James Clerk MaxwellThe quiet genius who unified electricity, magnetism and light

A Breadth of Genius — Before Electromagnetism

What makes Maxwell extraordinary is that the work for which he is most famous was only one peak in a whole mountain range.

The Kinetic Theory of Gases

In Illustrations of the Dynamical Theory of Gases (1860), Maxwell treated a gas as a swarm of molecules in random motion and used probability to derive the distribution of their speeds — the first time a physical law was given a statistical foundation. Refined by Ludwig Boltzmann, it became the Maxwell–Boltzmann distribution, a cornerstone of statistical mechanics.

Maxwell's Demon

In a letter to Peter Guthrie Tait on 11 December 1867, Maxwell imagined a "finite being" controlling a tiny door between two boxes of gas, letting fast molecules through one way and slow ones the other, seemingly cheating the second law of thermodynamics. The deeply religious Maxwell never used the word "demon" — that label was supplied by Lord Kelvin. The thought experiment still drives debate linking thermodynamics and information theory today.

The World's First Colour Photograph

Maxwell worked out that any colour could be reproduced by mixing red, green and blue light — the principle of additive colour, and the foundation of every screen you own. To demonstrate it, at a Royal Institution lecture in London in 1861, he had the photographer Thomas Sutton photograph a tartan ribbon three times, through red, green and blue filters, then projected the three positives through matching filters to recombine them into one full-colour image.

There is a delicious accident in the story: Sutton's plates were essentially blind to red light, and the demonstration only worked because the red dye in the ribbon happened to reflect ultraviolet that leaked through the red filter — a fact not understood until Kodak researcher Ralph M. Evans reproduced the experiment and explained it in Scientific American in November 1961. See our full article on Maxwell's first colour photograph.

The Equations — The Heart of It All

By the mid-19th century, electricity and magnetism were a patchwork of separate laws. Michael Faraday had shown that a changing magnetic field produces electricity, and had the visionary but un-mathematical idea of "lines of force" filling space. André-Marie Ampère had quantified the magnetism produced by currents; Carl Friedrich Gauss had described how charges produce electric fields. What was missing was a single mathematical framework. Maxwell supplied it.

In three great works he built the theory: On Faraday's Lines of Force (1855–56); On Physical Lines of Force (1861–62), which introduced his decisive new idea; and A Dynamical Theory of the Electromagnetic Field (1865), which stripped away the mechanical scaffolding to present the field itself as the reality. He gathered it all into the monumental two-volume Treatise on Electricity and Magnetism in 1873.

The crucial stroke was the displacement current. Ampère's law, as it stood, was mathematically inconsistent for changing fields. Maxwell added a new term — with no experimental evidence behind it at the time — to make the mathematics whole. The consequence was breathtaking: a changing electric field must itself generate a magnetic field. Combine that with Faraday's induction, and electric and magnetic fields can take turns regenerating one another and propagate through empty space as a wave.

When Maxwell calculated the speed of that wave from purely electrical measurements, he obtained about 310,740,000 metres per second — startlingly close to the measured speed of light. He drew the inevitable conclusion: light itself is an electromagnetic disturbance.

Maxwell did not live to see it proved. In 1887–88, eight to nine years after his death, the German physicist Heinrich Hertz generated and detected electromagnetic waves in his laboratory, measured their speed, and confirmed they behaved exactly as Maxwell's mathematics demanded.

The Man Behind the Mathematics

In 1858 Maxwell married Katherine Mary Dewar at Old Machar, Aberdeen. Katherine was no mere bystander to his science — she assisted in his colour-vision experiments and later made the painstaking temperature measurements for his work on the viscosity of gases. When Maxwell nearly died of smallpox, he credited her nursing with saving his life.

His career carried him across Britain: Professor of Natural Philosophy at Marischal College, Aberdeen (1856–1860); King's College London (1860–1865); five productive years back at Glenlair (1865–1871), where he wrote much of the Treatise; and finally, from 1871, the first Cavendish Professor of Physics at Cambridge, where he designed and oversaw the building of the Cavendish Laboratory — which opened in 1874 and went on to become one of the most famous laboratories in the world.

For all his seriousness of purpose, Maxwell was a famous wit who delighted in puns and parody. Tait suggested that Maxwell sign his letters "dp/dt", because in Tait's own thermodynamics textbook the equation dp/dt = JCM happened to spell out Maxwell's initials — and Maxwell duly adopted the joke. (It is a mathematical in-joke about his initials, not, as is sometimes claimed, a quip about a Scottish accent.) His faith was deep but unshowy, and threaded quietly through both his life and his letters.

Death and Legacy

Maxwell died on 5 November 1879, aged just 48, of abdominal cancer — the same disease, at a strikingly similar age, that had killed his mother. Most authoritative sources, including the MacTutor archive, record that he died in Cambridge; some popular accounts say Glenlair, where he was deeply rooted and near which, at Parton Kirk, he is buried. Hertz's confirmation of his waves came within a decade; special relativity was built on his equations a generation later.

His standing today is immense. In Physics World's 1999 millennium poll of 100 leading physicists, Maxwell was voted the third-greatest physicist of all time, behind only Einstein and Newton and ahead of Bohr, Heisenberg, Galileo and Feynman. A statue of him by Alexander Stoddart — seated with his dog Toby at his feet and a colour-top in his hands — was unveiled at the east end of George Street in Edinburgh on 25 November 2008. His birthplace at 14 India Street houses the James Clerk Maxwell Foundation.