James Clerk Maxwell

I

Introduction

James Clerk Maxwell (1831-1879), British physicist, best known for his work on the connection between light and electromagnetic waves (traveling waves of energy). Maxwell discovered that light consists of electromagnetic waves (see Electromagnetic Radiation) and established the kinetic theory of gases. The kinetic theory of gases explains the relationship between the movement of molecules in a gas and the gas’s temperature and other properties. He also showed that the rings of the planet Saturn are made up of many small particles and demonstrated the principles governing color vision.

Maxwell was born in Edinburgh, Scotland. He was educated at Edinburgh Academy from 1841 to 1847, when he entered the University of Edinburgh. He then went on to study at the University of Cambridge in 1850, graduating with a bachelor’s degree in mathematics in 1854. He became a professor of natural philosophy at Marischal College in Aberdeen in 1856. Then in 1860 he moved to London to become a 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. In 1871 he moved 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 illness forced him to resign.

II

Color Vision

Maxwell’s first important contribution to science began in 1849, when he applied himself to examining how human eyes detect color. He built on the ideas of British physicist Thomas Young and German scientist Hermann Helmholtz on color vision. Maxwell spun disks painted with sectors of red, green, and blue to mix those primary colors into other colors. He confirmed Young's theory that the eye has three kinds of receptors sensitive to the primary colors and showed that color blindness is due to defects in the receptors. He also fully explained how the addition and subtraction of primary colors produces all other colors. He crowned this achievement in 1861 by producing the first color photograph. Maxwell took this picture, the ancestor of all color photography, printing, and television, of a tartan-patterned ribbon. He used red, green, and blue filters to expose three frames of film. He then projected the images through the appropriate filters to project a colored image. See also Color.

Maxwell worked on several areas of inquiry at the same time, and from 1855 to 1859 he took up the problem of Saturn's rings (see Saturn (planet)). No one had developed a satisfactory explanation that would result in the rings having a stable structure. Maxwell proved that a solid ring would collapse and a fluid ring would break up. However, he found that a ring composed of concentric circles of small satellites could achieve stability. Images from the Pioneer and Voyager spacecraft in the 1970s and 1980s proved beyond a doubt that Saturn’s rings are indeed composed of many small bodies orbiting the planet together.

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III

Electromagnetic Theory of Light

Maxwell's development of the electromagnetic theory of light took many years. It began with the paper “On Faraday's Lines of Force” (1855–1856), in which Maxwell built on the ideas of British physicist Michael Faraday. Faraday explained that electric and magnetic effects result from lines of force that surround conductors and magnets. Maxwell drew an analogy between the behavior of the lines of force and the flow of a liquid, deriving equations that represented electric and magnetic effects. The next step toward Maxwell’s electromagnetic theory was the publication of the paper “On Physical Lines of Force” (1861–1862). Here Maxwell developed a model for the medium that could carry electric and magnetic effects. He devised a hypothetical medium that consisted of a fluid in which magnetic effects created whirlpool-like structures. These whirlpools were separated by cells created by electric effects, so the combination of magnetic and electric effects formed a honeycomb pattern.

Maxwell could explain all known effects of electromagnetism by considering how the motion of the whirlpools, or vortices, and cells could produce magnetic and electric effects. He showed that the lines of force behave like the structures in the hypothetical fluid. Maxwell went further, considering what would happen if the fluid could change density, or be elastic. The movement of a charge would set up a disturbance in an elastic medium, forming waves that would move through the medium. The speed of these waves would be equal to the ratio of the value for an electric current measured in electrostatic units to the value of the same current measured in electromagnetic units (see Electrical Units). German physicists Friedrich Kohlrausch and Wilhelm Weber had calculated this ratio and found it the same as the speed of light. Maxwell inferred that light consists of waves in the same medium that causes electric and magnetic phenomena.

Maxwell found supporting evidence for this inference in work he did on defining basic electrical and magnetic quantities in terms of mass, length, and time. In the paper “On the Elementary Regulations of Electric Quantities” (1863), he wrote 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 developing a mathematical system. He achieved this in “A Dynamical Theory of the Electromagnetic Field” (1864), in which he developed the fundamental equations that describe the electromagnetic field. These equations showed that light is propagated in two waves, one magnetic and the other electric, which vibrate perpendicular to each other and perpendicular to the direction in which they are moving (like a wave traveling along a string). Maxwell first published this solution in “Note on the Electromagnetic Theory of Light” (1868) and summed up all of his work on electricity and magnetism in Treatise on Electricity and Magnetism in 1873.

The treatise also suggested that a whole family of electromagnetic radiation must exist, of which visible light was only one part. In 1888 German physicist Heinrich Hertz made the sensational discovery of radio waves, a form of electromagnetic radiation with wavelengths too long for our eyes to see, confirming Maxwell’s ideas. Unfortunately, Maxwell did not live long enough to see this 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 German-born American physicist Albert Michelson and American chemist Edward Morley in 1881 and 1887. Maxwell had suggested an experiment much like the Michelson-Morley experiment in the last year of his life. Although Maxwell believed the ether existed, his equations were not dependent on its existence, and so remained valid.

IV

Kinetic Theory of Gases

Maxwell's other major contribution to physics was to provide a mathematical basis for the kinetic theory of gases, which explains that gases behave as they do because they are composed of particles in constant motion. Maxwell built on the achievements of German physicist Rudolf Clausius, who in 1857 and 1858 had shown that a gas must consist of molecules in constant motion colliding with each other and with the walls of their container. Clausius developed the idea of the mean free path, which is the average distance that a molecule travels between collisions.

Maxwell's development of the kinetic theory of gases 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 (speeds and the directions of the speeds) 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. He showed that gases store heat in the motion of their molecules, so the molecules in a gas will speed up as the gas’s temperature increases. Maxwell then applied his theory with some success to viscosity (how much a gas resists movement), diffusion (how gas molecules move from an area of higher concentration to an area of lower concentration), and other properties of gases that depend on the nature of the molecules’ motion.

Maxwell's kinetic theory did not fully explain heat conduction (how heat travels through a gas). Austrian physicist Ludwig Boltzmann modified Maxwell’s theory in 1868, resulting in the Maxwell-Boltzmann distribution law. Both men 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 was derived using statistics, so it also revised opinions on the validity of the second law of thermodynamics, which states that heat cannot flow from a colder to a hotter body of its own accord. 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. Although this event is very unlikely, it is not impossible, and the second law is therefore not absolute, but highly probable.

Maxwell is generally considered the greatest theoretical physicist of the 1800s. He combined a rigorous mathematical ability with great insight, which enabled him to make brilliant advances 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 also paved the way for the discovery and application of the whole spectrum of electromagnetic radiation that has characterized modern physics. Physicists now know that this spectrum also includes radio, infrared, ultraviolet, and X-ray waves, to name a few. 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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