Ampère's Current Experiments: Forces Between Wires

In 1820, Hans Christian Ørsted of Denmark reported that a current-carrying wire deflected a nearby magnetic needle. François Arago brought the news to Paris and presented and reproduced related experiments at the French Academy of Sciences in September. André-Marie Ampère then began an intensive investigation: if a current could act on a magnetic needle, could currents also exert directly visible mechanical effects on one another?

This question changed the object of experimentation. Rather than merely repositioning a needle and wire, Ampère made the current-carrying wire itself movable and examined whether another circuit could move it. Batteries, supports, and wires thus formed an apparatus for studying force, opening a new experimental approach to the relationship between electricity and magnetism.

References: [3] [1]

On September 18, Ampère outlined his ideas at the Academy, including winding wires into flat spirals and helices and explaining magnetic action through currents. On September 25, he demonstrated attraction and repulsion between energized spirals. Reversing the current changed the action observed; replacing one coil with a magnet allowed a comparison between the behavior of the two kinds of object.

Coils combined the effects of several wire segments, making the result easier to see, but Ampère also needed to study simpler geometrical arrangements. Two straight wires placed parallel to one another, one fixed and the other able to swing, revealed attraction between currents flowing in the same direction and repulsion between opposite currents. This relationship became central to his reports in the autumn of 1820. Current direction was an important determinant of the effect, which could not simply be assimilated to the repulsion of like electrostatic charges.

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The difficulty was to let a wire move freely while keeping the circuit connected. Ampère mounted a movable conductor in a rotating frame supported on small pivots, with insulating components such as glass separating conducting parts that needed to remain apart. Counterweights shifted the frame's center of gravity so that even small current effects could produce a noticeable deflection. Suitable positioning of the fixed and movable wires made it possible to compare different connections.

The moving contacts also had to conduct reliably. Recalling his experiments, Ampère stressed that one previously unsuccessful trial worked only after drops of mercury were added at the contact points. Here mercury provided a movable electrical connection, showing that reproducibility depended not only on theory but also on whether the contacts truly completed the circuit. He also proposed shielding the apparatus with a glass cover to prevent air currents from moving the lightweight frame.

The experimenter could then reverse the connections of just one circuit, see whether attraction became repulsion, and break the current to check whether the movement ceased. Such comparisons separated current-induced motion from imbalance in the supports, drafts, or poor contacts. An apparently simple phenomenon required a whole set of arrangements before it could become evidence open to discussion.

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From the similar behavior of current-carrying coils and magnets, Ampère developed the hypothesis that closed currents also existed within magnets. This did not mean that wires had actually been observed inside a magnet. It was an attempt to explain wires, magnetic needles, and terrestrial magnetism through the same interaction. The hypothesis in turn suggested new coil shapes and experimental configurations.

Moving from attraction and repulsion to a quantitative law required an account of how force varied with current strength, distance, and conductor direction. Over the following years, Ampère studied different equilibrium arrangements, dividing conductors into very short current elements and seeking a mathematical expression whose summed effects would give the action of a complete circuit. Comparing effects through the equilibrium of a movable component avoided some of the difficulties of directly measuring extremely small forces.

The new experimental facts established in 1820 and the later formula for forces between current elements therefore belong to different levels of explanation. The former showed that currents genuinely interacted; the latter also involved mathematical modeling of tiny constituent parts and the manner of their action. The sequence of derivation presented in a finished book cannot be treated as a daily record of discovery.

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Ampère published his early research progressively and reorganized it in later collections. The surviving 1821 separate publication on the mutual action of two electric currents is a revised version of journal papers from 1820. His systematic theory appeared as a separate Paris publication in 1826 and was also included in an Academy memoir volume published in 1827 but bearing the nominal year 1823. The year of a report, a volume's designated year, and its actual publication date must therefore be distinguished.

These studies provided a foundation for research into producing controllable mechanical effects with electric currents and using mechanical effects to compare currents. Later electromagnetic instruments and motor theory developed this approach further, but Ampère had not already completed those devices in 1820. The specific achievement at this point was to make attraction and repulsion between currents reproducible experimental phenomena in Paris, with conditions that could be varied, and to establish a new direction of electrodynamic research.

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