James Clerk Maxwell introduced the displacement-current term that completed the mathematical symmetry of classical electromagnetism.
Maxwell added this term to Ampère’s circuital law in the 1860s. It represents the magnetic effect associated with a changing electric field, including the field between capacitor plates while a capacitor is charging. This addition made the equations compatible with charge conservation.
The completed equations predicted that changing electric and magnetic fields could sustain one another and travel through space as electromagnetic waves. Maxwell calculated that their speed matched the measured speed of light, leading him to identify light as an electromagnetic phenomenon.
Heinrich Hertz later produced and detected radio waves experimentally, confirming a major consequence of Maxwell’s theory. Oliver Heaviside subsequently helped express Maxwell’s original equations in the compact vector form commonly taught today.