Ørsted's Compass Deflection
Electricity and magnetism are connected
First published: H. C. Ørsted, "Experimenta circa effectum conflictus electrici in acum magneticam" (1820).
A current-carrying wire deflects a nearby compass needle. Electricity and magnetism, long thought independent, are unified.
During a lecture in April 1820, Ørsted noticed that a compass needle near a current-carrying wire deflected perpendicular to the current. The phenomenon was a complete surprise — electricity and magnetism had been studied as independent domains. The discovery launched electromagnetism as a unified field: Ampère soon derived the quantitative law relating current to magnetic force; Faraday demonstrated the reciprocal effect (induction) in 1831; Maxwell unified the whole theory in 1865. Ørsted's observation is the empirical seed of the electrical and electronic civilisation that followed.
Formulation
Compass needle placed near straight conductor. Current switched on: needle deflects to align perpendicular to wire (along the circular magnetic field lines around it). The current generates a magnetic field; magnetism and electricity are coupled.
Dimensions Engaged
Matter
Two previously distinct classes of phenomena (electric, magnetic) are revealed as aspects of one underlying interaction.
Energy
Electrical and magnetic forms of energy are interconvertible — laying the foundation for the electrical revolution.
Space
Magnetic field structure around a current — the first empirical anchor for field-theoretic physics.
Responses — How Schools Engage
Affirms / takes the bait 5
A canonical empirical discovery and the foundational moment of electromagnetic physics. Unification of forces becomes a guiding methodological principle.
Electromagnetism is a real unified phenomenon, not merely a calculational convenience.
The unification reveals structural identity beneath apparent diversity: a recurring theme in fundamental physics.
Field-theoretic physics begins here: electromagnetic phenomena are best understood as processes in spatially extended fields rather than as static configurations of substantial particles.
A clean operational discovery: a previously unsuspected dependence between measurable quantities is revealed by direct experiment.
Related Experiments
Experiments engaged by an overlapping set of schools — likely to surface the same fault lines.
Further reading
- Ørsted (1820), op. cit.
- Williams, *The Origins of Field Theory* (1966)
Related Historical Debates
Debates that share dimensions and/or aligned schools with this experiment.
Personas Most Aligned With This Experiment
Ranked by total declared-influence weight in the schools that respond to this experiment.
Works Most Aligned With This Experiment
Ranked by total declared-influence weight in the schools that respond to this experiment.
Related Contemporary Dilemmas
Dilemmas that engage the same dimensions as this experiment.