Other Free Encyclopedias » Online Encyclopedia » Encyclopedia - Featured Articles » Contributed Topics from K-O

Maxwell, James Clerk

electromagnetic waves theory gas

(1831–79) British physicist: produced the unified theory of electromagnetism and the kinetic theory of gases.

Maxwell went to school at the Edinburgh Academy, a harsh institution where his country accent, home-designed clothes and sense of humour gained him the undeserved nickname of ‘Dafty’, and possibly caused his shyness; he was happier at Edinburgh University, which he entered at 16. The previous year he had developed the known method of drawing an ellipse using pins and thread, to generate a series of novel curves. The work was published by the Royal Society of Edinburgh in 1846.

In 1850 he entered Trinity College, Cambridge and graduated as Second Wrangler, winning the Smith’s Prize (1854). Then 2 years later he secured a professorship at Marischal College, Aberdeen, where he married the principal’s daughter, but an administrative reorganization made him redundant and in 1860 he moved to King’s College, London. After the death of his father (1865), who had cared for him since his mother died when he was 8 and of whom he was very fond, he resigned his post at King’s and remained at the family home in Scotland as a gentleman-farmer doing research. However, he was persuaded to become the first Cavendish Professor of Experimental Physics in Cambridge, setting up the laboratory in 1874. He contracted cancer 5 years later and died soon afterwards, aged 48. In setting up the Cavendish Laboratory, he formed an institution unique in physics, to be headed by a succession of men of genius and producing graduates who dominated the subject for generations.

Maxwell was the most able theoretician of the 19th-c, perfectly complementing , who was its most outstanding experimentalist. He began research on colour vision in 1849, showing how all colours could be derived from the primary colours red, green and blue. This led, in 1861, to his producing the first colour photograph using a three-colour process; the photograph was of a tartan. Other early work (1855–9) showed that Saturn’s rings must consist of many small bodies in orbit rather than a solid or fluid ring, which he showed would be unstable. He casually referred to this as ‘the flight of the brickbats’.

His monumental research on electromagnetism had small beginnings. Faraday viewed electric and magnetic effects as stemming from fields of lines of force about conductors or magnets, and Maxwell showed that the flow of an incompressible fluid would behave in the same way as the fields (1856). Then, in 1861–2, he developed a model of electromagnetic phenomena using the field concept and analogous vortices in the fluid which represented magnetic intensity, with cells representing electric current. Having explained all known electromagnetic phenomena, Maxwell introduced elasticity into the model and showed that transverse waves would be propagated in terms of known fundamental electromagnetic constants. He calculated that the waves would move at a speed very close to the measured speed of light. He unhesitatingly inferred that light consists of transverse electromagnetic waves in a hypothetical medium (the ‘ether’).

To study electromagnetic waves further, the fluid analogy was taken over into a purely mathematical description of electromagnetic fields. In 1864 he developed the fundamental equations of electromagnetism (Maxwell’s equations) and could then show how electromagnetic waves possess two coupled disturbances, in the electric and magnetic fields, oscillating at right angles to one another and to the direction in which the light is moving. The original mechanical model was now rightly cast off.

Furthermore, Maxwell stated that light represented only a small range of the spectrum of electromagnetic waves available confirmed this in 1888 by discovering another part of the spectrum, radio waves, but by this time Maxwell was dead. Maxwell also suggested the experiment (1881, 1887) to search for an absolute electromagnetic medium (the ether). Its proven absence prompted research on relativity (1905) and the era of modern physics.

Maxwell also contributed to the kinetic theory of gases, building on the existing picture of a gas as consisting of molecules in constant motion, colliding with their container and with each other; this picture was due to and to two little-known men, J Herapath (1790–1868) and J J Waterston (1811–83). As gases diffuse into each other rather slowly, deduced that although they travel fast, the molecules must have a very small ‘mean free path’ between collisions.

From 1860 Maxwell (and independently ) used statistical methods to allow for the wide variation in the velocities of the various molecules in the gas, deriving the Maxwell–Boltzmann distribution of velocities. Maxwell showed how this depends on temperature, and that heat is stored in a gas in the motion of the gas molecules. The theory was then used to explain the viscosity, diffusion and thermal conductivity of gases.

Maxwell and his wife found experimentally (1865) that gas viscosity is independent of pressure and that it is roughly proportional to the temperature, and rises with it (the reverse of the behaviour of liquids). This did not agree with Maxwell’s theory, and he could only gain agreement by assuming that molecules do not collide elastically but repel one another with a force proportional to their separation raised to the fifth power. This and further work by Boltzmann from 1868 allowed the full development of the kinetic theory of gases.

Maxwell was a shy and mildly eccentric person, who was deeply religious, with a strong sense of humour and no trace of pomposity. Like Einstein, and in contrast to or Faraday, Maxwell made his enormous advances in physics without excessive mental strain. He excelled in his sure intuition in physics, in applying visual models or mathematical methods without being tied to them, and above all in freeing himself from preconceptions and in exercising his creative imagination. Maxwell’s summary of electromagnetism in his field equations is an achievement equalled only by that of Newton and Einstein in mechanics.

May, Sir Robert [next] [back] Maury, Matthew Fontaine

User Comments

Your email address will be altered so spam harvesting bots can't read it easily.
Hide my email completely instead?

Cancel or