James Clerk Maxwell

James Clerk Maxwell (June 13, 1831 to November 5, 1879) was a Scottish physicist. He showed that electric and magnetic fields travel through space as waves at a speed that matched the measured speed of light, and concluded that light is one of them. He derived the distribution of molecular speeds in a gas, which Ludwig Boltzmann extended; he demonstrated the principle of three-color photography; and he showed by calculation that the rings of Saturn can be stable only if they are made of separate small particles. In 1867 he described, in a letter to a friend, a being that sorts molecules through a small hole and so appears to break the second law of thermodynamics, a figure that William Thomson later named a demon. Maxwell died at 48, of the cancer that had killed his mother at the same age.^[1]

The demon gave demonpore, inc., the first name of Molecular Reality Corporation, its name, and it is the subject of its own article, Maxwell's demon. This article is about the man.

Maxwell was born at 14 India Street in Edinburgh, in a house his parents had built, and grew up at the family's estate of Glenlair in Kirkcudbrightshire. His mother died when he was eight. In 1841 he went to the Edinburgh Academy, where he was known as "Dafty". Peter Guthrie Tait, one class below him, became a lifelong friend. At Cambridge he graduated from Trinity College in 1854 as Second Wrangler, and was bracketed equal with the Senior Wrangler, Edward Routh, in the competition for the Smith's Prizes.^[1]

He became professor of natural philosophy at Marischal College, Aberdeen, in November 1856, and after the merger of the Aberdeen colleges moved in 1860 to King's College London. In the spring of 1865 he left London and returned to Glenlair, where he wrote most of his Treatise on Electricity and Magnetism (1873). In 1871, reluctantly by one account, he accepted the first Cavendish chair of physics at Cambridge. He designed the Cavendish Laboratory, which opened on June 16, 1874, and from 1874 to 1879 he edited the unpublished papers of Henry Cavendish, who had left twenty packets of manuscript on electricity and published two papers. Maxwell copied them out by hand.^[1]

Maxwell's first work on color in the 1850s used a spinning top with paper discs of three colors in adjustable proportions, and supported the theory that the eye has three kinds of color perception. George Gabriel Stokes wrote to him on November 7, 1857 that the results "afford most remarkable and important evidence in favour of the theory of three primary colour-perceptions".^[1]

On May 17, 1861, Maxwell lectured at the Royal Institution on the theory, and showed a tartan ribbon photographed three times by Thomas Sutton, through red, green and blue-violet filters, and projected through the same filters on a screen so that the images overlapped. The colors came out, and the result is often called the first color photograph. Sutton's plates were almost blind to red and weak in green. In 1961 Ralph Evans suggested that the red record may have come from ultraviolet light that the dye reflected, so that the red channel would show where the ribbon reflected ultraviolet and not where it looked red.^[2]

The topic for the Cambridge Adams Prize of 1857 was the motion of Saturn's rings. Maxwell and Tait had talked about the problem as schoolboys. He worked on it at Aberdeen for two years and showed that the rings could be stable only if they consisted of a large number of small solid particles. The astronomer royal, George Airy, wrote that it was "one of the most remarkable applications of mathematics to physics that I have ever seen".^[1] In 1895 James Keeler's spectra showed that the inner part of the rings orbits faster than the outer, as separate particles obeying Kepler's laws would.^[3]

In 1860 Maxwell published a derivation of the distribution of speeds among the molecules of a gas at one temperature, an early statistical law in physics. The same paper predicted that the viscosity of a gas should not depend on its density, a result that seemed absurd.^[4] In the mid-1860s he measured the damping of oscillating discs in air at different pressures and found that it did not change, and in 1867 he gave the full theory.^[5][6] Viscosity gave the mean distance a molecule travels between collisions, and that distance, with the density of the gas, gave a size for the molecule; this was one of the routes by which the number of molecules in a given volume was first estimated, as described in Molecular reality (history of science).

The being that sorts molecules through a hole was a device for making a point about these statistics. Maxwell's own summary of its purpose, in a note to Tait, was "To show that the 2nd Law of Thermodynamics has only a statistical certainty".^[7]

Michael Faraday had pictured electric and magnetic effects as lines of force. Maxwell's first paper on the subject (1855) gave the lines a mathematical form by analogy with the flow of an incompressible fluid. In "On Physical Lines of Force" (1861 to 1862) he went further and built a model of the medium, a sea of tiny rotating vortices separated by particles that act as idle wheels, and from it obtained a term for the changing electric field that is now called the displacement current.^[8]

The model gave a speed for the transverse waves in the medium that depended only on electrical measurements. Using the values of Wilhelm Weber and Rudolf Kohlrausch, Maxwell calculated 310,740,000 meters per second. Hippolyte Fizeau had measured the speed of light as 314,858,000 meters per second. Maxwell wrote that the two agreed so exactly "that we can scarcely avoid the inference that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena."^[8] The modern speed of light is 299,792,458 meters per second, so his figure was 3.7 percent high and Fizeau's 5.0 percent.

The mechanical model was dropped in the paper that followed. "A Dynamical Theory of the Electromagnetic Field" was read to the Royal Society on December 8, 1864 and published in 1865. It states the theory in twenty equations for twenty variables, and shows that an electromagnetic disturbance travels as a wave at the speed of light.^[9] The Treatise of 1873 gathered it into a textbook. Oliver Heaviside in the 1880s reduced the system to four vector equations, which are what students now call Maxwell's equations, and Heinrich Hertz generated and detected the waves in the laboratory in 1887 and 1888, eight and nine years after Maxwell's death.^[10][11]

Maxwell wrote verse all his life. "Rigid Body Sings" is a song about colliding bodies, set to the tune of "Comin' Thro' the Rye". Lewis Campbell and William Garnett printed a selection in their biography of him in 1882.^[12]

Two results of Maxwell's are used in the analysis of a nanopore, and neither had a nanopore in view. In the Treatise he derived the conductivity of a liquid holding a dilute suspension of insulating spheres. If the spheres fill a fraction φ of the volume, the conductivity is that of the liquid multiplied by (1 − φ)/(1 + φ/2), or about 1 − 3φ/2 for small φ. Graham's history of the Coulter principle reports that Wallace Coulter, as a student of electrical engineering, had read of a method for calculating the resistance of such suspensions, and cites the Treatise for it.^[13][14] The second is the resistance of the current spreading out from a small opening, which Maxwell treated in the nineteenth century and which nanopore researchers call the access resistance.^[15]

The company's account of the name is that a colleague, on hearing Kent Kemmish describe sliding-occlusion nanopores, was reminded of cartoons of Maxwell's demon.^[16] A flyer posted in March 2026 adds the sentence "other work by Maxwell inspired the Coulter Counter."^[16] The founder's essay for the Sparky class of Epic Quest Bio lists Maxwell among the authors of the "spellbooks" that a Sparky commands, beside Faraday, Nyquist and Shannon, and, further on, Neher and Sakmann.^[17] The company's cartoon demon is Maxine, a small red imp.^[18]

The CGS unit of magnetic flux is the maxwell. Maxwell Montes, a mountain range on Venus, was named in 1979 and is the only feature on that planet named for a man.^[19][20] A submillimeter telescope on Mauna Kea is called the James Clerk Maxwell Telescope.

• Maxwell's demon
• Ludwig Boltzmann
• Coulter principle
• Jean Perrin
• Ernst Mach
• Sparky