For centuries, electricity and magnetism were understood as distinct and unrelated phenomena. Electricity was primarily observed in the form of static discharges or the steady flow generated by chemical batteries. Magnetism, conversely, was solely associated with lodestones and the Earth’s natural magnetic field. The groundbreaking discovery that linked these two forces into a single concept, electromagnetism, was made by Hans Christian Ørsted in 1820.
Electricity and Magnetism as Separate Forces
Before the nineteenth century, scientific study treated electricity and magnetism as separate branches of physics with no theoretical connection. Electricity was mostly investigated through experiments involving static charges, such as rubbing materials to generate sparks or using Leyden jars to store charge. The invention of the Voltaic pile, or chemical battery, around 1800 allowed for the first sustained, constant flow of electric current, providing a powerful new tool for researchers.
Magnetism, by contrast, was an ancient study focused on the properties of naturally occurring magnetic minerals like lodestone. Researchers understood that magnets possessed two poles, north and south, and that these poles exerted forces on each other, attracting opposites and repelling likes. This field of study was considered entirely distinct from the forces associated with electric charge and current. No experiment had successfully demonstrated that the two were intrinsically linked.
Hans Christian Ørsted and the Breakthrough Experiment
The Danish physicist Hans Christian Ørsted made the foundational observation in April of 1820 during a lecture demonstration. He had prepared an electric circuit powered by a Voltaic pile and had a magnetic compass nearby. When he connected the circuit, sending a current through a wire situated parallel to the compass needle, the needle immediately moved.
This deflection of the magnetic compass needle was a clear sign that the electric current in the wire was generating a magnetic field. Crucially, the needle did not point toward the wire. Instead, the needle rotated to stand nearly perpendicular to the wire, demonstrating that the magnetic field lines formed circles around the current path. Ørsted quickly repeated the experiment, confirming that reversing the direction of the electric current caused the compass needle to rotate in the opposite direction.
The discovery was not purely accidental, as Ørsted had long suspected a relationship between electricity and magnetism. However, the precise nature of the interaction—that an electric current created a circular magnetic field—was entirely unexpected. His initial publication in July 1820, a brief four-page pamphlet, immediately alerted the European scientific community to the magnetic effect of an electric current. This single observation established the qualitative link and ushered in the era of electromagnetism.
Quantifying the New Relationship
Ørsted’s qualitative discovery prompted an immediate reaction from scientists across Europe, particularly in France. The most significant follow-up work came from the French physicist André-Marie Ampère. Ampère rapidly began a series of experiments and mathematical analyses that established the quantitative laws governing this new interaction, which he termed “electrodynamics.”
Ampère demonstrated that magnetism was a property of moving electric charges, not permanent magnetic materials. He showed that two parallel current-carrying wires would attract each other if the currents flowed in the same direction, and repel if the currents flowed in opposite directions. This established that electric currents could exert forces on one another through the magnetic fields they produced.
Furthermore, Ampère developed a mathematical framework that described the force between current elements, providing the first quantitative law of electromagnetism. This work, alongside the contributions of Jean-Baptiste Biot and Félix Savart, provided the necessary mathematical tools. The Biot-Savart Law offered a way to determine the strength and direction of the magnetic field at any point in space based on the geometry of the current-carrying wire. These efforts transformed Ørsted’s observation into a rigorous, predictable physical science.
The Final Unification of Light and Electromagnetism
The laws developed by Ampère, Biot, and Savart defined how electricity produced magnetism, but the relationship was not yet symmetrical. A decade later, in 1831, Michael Faraday demonstrated the reverse phenomenon: electromagnetic induction. He showed that a changing magnetic field could induce an electric current in a nearby wire. This discovery is the physical principle behind nearly all modern electrical generation.
The final unification of these discoveries was achieved by the Scottish physicist James Clerk Maxwell in the 1860s. Maxwell synthesized the experimental laws of Ørsted, Ampère, and Faraday into a coherent set of four equations. These equations showed that electric and magnetic fields are two inseparable components of a single electromagnetic field.
Maxwell recognized a symmetry in the relationship: a changing magnetic field creates an electric field, and a changing electric field must also create a magnetic field. Solving his equations predicted the existence of self-propagating electromagnetic waves traveling through space. When he calculated the speed of these waves, he found it was identical to the known speed of light. This led to the conclusion that light itself is an electromagnetic wave, unifying optics, electricity, and magnetism into one grand theory.