E2. Early Links Between Electricity And Magnetism

Hans Christian Oersted is a hero to physics teachers everywhere because he made an important discovery in physics while teaching. In 1820, in Copenhagen, at the end of a lecture on the subjects of electricity and magnetism, he happened to place a compass needle—a small magnet—near a wire carrying an electric current. The compass needle swung to a new direction, indicating the existence of a magnetic force (more exactly, a torque) that could be attributed only to the nearby electric current. Apparently the flow of electric charge in the wire created magnetic effects (in later terminology, it created a magnetic field), notable because stationary charge had long been known to have no magnetic effects. Whether this experiment was carried out by chance or by design is uncertain. In any case, Oersted was prepared to capitalize on the discovery. As the subjects of his lecture suggest, he already believed in the existence of a connection between electricity and magnetism. Upon the discovery of the connection, he quickly established the pattern of what we now call the lines of magnetic field. They trace circular paths about a current-carrying wire, very different indeed from the radial lines of magnetic force emanating from a magnetic pole at rest. Other scientists, especially in France, were drawn to investigate Oersted’s discovery. Within a few months of the arrival in Paris of the news of Oersted’s discovery, Jean Baptiste Biot, Félix Savart, and André-Marie Ampère had worked out in mathematical detail many aspects of the links between electric currents and magnets.¹

What exactly is the content of Oersted’s discovery? I can best express it from a modern point of view. (1) A moving charge—or equivalently, an electric current—creates a magnetic field. Therefore the pole of a magnet placed near a current will experience a force (the magnet as a whole will experience a torque). Because of Newton’s third law, verified to be valid for this new effect, the magnetic pole in turn must cause the moving charge or current to experience a force, equal and opposite to that felt by the magnetic pole. Thus a corollary to the creation of a magnetic field by a moving charge is this: (2) A magnetic field exerts a force on a moving charge. Finally, because a moving charge both creates a magnetic field and feels a force in a magnetic field, no magnets are necessary at all to demonstrate the new effect: (3) One current will exert a force on another current. Since a wire carrying current contains moving charge but normally has no net electrification, any force exerted by one current-carrying wire on another is necessarily distinct from the electrostatic force of Coulomb. These were the important new discoveries of 1820 that launched the combined science of electromagnetism.

Eighty-five years before Oersted’s discovery, the following description of some evidence for a link between electricity and magnetism was published in England.²

A tradesman of Wakefield, having put up a great number of knives and forks in a large box, and having placed the box in the corner of a large room, there happen’d in July, 1731, a sudden storm of thunder, lightning, etc., by which the corner of the room was damaged, the Box split, and a good many knives and forks melted, the sheaths being untouched. The owner emptying the box upon a Counter where some Nails lay, the Persons who took up the knives, that lay upon the Nails, observed that the knives took up the Nails.

The electrical discharge had turned the knives into magnets—it had created magnetism. Despite this and earlier indirect pieces of evidence for a connection between electricity and magnetism, the scientific study of this link had to await the development of an important new tool, the chemical cell, which made steady currents possible.

---

---

⇐ PREVIOUS ESSAY | NEXT ESSAY ⇒