of magnetism in greater depth and give it a quantum interpretation. It is impossible to present all the conceptual complexity concerning the development of magnetism in a few pages. Thus, we will limit to synthesizing the most discoveries related to magnetism from the ancient time, five centuries before Jesus Christ, to the beginning of the 20th century. Thus, we will mainly focus on the erroneous theories developed throughout history by renowned scientists and the conceptual difficulties related to the study of magnetism.

The Greeks knew about magnetism in the 6th and 4th centuries BCE. According to several historical writings, it is up to the Greek Thales (624 BC-546 BC) of Miletus in the 6th century BCE to have attempted to explain magnetism [1-4]. For him, the type of rock capable of attracting distant iron objects and the firm adherence of iron nails on the stone can be explained by its "soul power" of attraction [5,6].

The philosopher, Plato (428 BC-384 BC), criticized the animist conception of Thales by assuming the impossibility of an attractive movement in the void since this attraction requires the action of invisible matter between the magnet stone and the iron. Since the pneumatique machine's creation by the Prussian physicist Otto von Guericke (1602-1686), we know that magnetic attraction occurs in a vacuum, so Plato's statement that "matter abhors a vacuum" is inconsistent.

To explain the attraction of iron by the "magnet stone," the ancient Greek philosopher Empedocles (490 BC-430 BCE) referred to the "atomic" theory of Democritus (460 BC-370 BC) and Leucippus (480 BC-420 BC).

We owe Empedocles an essay explaining the action of the magnet on the pore structure of the iron mechanically. Democritus composed treatise-related magnet phenomena in which he says that atoms penetrate in the middle of the less sensitive particles of metal and that the iron atoms move to be absorbed by those of the magnet because of their resemblance. Plutarch thinks a little like Epicurus on the attraction when he says the magnet stone emits fragrances that form a whirlwind around it; from there comes the strength with which this stone attracts iron.

Attempting to explain the magnetization of iron goes back to studying the magnet's magnetism. In this regard, the explanations advanced by Democritus in referring to an atomic conception of matter were not shared by most philosophers and scientists of his time because the dominant theory was that of the four elements (air, water, fire, and Earth) developed by Aristotle and his followers.

According to this thesis, the magnet stone emits something that we do not see, called fragrance. It seems that we found the magnet stone in Magnesia, an ancient city of Asia Minor (today part of Turkey), which derives the term "magnetism" and was named "stone of Magnesia" or "magnet stone." Point out that nowadays, we know that this stone is a mineral composed of iron and oxygen whose chemical formula is Fe2O3, and we find it in many regions around the world.

Thereby, the Greeks could not explain the attractive power of this stone except by arguing that it had, for example, a "soul" that conferred this property. In this era, we associate the soul with movement; all bodies in motion were endowed with life.

The Chinese have long known about the lodestone attraction of iron particles, and they knew that when freely suspended on a string, it will align itself in the same direction. This observation led the priests to invent a magnetite stone spoon to predict the future. The spoon used for this purpose has a short tail and balances on its rounded base. We placed it in the center of a polished plate engraved with various signs to read the future. A sharp blow on the tail and the spoon turns. When it stops, it remains to interpret the inscriptions indicated by the direction of its sleeve. Also, the Chinese used the magnetite stone spoon to build houses in the direction the spoon gave when it stopped spinning.

Thus, it would seem that the Chinese used the magnetite stone later in navigation. It was much later, between the ninth and twelfth century AD, that the Chinese replaced their divining spoon with a floating needle that they would use in navigation. How was it possible to make needle compasses when the only magnetism that existed was natural (the stone of Magnesia)? We knew that the magnet stone attracts iron and can turn it into a magnet. Thus, by laying down a thin iron rod on the magnet stone for a specific time, it acquires the ability to attract iron like the magnet stone. We can also magnetize the iron rod by rubbing it against the rock. We must rub it in the same direction; otherwise, it does not attract. The magnetization time of the rod by friction is much faster than the previous method.

Does the magnetized rod turn into a permanent magnet? The magnetism acquired by the stem is not stable because it loses its magnetic property after a while, and we must start again. Once the rod is magnetized, we can use it as a compass by suspending it with a thin string or, even better, by placing it on a pivot or placing it on a piece of cork floating on the water contained in a boll. When the cork is in equilibrium, the needle takes a given direction as the suspended magnet stone. If we move them to different places at night, the needle makes the same orientation relative to a fixed Polaris star. Thereby, when freely suspended on a string, pieces of lodestone served as the first magnetic compasses. We could navigate thanks to this lodestone for many years without understanding why the lodestone aligns in the same direction.

We attributed unfounded properties to magnet stone, such as the stone having a soul as postulated by the Greek Thales to explain its attraction capacity. Also, we thought the magnetic stone had the power to predict the future. Other beliefs were advanced about the direction of the compass's needle toward the Polaris star (a fixed star) in the Middle Ages and Renaissance.

However, the compass's needle direction towards this fixed star was not understood. In this regard, many thought the compass's orientation in a fixed direction was the "Work of God." Also, in some fables, we mentioned that the magnet stone would be able to get unfaithful husbands out of their bed at night if we put it under their pillow.

These conceptions about magnetic force are generally based on mythology and religion; that lasted centuries. In this regard, Blundell [4] points out the following: "The struggle to understand magnetism has been long and tortuous. At various times, magnets have been claimed to be useful in detecting adultery, healing the sick, and also unlocking the secrets of the life force of the Univers."

Blundell [4] emphasizes that lodestones also had many medicinal properties: "[…] it variously caused a mental disturbance, melancholia, it preserved youthfulness, purged the bowels, or worked to 'stay the purging,' corrected 'excessive [senses] of humor of the bowels and putrescence of the same, and could be used to cure headaches or stab wounds."

Despite these unsubstantiated beliefs, the magnet stone had a utilitarian property as it paved the way for navigation.

It is important to note that up to our Epoque, in the pseudoscience books, we affirm that the magnet can cure cancer, deafness, visual acuity, rheumatism, nerve diseases, and many other diseases.

For example, the famous magnetizer Mesmer, Friedrich-Anton Mesmer (1734-1815), a German physician, could, he claimed, magnetize water or trees and thus communicate a magnetic healing virtue. According to Mesmer, all disease stems from improper fluid distribution within the human body. Therefore, thanks to a magnet, it is sufficient to drain the fluid adequately to rebalance human bipolarity [7].

However, magnets have many medical applications, especially in ocular surgery. The same applies to protecting the health of cows that swallow metals found, for example, in the hay or grass.

Despite these beliefs regarding magnet stone and the compass, the latter constituted a revolutionary breakthrough in navigation, even if its orientation toward the sky remained mysterious. It is up to Peter Peregrinus of Maricourt (13th century - 13th century after 1269), a French military engineer who lived in the 13th century, Robert Norman (1560-1584), an English Sailor, writer, and geophysicist, and William Gilbert (1540-1603), an English physician, to have developed scientific experiments. They studied the properties of the magnet stone and those of the stone compass. Below, we will present a summary of their revolutionary contributions.

Petrus Peregrinus elucidated part of the mystery of the Chinese compass. They conducted the first systematic study of the magnet stone properties through experimentation to study the magnet stone property and the stone compass orientation toward the sky. In 1269, he explained in his famous book Letter on the magnet [8], summarized below.

Firstly, he cut magnet stones in the shape of spheres for his experiments. Why did he choose this geometric shape? Because he shared Aristotle's conception that the sky is spherical and also believed that the sky oriented the compass. He experimented with his spherical stone and an iron needle to show that it had two different ends, which he called the "North and South poles." The term poles used by Petrus Peregrinus will be kept in the magnetism vocabulary, even if their signification differs from our day, as we will see below.

Thus, according to Peter Peregrinus, the compass indicates the celestial North. How did he proceed to distinguish these two poles? First, he established certain parallelism between the magnet stone and the starry sky. Following Borvon [9], Peregrinus wrote, "in the sky, there are two remarkable points because the celestial sphere moves around them as around an axis. One is called the North Pole, the other the South Pole. Thus, in this stone, you find the same two points, one of which is called the North Pole and the other the South Pole."

Secondly, Peregrinus identified the properties of the North and South poles of the stone by performing an experiment where he placed the sphere in a wooden bowl on the water's surface. Once the stone was oriented, he named the pole of the sphere, which went towards the celestial North pole and the other South Pole. Then, he did the same experiment with another stone and, approaching it to the first, established the following rule: the North pole of a stone can attract the South pole of the other, and the South Pole is the North Pole. If, on the contrary, we approach the North Pole from the North Pole, we will see the stone that we carry flee on the water, the stone that we hold, and the same if we approach the South Pole from the South Pole. He also made the following observations: (1) by breaking the stone in two, each piece had its two North and South poles, and he never separated the poles and (2) magnetization of iron by contact with a magnet [10].

According to many historians, these experiments carried out the first systematic study through an experimental method to elucidate the properties of magnet stone orientation of the compass toward the sky.

According to many historians, these experiments carried out the first systematic study through an experimental method to elucidate the properties of magnet stone orientation of the compass toward the sky.

It is up to Norman to have studied the magnetic compass inclination and to measure its deviation. Indeed, thanks to his talent as an experimenter, he created a compass for such measure. How did he come to be interested in the problem of magnetic inclination? He was a skilled maker of navigational instruments, and he knew that for the compass needle to remain horizontal, it had to be balanced by placing a counterweight on the South side or shortening it from the North. He observed that the magnetic needle threaded into a ball of cork, in indifferent equilibrium in a bowl of water, bowed in the direction of the magnetic force; he measured this deviation to be 71°50' in London. He notes that the compass's orientation was due to turning toward, rather than being attracted to, a certain point and related his newly discovered deviation of the needle from the horizontal.

In his famous book The Magnet, written in Latin, which had a significant impact on the scientific world, published in 1600, Gilbert mentioned the works of Peregrinus and Norman [11]. The essential Gilbert's contribution in the study of magnetism remains to have attributed the directive power of the magnetic needle, not to the sky or the star Polar, like Peregrinus, but the Earth. Le Mouël and Poirier [12], in their book on the history of magnetism, quote Gilbert's words to explain why he referred to the Earth: "As the spherical form, which is the most perfect, agrees better with the Earth, which is a globe, and also as it is the most proper form for experiments, we propose to give our main demonstration by the aid of a magnet stone in the shape of a globe, as being the best and most appropriate. Then take a strong magnet stone, solid, of the right size, uniform, without blemish; on a turn [...] give it the shape of a ball."

How did Gilbert demonstrate the link between the compass's orientation and his magnet, a 'terrella'? For that, he made the following assumption: The Earth being magnetic in all its parts and the water not being, the needle must turn towards the Earth wherever it is because of its more significant quantity of magnetic matter. Gilbert procured a lodestone and fashioned it into a sphere using his lathe. He provocatively termed the resulting round magnet a 'terrella,' a 'little Earth.' Passing a compass needle around the terrella, he found that the needle pointed in different directions as the compass needle moved around the sphere, and he realized that this behavior mimicked the behavior of a compass needle at different locations around the Earth. The logic was inescapable: a plausible mechanism for the origin of the magnetic effect on a compass needle was the magnetism of the Earth itself. Planet Earth behaves like a giant terrella [4].

Earlier, we pointed out that Thales had an animistic view of the magnet stone when referring to his "soul." Gilbert also seems to have an animist conception since, for him, the magnet stone had "the power of the heart of the Earth." On this subject, Le Mouël and Poirier [12] point out that Gilbert's animist conception did not prevent her from "developing her idea within a scholastic framework."

Gilbert does not seem to consider the problem of the inclination of the compass needle studied by his compatriot Norman, even though he was aware of his work, as we have pointed out above. Thus, Gilbert commits two errors. Firstly, he considered that the Earth is magnetic in all its parts; secondly, he thought that the magnetic needle was pointing toward the poles of the Earth and, therefore, that the magnetic poles were confused with the geographical poles. He justified the phenomenon of declination by saying that it was due to uneven relief on the surface of the Earth. According to Le Mouël and Poirier [12], Gilbert justified his idea by experiments showing that the magnetic needle passed over a terrella presenting surface irregularities deflected by the excesses or the defects of magnetic material corresponding respectively to the bumps and hollows. Thus, according to Gilbert, the compass needle must point towards the Earth wherever it is. The astronomer Edmond Halley (1656-1742) showed that this "is not true in many cases, and most remarkably on the coast of Brazil, where the needle is so far from being attracted by the Earth that it departs from the meridian and lies N. by E., which is just parallel to the coast [12].

Without a doubt, significant progress has been made thanks to the work of Perigrinus, Norman and Gilbert, and other scientists on the magnetic stone. However, several questions remained unanswered, including the phenomenon of induced magnetism as the question of iron which acquired magnetic properties in rubbing it, for example, against the magnet stone, and that it lost its magnetism after a while. We will see in the following that, thanks to the strenuous efforts of many researchers, we will learn more about the magnetism of the magnet stone.

At the end of the 18th century, magnetism made significant progress thanks, among others, to the French engineer and physicist Coulomb (1736-1806); who demonstrated experimentally that magnetic attraction varied as the inverse of the square of the distance thanks to its "magnetic balance" built on the same principle as the electric balance with which he had measured the force of attraction between two electric charges of opposite signs. Note that in the case of "electric balance," the measurement was more accessible than with the magnetic scale. Why? We can separate the electric charges and know the charge quantity in electrostatics. Thus, the essential physical quantity in electrostatics is the electric charge. However, pour le magnétisme, Coulomb measured the force with which one pole of a magnet (for example, the North Pole) attracts the pole of the opposite name of another magnet (in this example, the South Pole), which poses a problem unlike electrical charges, one cannot separate the two poles in question. So he could not isolate "magnetic masses," South or North. However, to measure the attraction between the N and S poles between the two magnets, it was necessary to ensure that the magnets used were long enough so that the repulsive action of the other poles did not disturb the attractive action that Coulomb sought to measure.

Coulomb considers a short bar magnet a dipole of length l, carrying two "masses magnetic" m opposite at its ends to circumvent this conceptual difficulty. Thus, the physical quantity is not the "magnetic mass" but the "magnetic moment vector" of modulus M with M = ml.

By definition, the body's magnetic moment characterizing by its tendency to align with the direction of the magnetic field. Thus, the compass needle is a "magnetic double": it is therefore not attracted to a point on the Earth as Gilbert thought or a point in the sky as Peregrinus thought; it is instead subjected, in the uniform field of the Earth, to two parallel forces of opposite directions applied to the poles, that is to say to a couple, which made it orient itself by turning. Above we specified that Norman had made this observation while studying the inclination of the compass.

Also, we know from the experiments made by the Danish physicist and chemist Hans Christian Œrsted (1777-1851) and the English physicist Michael Faraday (1791-1867) that an electrical current produces a magnetic field. For example, placing a compass near an electric wire; its needle turns as a current runs through the wire. Also, wounding electric wire in several turns around the compass, its needle takes the direction of the solenoid axis, thus constituted as soon as the current circulates. These observations are at the origin of electromagnetism development, the merit of which goes to the French physicist and mathematician Marie-André Ampère (1775-1836) and the Scottish physicist and mathematician Maxwell (1831-1879).

Thus, the electric current creates a magnetic field like the Earth. For example, a surface S of a circular loop of wire traversing by a current I constitutes a magnetic pole whose moment M is proportional to the intensity of this current with M = S.I.

This revolutionary observation relates magnetism to moving electric charges. What about this phenomenon at the atomic level? It is the same. Indeed, an electron first has a magnetic moment associated with its intrinsic angular momentum, the spin (i.e., quantum characteristic of particles intimately linked to their rotational properties. It plays an essential role in the properties of matter.

Thereby, magnetism is a property of the incomplete electronic layers of atoms. For example, the hydrogen atom comprises an electron (i.e., negative electric charge) and a nucleus (i.e., positive electric charge). The electron is moving around the nucleus and around itself. However, a moving charge creates a magnetic field; therefore, the hydrogen atom is magnetic. However, the hydrogen molecule (H2) is not magnetic since its electronic layer is complete. It contains two electrons, and their movements are opposed, which cancels their magnetism. Also, we know that lodestone make of iron oxide Fe3O4.

The electrons of the atoms generally group in pairs of opposite spins, so the total magnetic moment is zero. These substances are said to be diamagnetic (i.e., a quantum phenomenon where the orbital movement of electrons around the atomic nucleus is modified) and only become magnetized in an external magnetic field. In this case, they induce a magnetic moment opposite the magnetic field's direction, which explains why the diamagnetic materials (e.g., zinc, lead, sulfur, metallic bismuth, and organic matter like benzene) are repelled by the magnets. In the case of iron, nickel, cobalt, and rare Earth, due to their electronic structure, their total magnetic moment is not null; therefore, they are a magnet. Among these magnetic materials, we distinguish paramagnetic, ferromagnetic, ferrimagnetic, and antiferromagnetic materials [13]; their essential properties are synthesized in table 1.

Note that the magnetic properties of a material are the result of the sum of its magnetic moments and the amount of the external magnetic field applied.

These materials (e.g., iron, nickel, cobalt, their alloys, and some rare earth) possess the property of becoming magnetic, i.e., becoming magnetized, when placed in a magnetic field and retaining some of this magnetism when the field is null. Moreover, the ferromagnetic substance magnetization depends not only on the magnetic field applied to it but also on the fields to which it has been previously subjected, on the remanent magnetization: this is the phenomenon of hysteresis (i.e., the properties of the material at a given instant depend not only on the parameters which describe it at this instant but also on its previous state).

Ferrimagnetic materials, i.e., the directions of the magnetic moments are parallel, and the directions of the neighboring magnetic moments are opposite, which should lead to zero global magnetism. However, here the amplitudes of the magnetic moments are slightly different. We observe spontaneous magnetization of the material, even without an external magnetic field. The material loses its magnetization at the Curie temperature (TC). French physicist Pierre Curie discovered this phenomenon in 1895. Many iron oxides (including magnetite: Fe2O3) or compound metal oxides (ferrites, such as PbFe12O19) are ferrimagnetic. Note that we build USB cables with ferrimagnetic materials to reduce high-frequency noise.

These materials (e.g., iron oxide: FeO, chromium: Cr) can be considered ferrimagnetic substances for which the magnetic moments of the two crystal lattices are equal and opposite. Consequently, the resulting magnetic magnetization is zero. There is a temperature analogous to the Curie temperature, called the Néel temperature for Louis Néel, a French physicist who, in 1936, gave one of the first explanations of antiferromagnetism. Above the Néel temperature at 132.26 K, the antiferromagnetic substance becomes paramagnetic.

We have known for several years that the Earth's core, located at a depth of about 5,100 km, is composed of two layers: an inner layer and an outer layer. The inner layer, referred to as the inner core, is solid and essentially composed of iron and nickel, ferromagnetic materials. We also know that its temperature is estimated between 3800°C and 5500°C depending on the depth.

Despite this high temperature, the core retains a solid shape due to the high pressure at this depth. As for the outer layer, the outer core is mainly composed of iron (between 80 and 85%) in the liquid state.

Liquid iron is subject to movements, i.e., the Earth moves around its geographical axis, and the Sun develops electric currents, as we have studied above with Oersted and Faraday. These currents generate a magnetic field similar to a bar magnet in Earth's heart.

Thanks to this terrestrial magnetic field, sailors and migratory birds have oriented themselves for millennia, not concerning the sky as Perigrinus thought. If the Earth lost its magnetism, would we be in danger? Unfortunately, the answer is yes because the Earth's magnetic field protects us from space, weather, and radiation. The winds that blow through the galaxy are radioactive, with the most dangerous coming from exploding stars or disintegrating black holes.

The danger also comes from the Sun, a thermonuclear furnace that throws out enormous, dangerous quantities during its eruptions. The Sun periodically ejects billions of tons of charged particles; it is the Solar wind. The Earth is often on its trajectory, but the magnetism deviates from these particles. Thus the Solar wind does not manage to cross the magnetic shield and circumvents it without threatening it. The simple incidences for us are the spectacular lights of the poles. These polar auroras are caused by solar particles trapped by our magnetic field and attracted by the poles once in the atmosphere. Nothing would stop the radiation if this field disappeared, which would be dangerous.

In the center of the Earth exists a fusion of iron and nickel. The movement of the Earth creates a magnetic field on the surface, and a contribution also comes from the upper layers of the Earth's atmosphere. Thus, the magnetic poles have moved a lot over the millennia. The study of an iron-containing stone produced during volcanic activities has concluded that there have even been several pole reversals in the last million years (compared to the geographical poles, which are fixed and located on the axis of rotation of the Earth). We are today in a period corresponding to an inversion; that is to say, the magnetic North Pole locating near the geographic South Pole (at almost 2000 km), and the magnetic South Pole locating at almost 2000 km from the geographic North Pole. Note that every hundred thousand years, there is a reversal of the Earth's magnetic poles; the North Pole becomes the South Pole and vice versa.

In conclusion, the path taken by scientists to understand the properties of the magnetic stone was long and arduous. It would be appropriate to study these developments with science students. Such studies show that the construction of scientific laws and theories does not follow a logical and continuous process as highlighted in traditional teaching and known periods of the scientific revolution (i.e., paradigm shift) in the sense of the historian of science Kuhn [14]. Also, such a study makes it possible to confront students' conceptions about the nature of science with those developed in the course of history [15-18].

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