Essay on the Theory of Electricity and Magnetism

Date 1759

Inspired by Sir Isaac Newton’s mathematical explanation of gravitational force, Aepinus’s Essay on the Theory of Electricity and Magnetism provided the first systematic, mathematical analysis of the forces of electricity and magnetism.

Locale St. Petersburg, Russia

Key Figures

• Franz Maria Ulrich Theodor Hoch Aepinus (1724-1802), German natural philosopher
• Sir Isaac Newton (1642-1727), English natural philosopher and mathematician
• Benjamin Franklin (1706-1790), American printer, statesman, and scientist
• Leonhard Euler (1707-1783), Swiss mathematician and Aepinus’s patron in Berlin

Summary of Event

In addition to being called the “Age of Reason,” the eighteenth century Enlightenment is often called the “Age of Newton” because of the immense influence Sir Isaac Newton’s ideas had not only on science but also on intellectual and cultural history. Following Newton’s great success in explaining the fundamental laws of the universe in his Philosophiae naturalis principia mathematica (1687; The Mathematical Principles of Natural Philosophy, 1729; best known as the Principia), other scientists tried to apply his synthetic, mathematical approach to chemistry and, in the case of Franz Maria Ulrich Theodor Hoch Aepinus, to electricity and magnetism.

Benjamin Franklin, who also was influenced by Newton, made experimental and theoretical contributions to the study of electricity that had a transformative effect on Aepinus. Franklin believed that he could explain electrical phenomena through the properties and actions of a subtle electrical fluid that could exist in material bodies either in surplus or in deficiency. Franklin used the mathematical terms “positive” (or “plus”) for objects with more than the normal amount of this electrical fluid and “negative” (or “minus”) for objects with less than the normal amount. Implied in Franklin’s analysis is the idea that electricity is a measurable quantity that can be neither created nor destroyed but can be moved from object to object. This idea was later generalized as the principle of the conservation of electric charge.

Franklin described the electrical fluid as “subtle,” since it could easily permeate such dense materials as metals, and he used this single fluid to explain such phenomena as electrical neutralization (that is, a parity of positive and negative electrical charge), the Leyden jar, and his lightning rod. In his explanation of the Leyden jar, a primitive device for storing electricity, Franklin used the Newtonian notion of electrification by influence (rather than by contact), and his lightning rod made use of his observation that pointed conductors were more efficient in attracting electricity than were rounded ones.

Despite Franklin’s successes, his electrical ideas had some serious deficiencies. For example, his ideas failed to explain the repulsion that existed between negatively charged objects (how could a lack of fluid cause a repulsion?). Furthermore, other scientists, including Aepinus, found Franklin’s notion of an “effluvia,” an electrical atmosphere surrounding charged objects, unsatisfactory.

Although Aepinus was appointed as an astronomer at the Berlin Academy of Sciences in 1755, he became fascinated by electricity following Franklin’s invention of the lightning rod in 1750. Aepinus lived with Swiss mathematicianLeonhard Euler, who supported his research. Johann Albrecht Euler, Leonhard’s son, and Johan Carl Wilcke, a Swede who was first Aepinus’s student and then his collaborator, helped conduct the experiments that refuted Franklin’s notion of electical effluvia. However, Aepinus’s intention in these experiments was not to overthrow Franklin’s single-fluid theory of electricity but to save it. Aepinus explained the troubling experimental fact that negatively charged objects repelled each other by assuming that the particles of ordinary matter possess inherent repulsive forces. Electrical fluid, Aepinus hypothesized, somehow hindered the natural repulsion of matter. When experimenters removed electrical fluid from objects, the repulsive forces between the particles of ordinary matter were unmasked, leading to the observed repulsion.

While at Berlin, Aepinus, following a suggestion by Wilcke, began research on tourmaline, a silicate crystal that had unusual thermoelectric properties. When Aepinus and Wilcke heated one face of a tourmaline crystal, it acquired an electric charge, but, to their surprise, the opposing face had the opposite charge. Aepinus became fascinated with the similarity between the positive and negative sides of this crystal, on one hand, and the north and south poles of an iron magnet, on the other. He theorized that, just as electrification was a consequence of the movement of electrical fluid, so magnetization was the result of a relocation of a subtle magnetic fluid within the iron. Aepinus pursued these ideas, without Wilcke, in St. Petersburg, Russia, where, in 1757, he became a member of the Imperial Academy of Sciences and where he tutored the son of Catherine the Great.

To solidify his prestigious position, Aepinus published, more quickly than he had originally intended, his Tentamen theoriae electricitatis et magnetismi (1759; Essay on the Theory of Electricity and Magnetism, 1979, also known as the Tentamen). The basic theme of the Tentamen is the analogy between electricity and magnetism. Like Newton, who used mathematics to explain terrestrial and celestial motions, Aepinus attempted to use mathematical methods to develop a cohesive explanation of electrical and magnetic phenomena from certain forces, without speculating about the mechanisms behind these forces.

For Aepinus, the forces guiding electrical and magnetic phenomena included attractions between the electric fluid and matter, repulsions between particles of the electric fluid, and repulsions between the particles of ordinary matter. He realized that these attractive and repulsive forces were proportional to the excess or deficiency of the electric fluid. He also analyzed induction, the phenomenon in which an object becomes electrified by a charged object that is brought near to it but prevented from making contact with it. Aepinus explained this phenomenon through his interpretation of the electrical force, which could act at a distance like gravity, and the mobility of the electric fluid in the approached object.

The Tentamen also contains Aepinus’s theory of magnetism. He believed that the north and south poles of a magnet were locations at which the magnetic fluid either surpassed or fell short of its nonpolar amount. He attributed the permanence of polarity in iron magnets to the strong attraction between the magnetic fluid and the particles of iron. Strong similarities therefore existed between Aepinus’s analysis of electricity and his treatment of magnetism.

Significance

Aepinus’s Essay on the Theory of Electricity and Magnetism is considered one of the most original and significant books in electricity. This work was important for many reasons. Using Newton’s analysis of forces that act at a distance, Aepinus provided the first mathematical treatment of electricity and magnetism. Indeed, some scholars see this application of the Newtonian concept of force as his principal contribution to the field. Although Aepinus did not discover the inverse-square law for the electrostatic force, he developed the ideas and methods that would enable later physicists to make this and other discoveries.

That it would take more than a quarter century for Charles Coulomb mathematically to describe the force between particles of the electric “fluid” shows that Aepinus was ahead of his time. Because of the highly mathematical approach of the Tentamen, it initially attracted few readers. René Just Haüy, a French mineralogist, aided the diffusion of Aepinus’s ideas when he published an excellent summary of them in 1780. This popularization stimulated some scientists to read the Tentamen, where they discovered models of how to mathematize the analysis of electricity and magnetism.

Henry Cavendish became an important disciple of Aepinus’s single-fluid theory of electricity and Alessandro Volta, famous for inventing the battery, had a deep respect for Aepinus’s achievements, even though Volta, who did not understand advanced mathematics, took a different approach to the quantification of electricity. A major characteristic of the Enlightenment was its quantifying spirit, the quest to put ideas about nature into mathematical form in order to understand them better. Aepinus’s work was very much a part of this quest, and his work continued to stimulate crucial new experiments and discoveries well into the nineteenth century.

Bibliography

Heilbron, John L. Electricity in the Seventeenth and Eighteenth Centuries: A Study of Early Modern Physics. New York: Dover, 1999. This reprint, with a new preface by the author, makes widely available a book originally published by the University of California Press in 1979. Aepinus’s contributions to electricity are extensively and insightfully discussed. Sixty-eight-page bibliography of primary and secondary sources. Index.

Home, Roderick W. The Effluvial Theory of Electricity. New York: Arno Press, 1981. Home, who has published extensively on Aepinus and eighteenth century electricity, began his interests with his Indiana University dissertation (1967), which he expanded and updated in this book. Bibliography and index.

Pancaldi, Giuliano. Volta: Science and Culture in the Age of Enlightenment. Princeton, N.J.: Princeton University Press, 2003. Besides being an excellent scientific biography of Volta, this book contains much analysis of electrical activities in the eighteenth century, including experiments, ideas, and theories of Aepinus. Extensive notes to each of the chapters, twenty-eight-page bibliography of primary and secondary sources, and an index.

Roller, Duane, and Duane H. D. Roller. The Development of the Concept of Electric Charge: Electricity from the Greeks to Coulomb. Cambridge, Mass.: Harvard University Press, 1967. A succinct history of early electricity, with an analysis of the “Franklin-Aepinus one-fluid conceptual scheme.” Questions for students on each of the sections, a bibliography, but no index.

Whittaker, Edmund. A History of the Theories of Aether and Electricity. New York: Dover, 1989. This reprint is a combination of a two-volume work originally published in 1951 and 1953. Whittaker discusses Aepinus’s contributions in chapter 2, “Electric and Magnetic Science Prior to the Introduction of Potentials.” Each volume ends with indexes of subjects and of authors cited.