Science › Classical physics
James Clerk Maxwell
1865· Person
ProvenTested many times; part of working physics.
Scottish physicist (1831–1879) who wrote the equations of electricity and magnetism and showed that light is an electromagnetic wave.
See it on the Science map →How it works
Maxwell took Faraday's pictures of lines of force and expressed them in mathematics. Between 1861 and 1865 he added one missing term, which revealed that changing electric and magnetic fields can sustain each other as a travelling wave. He also founded the statistical theory of gases, produced the first colour photograph (1861), and was the first director of the Cavendish Laboratory at Cambridge.
Why it matters
His equations underpin radio, power grids, radar, Wi-Fi and mobile phones. Einstein built special relativity directly on them.
How the work held up
Heinrich Hertz generated and detected radio waves in 1887–1888, moving at the speed Maxwell predicted. His gas theory correctly predicted that gas viscosity does not depend on pressure.
Still a mystery
He assumed his waves travelled through an 'ether'. The Michelson–Morley null result (1887) and Einstein's relativity (1905) did away with it.
Connections (2)
▸James Clerk Maxwell unified into four equations Electromagnetism (Maxwell's equations)Proven
The relationship
Maxwell gathered the known laws of electricity and magnetism, from Coulomb, Ampère, Gauss and Faraday, into one mathematical system. He found them inconsistent when currents change, and added a term (the 'displacement current'). With it the system closed into a complete theory in which a change in each field creates the other.
How the link was found
Maxwell, in papers of 1861–1862 and his 1865 'A Dynamical Theory of the Electromagnetic Field'. Oliver Heaviside rewrote them as today's compact four equations in the 1880s.
Why it's proven
Hertz's radio waves (1887–1888) confirmed the predicted waves. The equations still design every antenna, motor and circuit.
Open questions and mysteries
The equations survive Einstein's relativity unchanged, but at atomic scales they give way to quantum electrodynamics. Why electric charge comes in fixed units remains unexplained.
▸James Clerk Maxwell calculated the speed of light, revealing Light is an electromagnetic waveProven
The relationship
Maxwell used electric and magnetic constants measured by Wilhelm Weber and Rudolf Kohlrausch in 1856 to compute how fast his waves travel. The result, about 310,000 km per second, was very close to light speeds measured by Fizeau and Foucault. He concluded light is an electromagnetic disturbance, uniting optics with electricity.
How the link was found
Maxwell, in 'On Physical Lines of Force' (1861–1862), and fully in his 1865 paper.
Why it's proven
Hertz confirmed in 1887–1888 that electromagnetic waves travel at light speed. Today light's speed is fixed by definition at exactly 299,792,458 metres per second.
Open questions and mysteries
Maxwell assumed an ether. Why the speed is the same for every observer puzzled physicists until Einstein in 1905.