Maxwell, James Clerk (DNB00)
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Maxwell, James Clerk
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MAXWELL, JAMES CLERK (1831–1879), first professor of experimental physics at Cambridge, was born in Edinburgh 13 Nov. 1831. His father, who died in 1856, was John Clerk, brother of Sir George Clerk, of Penicuik in Midlothian. John Clerk adopted the surname of Maxwell on succeeding to an estate in Kircudbrightshire, which had come into the family by a marriage with a Miss Maxwell. Clerk Maxwell's mother was Frances, daughter of Robert Cay, of Charlton, Northumberland. His early childhood was passed in his father's country house of Glenlair, near Dalbeattie. In 1839 his mother died, and two years later Maxwell became a pupil at the Edinburgh Academy, and in 1847 entered the university of Edinburgh, attending lectures on mathematics, natural philosophy, chemistry, and mental philosophy. He had already, at the age of fifteen, communicated to the Royal Society of Edinburgh a paper ‘On the Description of Oval Curves’ (Proc. Roy. Soc. Edin. 1846, vol. ii.). A second paper ‘On the Theory of Rolling Curves’ (Trans. Roy. Soc. Edin. vol. xvi. pt. v.), was read in 1849, and a third ‘On the Equilibrium of Elastic Solids’ (Trans. Roy. Soc. Edin. vol. xx. pt. i.), in 1850. The last paper was the outcome of a visit paid in 1848 to Nicol, the inventor of Nicol's prism, who showed him the beautiful chromatic effects exhibited by unannealed glass in polarised light. It occurred to Maxwell to study by their aid the strains set up in an elastic substance such as gelatine when subject to stress, and to compare his experimental results with theory. In obtaining his theory Maxwell discarded the hypotheses of Navier and Poisson as to the action between the molecules of an elastic body, since they had led to results inconsistent with experiment, and starting afresh arrived at equations which, as he states, had been already obtained in a different way by Stokes and Candy. They had also been given in 1837 by Green, who based his work on the fundamental principle of the conservation of energy.
In October 1850 Maxwell left Edinburgh for Cambridge, entering as an undergraduate at Peterhouse, but in December of the same year he migrated to Trinity; Dr. Thompson, afterwards master, was his tutor. He became a pupil of the great ‘coach,’ Hopkins, in 1851, and in April 1852 was elected a scholar of his college. He graduated in 1854 as second wrangler, the senior being Dr. Routh of Peterhouse, with whom he was bracketed as first Smith's prizeman. In 1855 he was elected a fellow of Trinity, and was placed on the staff of lecturers. During the next year he was appointed professor of natural philosophy in Marischal College, Aberdeen. The college was amalgamated in 1860 with King's College, to form the university of Aberdeen, and Maxwell vacated his chair, but almost immediately afterwards became professor of natural philosophy in King's College, London. This post he resigned in 1865, retiring to private life at Glenlair, but in 1871 he was induced to come forward as a candidate for the new chair of experimental physics, which the university proposed to found at Cambridge. He was elected without opposition, and delivered his inaugural lecture 25 Oct. 1871.
The Duke of Devonshire, the chancellor, had just offered to present the university with a physical laboratory, and Maxwell's first work was to arrange the details of the plans and to superintend the building. The laboratory was opened in June 1874. The work of the professorship occupied him during term time for the next five years; the long vacation was usually spent at Glenlair. While staying there during the summer of 1879, he became seriously ill, and returned to Cambridge in October, only to succumb to a painful malady on 5 Nov. of the same year, at the early age of forty-eight.
In 1858 he married Katherine Mary Dewar, daughter of the principal of Marischal College.
Maxwell's power as an original investigator, of which he gave the first proofs at the age of fifteen, was signally illustrated shortly after he obtained his fellowship at Trinity. A paper on ‘Faraday's Lines of Force’ was read before the Cambridge Philosophical Society on 10 Dec. 1855 and on 11 Feb. 1856 (Camb. Phil. Soc. Trans. vol. x. pt. i.), and contains the germs of much of his future work. He had read Faraday's ‘Experimental Researches,’ and set himself ‘not to attempt,’ quoting his own words, ‘to establish any physical theory of a science in which I have hardly a single experiment, but to show how by a strict application of the ideas and methods of Faraday, the connection of the very different order of phenomena which he has discovered, may be placed before the mathematical mind.’ Following a suggestion of Sir William Thomson (now Lord Kelvin), he worked his endeavour out by the aid of analogies with corresponding phenomena in hydrodynamics and heat. In later papers the ideas here originated received further development. Meanwhile other phenomena were interesting him. He had already (1855) written on the theory of colours in relation to colour-blindness, and in a paper on ‘Experiments on Colour as Perceived by the Eye’ (Phil. Trans. Roy. Soc. Edin. vol. xxi. pt. ii.), he had investigated the effects of combinations of various colours by means of the rapid rotation of discs coloured differently in different parts. Maxwell's colour-top is now well known. The main results of his work on colour are summed up in his paper ‘On the Theory of Compound Colours,’ read before the Royal Society 22 March 1860 (Phil. Trans. 1860). His instrument, the colour-box, by which he investigated the effect of mixing in given proportions light taken from different parts of the spectrum, is first described, and then it is shown that any given colour sensation may be produced by combinations in due proportion of rays taken from three parts of the spectrum, and also that if we select three definite rays as standards, all other colours may be produced by proper combinations of these. In the most general case it may be that, to produce a given colour, we should have to subtract a certain amount of the third colour C, from the two other colours A and B, taken arbitrarily. This would mean that the effect of mixing the given colours, and a proper amount of C, just matches the mixture of A and B, but it is further shown that there are three primary colours by arithmetical addition of which, in proper proportions, any other colour may be produced. Probably these three different elements of colours correspond to three different sensations in the eye, and a body appears to us of a definite colour because it excites these sensations each in its proper proportion. The experiments tended to confirm the conclusion that colour-blindness is due to the absence of one of the three primary sensations. For this work Maxwell was awarded the Rumford medal of the Royal Society 30 Nov. 1860.
Meanwhile Maxwell had been engaged on his essay ‘On the Stability of Motion of Saturn's Rings,’ which gained the Adams prize in 1857. Laplace had shown that the ring could not be solid, for if so it would be unstable, the slightest displacement of its centre from the centre of the planet would originate a motion, which would ultimately destroy the whole.
Maxwell considered the effect of loading the ring at one or more points, and showed that if the load were great enough we could account for the motion on known laws, but if this were so, the load must be so great, that it would be visible as a satellite, and this is not the case. There then remained the assumption that the ring is fluid, or else consists of a large number of very small separate solid particles. Either of these hypotheses was proved to give a possible form of motion, and the latter in all probability is the nature of the ring.
It may have been the discrete particles of Saturn's rings that led Maxwell to study the kinetic theory of gases. According to this theory, the pressure which a gas exerts is due to the impact of its molecules on the walls of the enclosing vessel; the temperature depends on the average energy of the motion. This had been clearly pointed out by Herapath in 1847, and in 1848 Joule, assuming that all the molecules of the gas possessed the same velocity of agitation, determined the relation between the velocity and the pressure, and calculated the former for hydrogen and other gases at a definite pressure and temperature. Clausius in 1857 and 1859 extended the work, making the same hypothesis as to the velocity of the individual molecules, and introduced the idea of the mean free path.
Maxwell's first papers on the subject appeared in the ‘Philosophical Magazine’ (January and July 1860). He pointed out that the velocities of the different molecules, even if equal to start with, would become unequal immediately in consequence of the collisions. He therefore devised the statistical method of treating the problem. On this method the whole number of molecules are divided into a series of groups, the velocities of all the molecules constituting a group, being the same within narrow limits, and the average velocity of each group is considered. He also found the law connecting this average velocity with the number of molecules in the group, and showed that when a state of permanence, that is of uniform temperature, has been reached, in the case either of a single gas or of a mixture, the average energy of agitation is the same throughout. From these considerations and on the supposition that the mean energy of agitation measures the temperature, the laws of Gay Lussac and Charles are deduced. The theory of diffusion had been given by Herapath, Maxwell extended it, and by applying similar reasoning to the diffusion of the momentum and the diffusion of the energy, explained the phenomena of viscosity and of conduction of heat respectively. The law of Dulong and Petit connecting the specific heat and the molecular weight was shown to follow, but difficulties of a serious nature were met with when the theory was applied to deduce the elation between the specific heats of a gas at constant pressure and volume respectively. These difficulties led Maxwell to abandon the hypothesis of collisions between hard spherical molecules, and to attack the problem on the assumption of action of a more general character between the particles. This is done in his paper ‘On the Dynamical Theory of Gases’ (Phil. Trans. 1866). Some of his conclusions he had attempted to verify by direct experiments, which are described in the Bakerian lecture ‘On the Viscosity of Air and other Gases’ (Phil. Trans. 1866).
The theorem as to the distribution of velocity in a gas was extended by Boltzmann (Vienna Proceedings, 1871–2), and still further by Maxwell in a paper ‘On Boltzmann's Theorem’ (Camb. Phil. Soc. Trans. 1878). Various objections have been urged against the theorem, and it seems now to be established that in the most general form given to it in his last paper, it does not hold (see Bryan, ‘On our Knowledge of Thermodynamics,’ Brit. Assoc. Report, 1891, where the points at issue are clearly stated). Another paper on the same subject, ‘On Stresses in Rarefied Gases arising from Inequalities in Temperature’ (Phil. Trans. 1879), deals among other things with the theory of Mr. Crookes's beautiful instrument, the radiometer.
In Maxwell's collected papers are to be found many others which have a bearing on the constitution of matter and on the theory of gases. Among them may be mentioned his lecture before the British Association at Bradford (Nature, vol. viii.) on ‘Molecules;’ and another lecture before the Chemical Society (ib. vol. xi.) on the ‘Dynamical Evidence for the Molecular Constitution of Bodies;’ his articles in the ‘Encyclopædia Britannica’ on ‘Atom,’ ‘Attraction,’ ‘Capillary Action,’ ‘Diffusion,’ ‘Constitution of Bodies,’ and other subjects; together with his review of Van der Waal's important work ‘On the Continuity of the Gaseous and Liquid States’ (Nature, vol. x.)
But the researches for which Maxwell is best known are those dealing with electricity and magnetism. These commenced with the paper in 1856 on Faraday's lines of force. The next published paper of importance was that on ‘Physical Lines of Force’ (Phil. Mag. 1861, 1862). It was Maxwell's view that electrical and magnetic effects do not arise from the attractions of electric or magnetic matter distributed over the surfaces of conductors or magnetic bodies, but are the means by which changes of some unknown description in the ether which fills space or in some of its properties become known to us. In consequence of these changes energy is stored up in the ether, and electrical or magnetic forces are one form of the manifestation of changes in the distribution of the energy. The experiments of Quincke on electric stress and of Kerr on electro-optics have shown the reality of this stress in the ether, while the theory of Poynting enables us to understand one method by which the energy may travel from place to place. The paper we are now considering describes a mechanism which would have properties in many respects analogous to those possessed by the electro-magnetic medium, though it does not pretend to be a complete representation of the actual condition of the ether.
Similar ideas, though in a far more general form, are developed in the great paper ‘On a Dynamical Theory of the Electro-magnetic Field,’ read before the Royal Society, 8 Dec. 1864 (Phil. Trans. vol. clv.). In it Maxwell took the important and novel step of applying dynamical equations in the generalised form given to them by Lagrange to the problems of electro-magnetism, in dealing with which ‘we are led to the conception of a complicated mechanism capable of a vast variety of motions, but at the same time so connected that the motion of one part depends, according to definite relations, on the motion of other parts. … Such a mechanism must be subject to the laws of dynamics.’ Electro-magnetic action is shown to travel through space at a definite rate in waves, and these waves consist of disturbances which are transverse to the direction in which the waves are propagated. In this respect then they resemble waves of light. Moreover, it is found by experiment that the velocity of the electro-magnetic waves in air and in many other media is the same as that of light, and thus the electro-magnetic theory of light becomes possible. The experiments in Maxwell's time were indirect, though so far as they went conclusive enough. We owe it to the genius of Hertz that we are now able to measure directly the velocity of electro-magnetic waves and to show that they are propagated, and can undergo reflection, refraction, and polarisation exactly like waves of light, and we now feel able to say that the two are the same in character; they differ merely, as do the bass and treble notes of a musical instrument, in the rapidity with which they are executed. In light waves periodic changes in the ether are taking place at the rate of some five hundred billions per second; the most rapid electro-magnetic changes we have yet produced are some few millions per second. The laws of these vibrations, when they are completely known, will give us the secret of the ether, and will enable some disciple of Clerk Maxwell to take that step which the master himself in his ‘Electricity and Magnetism’ confessed himself unable to take, and to explain the mechanism at one time of light, electricity, and magnetism. The paper on the electro-magnetic field was in time expanded into the great ‘Treatise on Electricity and Magnetism,’ published in 1873, on the second edition of which Maxwell was at work at the time of his death.
But it is not only on the theoretical side of electricity that advance is due to Maxwell. He realised, like Lord Kelvin, that a carefully thought-out system of measurement was essential for its progress, and that accurate experiment was needed to form a foundation for his theory. Maxwell became a member of the newly formed electrical standards committee of the British Association in 1862, and was one of the sub-committee appointed to construct the standard of resistance. The necessary experiments were carried out in his own laboratory at King's College, and the results, which have been so fruitful to electrical science, are recorded in the ‘Reports’ of the committee for 1863 and 1864. The ‘Report’ for 1863 contains an appendix by Maxwell and Fleeming Jenkin ‘On the Elementary Relations between Electrical Measurements,’ in which the fundamental principles involved are stated with unrivalled accuracy and clearness.
Another important series of experiments, those on the velocity of propagation of electro-magnetic waves, is described in the paper ‘On a method of making a direct Comparison of Electrostatic with Electro-magnetic Force; with a Note on the Velocity of Light’ (Phil. Trans. vol. clviii). Maxwell's numbers showed that this velocity was nearly that of light; more recent work has proved that the two are, within the limits of error of very exact experiments, identical.
The theory Maxwell formulated is day by day gaining more and more acceptance; the foremost physicists throughout the world are engaged in working at it, and in developing ideas, the germs of which may nearly all be traced in the ‘Electricity and Magnetism’ or in the paper on the ‘Electro-magnetic Field.’
Besides the books already mentioned Maxwell published in 1879 the ‘Electrical Researches’ of Henry Cavendish, written between 1771 and 1781; edited from the original manuscripts in the possession of the Duke of Devonshire, K.G.; he also wrote a text-book of ‘Heat’ and a small treatise on dynamics called ‘Matter and Motion.’ After his death an elementary treatise on ‘Electricity,’ which was left unfinished, was completed and published by Professor Garnett. Among his other papers are some on ‘Geometrical Optics,’ which contain important results, and several published mostly in the ‘Transactions of the Royal Society of Edinburgh,’ ‘On Reciprocal Figures and Diagrams of Force.’ A memorial edition of his scientific papers, undertaken by a committee appointed soon after his death, was edited by Mr. W. D. Niven, and was issued from the Cambridge University Press in 1890, 4to.
As a man Maxwell was loved and honoured by all who knew him; to his pupils he was the kindest and most sympathetic of teachers, to his friends he was the most charming of companions; brimful of fun, the life and soul of a Red Lion dinner at the British Association meetings, yet in due season grave and thoughtful, with a keen interest in problems that lay outside the domain of his own work, and throughout his life a stern foe to all that was superficial or untrue. On religious questions his beliefs were strong and deeply rooted; the words which close his lecture on molecules, expressing his faith in ‘Him, who in the beginning created not only the heaven and the earth, but the material of which heaven and earth consist,’ have often been quoted.
There is a bust by Boehm in the Cavendish Laboratory, and also a portrait painted by his cousin, Miss Wedderburn. The bust was executed after his death from G. J. Stodart's engraving, which forms the frontispiece to his works; and a portrait by Mr. Lowes Dickenson, based on the same engraving, was presented to Trinity College by the subscribers to the memorial fund.
By his will he left funds to found a studentship in experimental physics open to members of the university of Cambridge. This was carried out in 1890, when, by the death of Mrs. Maxwell, the university came into possession of the property.[Life by Professor Lewis Campbell of St. Andrews, and Professor Garnett, his Demonstrator at the Cavendish Laboratory, 1882.]