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A chance discovery ?

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By Christine Blondel and Bertrand Wolff

Can electricity act on magnetism ?

In the article Feu électrique, Fluide électrique, ou Matière électrique in the Encyclopédie of Diderot and d'Alembert (1755), Louis-Guillaume Le Monnier wrote:

"A violent electric spark sometimes alters a compass or magnetizes small needles, according to the direction given by the spark. Furthermore it has long been observed that lightning (which is nothing but a big electric spark) is able to magnetize all kinds of iron & steel tools enclosed in boxes; to give to the nails of a vessel enough magnetic virtue to alter compasses quite from afar off; to change iron crosses of ancient bell towers, which have been repeatedly exposed to the vivid impressions of this terrible fluid, into real magnets."

There are many accounts in the 18th century on the magnetic effects caused by storms. Navigators have even observed an inversion of the poles of their compasses under the action of lightning.

Benjamin Franklin is one of those who have sought to reproduce these effects with electric sparks. Under the action of the discharge of powerful batteries of Leyden jars, he succeeded in magnetizing steel needles and inverting the poles of a magnet. But he believed he could also notice, contrary to Le Monnier’s assertion, that the direction of the magnetism acquired by the needle depends on its orientation in relation to the earth's magnetic meridian and not on the direction of the discharge. In 1773 he concluded that "these two powers [electricity and magnetism] have no relation to each other" and that "the apparent production of magnetism is only accidental". The ultimate cause would therefore be terrestrial magnetism, to which the thermal or mechanical effects of the discharge would give the possibility to act. This opinion was taken up by French physicists.

But another way to bring closer electricity and magnetism was based on the analogies presented by the interactions between the two types of electricity on the one hand, and between the two types of magnetism on the other. Two bodies charged of opposite electricities attract each other, as do a north pole and a south pole.

Wouldn’t the recently discovered battery - the "voltaic pile" -, with its two ends, positive and negative, have something in common with a magnetic bar? The term "poles", often used at that time to name the ends of a battery, finds here its origin. Couldn’t a magnet and a battery interact? Several physicists, in particular the German physicist Ritter, tried unsuccessfully to make an isolated battery act on a magnetic needle. Conversely, attempts were made to produce electrolysis using "batteries of magnets". Ritter made a battery float on the surface of water, in the hope that it will orient itself under the effect of earth's magnetism, to no avail. Wouldn't North and South poles be at the same time opposite electrical poles?

Ritter was known for his work in chemistry and galvanic electricity. By 1800 he had discovered electrolysis of water, independently of Nicholson and Carlisle. He also highlighted the principle of the accumulator. But the illusory character of his "discoveries" linking electricity and magnetism quickly appeared, hence a lasting distrust of the theses of scientists who, like Oersted, had supported Ritter’s assertions.

It would be noted that in the experiments looking for a possible interaction between a battery and a magnet, the terminals of the battery were not connected. As a matter of fact, one expected properties linked to the existence of isolated opposite electricity poles. Indeed the deviation of an electrometer connected to a pole of an isolated battery disappeared when the two poles were connected by a conductor (the voltage between the poles was too small to be detected by an electrometer) [See the vidéo La pile de Volta en court-circuit]. 

The failures of these attempts, however, were not enough to dissuade Oersted, guided by the conviction of a deep unity of the "forces" of nature. But his experimental process will be very different from that of Ritter, for Oersted experimented with a closed circuit.

Who was Hans-Christian Oersted (1777-1851) ?

Born in a small town without a school, the son of a poor apothecary, he received some teaching by a few educated inhabitants, and from the age of 12 he assisted his father, before entering the university of Copenhagen, where he studied pharmacy, medicine, physics, astronomy, and philosophy. He received his doctorate in philosophy at the age of 22, writing his thesis on the philosophies of Imannuel Kant. He began his professional life as an apothecary in Copenhagen, but as soon as Volta’s discovery was announced, he experimented on chemical decompositions using powerful voltaic batteries. From 1801 to 1803 he made a tour of Europe, meeting scientists, poets and philosophers - notably in Germany the philosophers Schelling and Fichte. He discussed at length with Ritter in Jena, then communicated his theses in Paris, and continued to correspond with him. Returning to Copenhagen in 1803, he obtained the chair of physics at the university in 1806.

Initial research

In a work translated into French in 1813 and with the title Researches on the Identity of Electric and Chemical Forces, Oersted included "Remarks on Magnetism" where one can read: "[From] the conviction that I was nourishing concerning the identity of electric and magnetic forces, [...] I resolved to test my opinion by experience. [...] It would be necessary to test whether electricity, in its most latent state, has no action on the magnet as such. This experience would not be without difficulty ".

Oersted knew Ritter's alleged "findings". The arguments in favor of the independence of electric and magnetic phenomena were well known to him. For example, whether a metal needle is magnetized or not, does not change its behavior in the vicinity of a body electrified by friction: in both cases it is electrified by influence and turns towards the electrified body. But Oersted's idea was that there is a hierarchy between the different forms of action of electricity : friction electricity - the manifestations of which are the most powerful -, "galvanic" electricity (that of the battery), and finally magnetism - the most "latent" form, that is to say the weakest. Only similar forms could interact, the other forms crossing "without interfering with each other". It is why it was useless to try to make friction (static) electricity act on a magnet. If Oersted, however, hoped to be able to detect an effect of galvanic electricity on "the magnet as such", while friction electricity did not distinguish between magnetized or non-magnetized needle, it was because in his opinion the "galvanic" form is closer to the magnetic form. But since it is only getting closer to it, we can expect that the effect will be weak and require particularly favorable circumstances: "this experience would not be without difficulty".

In fact, it was seven years later that Oersted took up the subject again. But in view of these reflections of 1813, whose obscure character he recognized in 1821, it is difficult to attribute to pure chance, as a tenacious legend does, the circumstances of his discovery.

The discovery

In the spring of 1820, during a private lesson given to a few students "already very advanced in science" - as he specified in one of his stories -, when he was bearing to incandescence a platinum wire connecting the two poles of a voltaic pile, Oersted found that the compass in the vicinity deviated slightly. According to his convictions on the electrical nature of heat, the incandescence of the wire manifested a favorable state of "electrical conflict", which then somehow overflowed the conductive wire. This is why he first made use of a very fine wire, while we know that the effect is much greater with a larger conductor, of low resistance and therefore passed through by a more intense current.
Under these conditions the magnetic effect is weak and seemed to Oersted, in his own words, "confusing". He waited to have "more leisure" to resume his research.


This engraving, published in Les Merveilles de la science by Louis Figuier (t. 1, 1867), contains inaccuracies: Oersted used a trough battery, not a Volta’s pile. But it shows Oersted’s interest in the study of heat: the parabolic reflector, in the right part of the engraving, allowed the study of the reflection and the focusing of "heat radiation", i.e. infrared radiation, which is reflected like light. This shows Oersted’s interest in the analogies between different physical phenomena.

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At the beginning of July he methodically resumed his experiments, in particular using a more powerful battery, until he could conclude in a brief four-page memoir, in Latin, published on July 21, 1820:

"the magnetic needle changes direction by the influence of the voltaic apparatus" [i.e. the battery], and "this effect takes place when the circuit is closed and not when it is interrupted". It is for having left the circuit open, he added, that famous physicists did not succeed, a few years ago, in showing this effect."

Several metals have been used "with equal success" for the "connective wire". Oersted also realized that a wire of significant diameter produces greater effects than the original fine platinum wire, and that incandescence is not required to create a magnetic effect. This is not explicit in the memoir but emerges from his laboratory notes and later accounts.

[See the vidéo L'expérience d'Oersted a7ec50d516ed625b786591b18bd05cb2.gif]

Oersted's interpretation of the experiment

For Oersted, the passage of electricity through the conducting wire, or connective wire, does not consist of a simple circulation of fluid: "We will designate the effect which manifests itself in the conductor and around it during the voltaic action, by the epithet of electric conflict. "
This conflict term is not further specified, but its effects are well defined: "the electric conflict only acts on the magnetic particles of matter", since it does not deflect non-magnetic needles. In addition, "it appears, from the facts presented, that the conflict is not limited to the conducting wire, but that it has around it a fairly extensive sphere of activity." The conflict between the two opposite electricities therefore extends in space.
Its way of acting is very surprising since "this conflict acts by spinning", forming a helix around the wire, since it orients the compass tangentially to a circle centered on the wire. And Oersted added: "negative electric matter" describes a right spiral and acts on the north pole while "positive electric matter" has a movement in the opposite direction and acts on the south pole without acting on the north pole.
This action was not at all what he was looking for! in 1827 he explained that he expected a radial effect emanating from the conductor, like luminous and calorific effects produced by the current. The compass would then have pointed towards the wire.
Ampère then slightly modified Oersted's interpretation, the magnetic action of the conductive wire no longer forming a helix but circles centered on the wire, following what are today called the magnetic field lines.

The reception of the experiment

Oersted's discovery undermined the schemes of "electrical science" of the time. Moreover, the philosophical opinions of its author aroused skepticism. If the reception by the learned world was enthusiastic, on the other hand few physicists adhered to the type of explanation given by Oersted. A period of intense theoretical and experimental turmoil began... [See the page Une expérience qui dérange... et passionne]

At the same time as it opened a new theoretical field, the discovery of Oersted was at the origin of the development of modern electricity.

Further reading


OERSTED, Hans Christian. Experiments on the Effect of a Current of Electricity on the Magnetic Needle, Annals of Philosophy, 1820, vol. 16, p. 273-277. [Read on Biodiversity Heritage Library]
OERSTED, Hans Christian. Expériences sur l'effet du conflict électrique sur l'aiguille aimantée. Annales de chimie et de physique, 1820, vol. 14, p. 417-425. [Read on Gallica]

MONNIER, Emmanuel. Les déviations inattendues de Christian Oersted. Les mathématiques expliquent les lois de la nature. Les Cahiers de Science & Vie, 67, février 2002, p. 12-19.
LOCQUENEUX, Robert. La naissance de l'électromagnétisme, une incompréhensible expérience. 200 ans de science, 1789-1989. Science & Vie Hors-série, 166, 1989, p. 46-53.
THUILLIER, Pierre. De la philosophie à l'électromagnétisme : Le cas Oersted. La Recherche, 21, 1990, p. 344-351.
DIBNER, Bern. Oersted and the Discovery of Electromagnetism. New York, 1961.
KIPNIS, Nahum. Chance in Science: The Discovery of Electromagnetism by H.C. Oersted. Science and Education, 14, 2005, p. 1-28.
CANEVA Kenneth L., The Form and Function of Scientific Discoveries, Dibner Library Lecture, Smithsonian Institution Libraries, November 16, 2000 [See the PDF]

 

A bibliography of "Secondary sources" on the history of electricity.



French version: March 2006 (English translation: January 2021)