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Summary Article: Maxwell, James Clerk (1831-1879)
from The Hutchinson Dictionary of Scientific Biography

Place: United Kingdom, Scotland

Subject: biography, physics

Scottish physicist who discovered that light consists of electromagnetic waves and established the kinetic theory of gases. He also proved the nature of Saturn's rings and demonstrated the principles governing colour vision.

Maxwell was born in Edinburgh on 13 November 1831. He was educated at Edinburgh Academy 1841-47, when he entered the University of Edinburgh. He then went on to study at Cambridge in 1850, graduating in 1854. He became professor of natural philosophy at Marischal College, Aberdeen, in 1856 and moved to London in 1860 to take up the post of professor of natural philosophy and astronomy at King's College. On the death of his father in 1865, Maxwell returned to his family home in Scotland and devoted himself to research. However in 1871 he was persuaded to move to Cambridge, where he became the first professor of experimental physics and set up the Cavendish Laboratory, which opened in 1874. Maxwell continued in this position until 1879, when he contracted cancer. He died in Cambridge on 5 November 1879, at the early age of 48.

Maxwell demonstrated his great analytical ability at the age of 15, when he discovered an original method for drawing a perfect oval. His first important contribution to science was made from 1849 onwards, when Maxwell applied himself to colour vision. He revived the three-colour theory of Thomas Young and extended the work of Hermann von Helmholtz on colour vision. Maxwell showed how colours could be built up from mixtures of the primary colours red, green, and blue, by spinning discs containing sectors of these colours in various sizes. In the 1850s, he refined this approach by inventing a colour box in which the three primary colours could be selected from the Sun's spectrum and combined together. Maxwell confirmed Young's theory that the eye has three kinds of receptors sensitive to the primary colours and showed that colour blindness is due to defects in the receptors. He also explained fully how the addition and subtraction of primary colours produces all other colours, and crowned this achievement in 1861 by producing the first colour photograph to use a three-colour process. This picture, the ancestor of all colour photography, printing, and television, was taken of a tartan ribbon by using red, green, and blue filters to photograph the tartan and to project a coloured image.

Maxwell worked on several areas of enquiry at the same time, and from 1855 to 1859 took up the problem of Saturn's rings. No-one could give a satisfactory explanation for the rings that would result in a stable structure. Maxwell proved that a solid ring would collapse and a fluid ring would break up, but found that a ring composed of concentric circles of satellites could achieve stability, arriving at the correct conclusion that the rings are composed of many small bodies in orbit around Saturn.

Maxwell's development of the electromagnetic theory of light took many years. It began with the paper ‘On Faraday's lines of force’ (1855-56), in which Maxwell built on the views of Michael Faraday that electric and magnetic effects result from fields of lines of force that surround conductors and magnets. Maxwell drew an analogy between the behaviour of the lines of force and the flow of an incompressible liquid, thereby deriving equations that represented known electric and magnetic effects. The next step towards the electromagnetic theory took place with the publication of the paper ‘On physical lines of force’ (1861-62). Here Maxwell developed a model for the medium in which electric and magnetic effects could occur (which might throw some light on the nature of lines of force). He devised a hypothetical medium consisting of an incompressible fluid containing rotating vortices responding to magnetic intensity, separated by cells responding to electric current.

By considering how the motion of the vortices and cells could produce magnetic and electric effects, Maxwell was successful in explaining all known effects of electromagnetism, showing that the lines of force must behave in a similar way. However Maxwell went further, and considered what effects would be caused if the medium were elastic. It turned out that the movement of a charge would set up a disturbance in the medium, forming transverse waves that would be propagated through the medium. The velocity of these waves would be equal to the ratio of the value for a current when measured in electrostatic units and electromagnetic units. This had been determined by Friedrich Kohlrausch (1840-1910) and Wilhelm Weber, and it was equal to the velocity of light. Maxwell thus inferred that light consists of transverse waves in the same medium that causes electric and magnetic phenomena.

Maxwell was reinforced in this opinion by work undertaken to make basic definitions of electric and magnetic quantities in terms of mass, length, and time. In On the Elementary Regulations of Electric Quantities (1863), he found that the ratio of the two definitions of any quantity based on electric and magnetic forces is always equal to the velocity of light. He considered that light must consist of electromagnetic waves, but first needed to prove this by abandoning the vortex analogy and arriving at an explanation based purely on dynamic principles. This he achieved in A Dynamical Theory of the Electromagnetic Field (1864), in which he developed the fundamental equations that describe the electromagnetic field. These showed that light is propagated in two waves, one magnetic and the other electric, which vibrate perpendicular to each other and to the direction of propagation. This was confirmed in Maxwell's ‘Note on the electromagnetic theory of light’ (1868), which used an electrical derivation of the theory instead of the dynamical formulation, and Maxwell's whole work on the subject was summed up in Treatise on Electricity and Magnetism (1873).

The treatise also established that light has a radiation pressure, and suggested that a whole family of electromagnetic radiations must exist, of which light was only one. This was confirmed in 1888 with the sensational discovery of radio waves by Heinrich Hertz. Sadly, Maxwell did not live long enough to see this triumphant vindication of his work. He also did not live to see the ether (the medium in which light waves were said to be propagated) disproved with the classic experiments of Albert Michelson and Edward Morley in 1881 and 1887, which Maxwell himself had suggested in the last year of his life. However this did not discredit Maxwell as his equations and description of electromagnetic waves remain valid even though the waves require no medium.

Maxwell's other major contribution to physics was to provide a mathematical basis for the kinetic theory of gases. Here he built on the achievements of Rudolf Clausius, who in 1857-58 had shown that a gas must consist of molecules in constant motion colliding with each other and the walls of the container. Clausius developed the idea of the mean free path, which is the average distance that a molecule travels between collisions. As the molecules have a high velocity, the mean free path must be very small, otherwise gases would diffuse much faster than they do, and would have greater thermal conductivities.

Maxwell's development of the kinetic theory was stimulated by his success in the similar problem of Saturn's rings. It dates from 1860, when he used a statistical treatment to express the wide range of velocities that the molecules in a quantity of gas must inevitably possess. He arrived at a formula to express the distribution of velocity in gas molecules, relating it to temperature and thus finally showing that heat resides in the motion of molecules - a view that had been suspected for some time. Maxwell then applied it with some success to viscosity, diffusion, and other properties of gases that depend on the nature of the motion of their molecules.

However, in 1865, Maxwell and his wife carried out exacting experiments to measure the viscosity of gases over a wide range of pressure and temperature. They found that the viscosity is independent of the pressure and that it is very nearly proportional to the absolute temperature. This later finding conflicted with the previous distribution law and Maxwell modified his conception of the kinetic theory by assuming that molecules do not undergo elastic collisions as had been thought but are subject to a repulsive force that varies inversely with the fifth power of the distance between them. This led to new equations that satisfied the viscosity-temperature relationship, as well as the laws of partial pressures and diffusion.

Maxwell's kinetic theory did not fully explain heat conduction, however, and it was modified by Ludwig Boltzmann in 1868, resulting in the Maxwell-Boltzmann distribution law. Both men thereafter contributed to successive refinements of the kinetic theory and it proved fully applicable to all properties of gases. It also led Maxwell to an accurate estimate of the size of molecules and to a method of separating gases in a centrifuge. The kinetic theory, being a statistical derivation, also revised opinions on the validity of the second law of thermodynamics, which states that heat cannot of its own accord flow from a colder to a hotter body. In the case of two connected containers of gases at the same temperature, it is statistically possible for the molecules to diffuse so that the faster-moving molecules all concentrate in one container while the slower molecules gather in the other, making the first container hotter and the second colder. Maxwell conceived this hypothesis, which is known as Maxwell's demon. Even though this is very unlikely, it is not impossible and the second law can therefore be considered to be not absolute but only highly probable.

Maxwell is generally considered to be the greatest theoretical physicist of the 1800s, as his forebear Faraday was the greatest experimental physicist. His rigorous mathematical ability was combined with great insight to enable him to achieve brilliant syntheses of knowledge in the two most important areas of physics at that time. In building on Faraday's work to discover the electromagnetic nature of light, Maxwell not only explained electromagnetism, but paved the way for the discovery and application of the whole spectrum of electromagnetic radiation that has characterized modern physics. In developing the kinetic theory of gases, Maxwell gave the final proof that the nature of heat resides in the motion of molecules.

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