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.
1 The names live on. The ampere is a unit of electric current; the oersted, in the no-longer-popular cgs system, is a unit of magnetic field strength; and the Biot-Savart law relates current to magnetic field strength.
2Philosophical Transactions, 39, 74 (1735).