The appointment of James ClerkMaxwell

The electors to the Cavendish Professorship of Experimental Physics naturally sought the best possible candidate for the new chair. The obvious first choice was Sir William Thomson, later to become 1st Baron Kelvin of Largs or the Lord Kelvin, who was regarded as the most distinguished British physicist of his day, both in theory and in experiment. As described in Section 1.4.4, he was knighted for his efforts in laying the first successful transatlantic submarine cable in 1866. With his new, well-furnished laboratory at the University of Glasgow and his excellent industrial and domestic arrangements in the area, it is not surprising that he was unwilling to leave Glasgow. Another factor in Thomson's decision was the absence in Cambridge of a network of instrument makers and industrialists upon which much of his work relied. It is said that the electors also tried to attract Hermann von Helmholtz from Germany, but he was too well established in Berlin to consider moving to Cambridge. Some measure of the ambitions of the electors to the Cavendish Chair is provided by the assessment of Helmholtz's stature by R. Steven Turner:

Next, they turned to Maxwell. Maxwell's health was always somewhat fragile and, after a bout of ill-health, he resigned his chair at King's College, London in 1865. He returned to manage the family estate at Glenlair in the Dumfries and Galloway region of southern Scotland and set about writing his monumental Treatise on Electricity and Magnetism, which was eventually published in two volumes in 1873 (Maxwell, 1873). Maxwell was well known in Cambridge and no one doubted his originality and brilliance.

The title and subtitle of this book deserve some explanation. To take the subtitle first, this book is a scientific history, rather than a history in the conventional sense – my interest and qualifications for writing it are as a scientist rather than as a historian. Specifically, my principal interest is in the content of the physics and how it came about, rather than a history of personalities, politics, administrative structures and so on. The latter topics cannot be ignored and obviously have a very significant bearing in framing the whole story, but that is not my prime goal. My objective is to set the scientific achievements in the context of the development of physics as a whole. This book is not a panegyric about the Laboratory, but an attempt to understand the areas in which it has been successful and what influenced the directions the research programme took.

The reason for the main title will become apparent as the story unfolds. James Clerk Maxwell was the first Cavendish Professor of Experimental Physics. His epochal scientific achievements in essentially all branches of physics need little emphasis here, but what is less well known is the profound effect his personality and research style had upon the early direction of the Laboratory. The achievement is all the more remarkable granted the rather barren field in which the seeds of future success were sown. Perhaps most significant of all is the remarkable agenda which Maxwell set for the Laboratory in his inaugural lecture of 1871. The subsequent history can be regarded as the working out of Maxwell's vision over succeeding generations of research workers. Tragically, Maxwell died in 1879 before the full impact of his agenda and reforms could be appreciated, but they were brought to fruition by his successors, Rayleigh, J.J. Thomson and Rutherford. By the time of Rutherford'sdeath in 1937, the Laboratory had more than 50 years of history behind it and had made revolutionary contributions to many areas of experimental physics. As the Laboratory continued to grow after the Second World War, the organisation of research had to change, but the essential Maxwellian philosophy was maintained at the research group level. At the same time, the Laboratory had to cope with the demands of ‘Big Science’ and to exploit the opportunities offered by large national and international facilities.

Thomson's election to the Cavendish Chair

William Thomson could still not be enticed back to Cambridge from Glasgow, despite a memorial, spearheaded by J.J. Thomson and sent to him with the signatures of a number of distinguished Cambridge scientists, urging him to stand. The Cavendish Chair was duly advertised and for the first time there was a competitive election. There were five candidates – Richard Glazebrook, Joseph Larmor, Osborne Reynolds, Arthur Schuster and Joseph John (J.J.) Thomson. Larmor was now Professor of Natural Philosophy at Queen'sCollege, Galway, while Reynolds and Schuster both held professorships, in Engineering and Applied Mathematics respectively, at Owens College, Manchester. Somewhat to his own and the University's surprise, the electors offered the chair to the 28-year old Thomson. Glazebrook was Rayleigh's choice, but Davis and Falconer (1997) argue that he was too wedded to the programme of the precise establishment of physical standards to appeal to the electors. Reynolds was thought to be more an engineer than an experimental physicist. The electors took a bold gamble on Thomson, but he undoubtedly had the potential to become a distinguished physicist.

Thomson had entered Owens College, Manchester at the age of 14 and was fortunate to be instructed by inspiring scientists – Thomas Barker lectured on mathematics, Balfour Stewart on natural philosophy and Osborne Reynolds on engineering physics. According to Arthur Schuster, Stewart made extensive use of argument by analogy, very much in the Maxwell tradition, and Reynolds, the pioneer of turbulent flow in fluid dynamics, lectured on the role of vortices in fluid motion. In Thomson's final years at Owens College, Schuster lectured on Maxwell's Treatise on Electricity and Magnetism and Poynting, who was to become a lifelong friend, was developing his insights into the interpretation of Maxwell'sequations. This training stood him in good stead when he was successful at his second attempt in obtaining funding in the form of an exhibition to Trinity College; in 1876 he matriculated studying for the Mathematical Tripos. Thomson was coached by Edward Routh and in 1880 he graduated second wrangler behind Joseph Larmor and joint first Smith'sPrize winner.

The period of Mott's tenure of the Cavendish Chair from 1954 to 1971 was a revolutionary period in astronomy, astrophysics and cosmology. The nature of these disciplines changed in fundamental ways, highlights including:

1. • the emergence of high-energy astrophysics, in which high-energy particles and cosmic magnetic fields played a key role

2. • the discovery of the cosmological evolution of the radio source population

3. • the discovery of the cosmic microwave background radiation

4. • the discovery of pulsars, which were shown to be magnetised, rotating neutron stars.

The Radio Astronomy Group was at the centre of these events. At the same time, the Nuclear Physics Group changed direction from the earlier period when it was feasible for a University group to construct its own particle accelerators. The group was renamed the High Energy Physics Group in the mid 1960s, and successfully developed instrumentation and data analysis facilities in support of major international projects.

The growth of the Radio Astronomy Group

The story of radio astronomy in Cambridge up to 1953–54 was described in Section 12.7.2, the pivotal years when the discipline was about to develop into a ‘Big Science’ discipline. Ryle was the driving force behind these developments. As he wrote to me just before his death, he thought of himself primarily as an electrical engineer with the ability to make complex radio receiving systems a reality. In addition, however, he had remarkable physical intuition, which guided all his research activities in radio astronomy and its technology. His great contribution was the practical implementation of the concept of aperture synthesis, the technique by which images of the radio sky are created by combining interferometric observations made with modest-sized radio telescopes located at different interferometer spacings. Ryle's ambitious programme was to determine both the amplitudes and the phases of the incoming radio signals so that, by Fourier inversion, the detailed brightness distribution of the radio emission could be reconstructed. Although understood in principle, the key technical issues concerned whether or not these concepts could be realised in practice, given the problems of receiver sensitivity, the need to preserve phase coherence over long periods and the stability of the overall system performance.

The changing frontiers of physics research

The outbreak of the First World War brought much of the research activity in UK universities to a halt. In particular, as indicated in Section 7.8, a whole generation of young men enlisted with tragic loss of life in the trenches of northern France, Belgium and the Low Countries. Although the attentions of experimental physicists had to turn to military-related topics, the theorists who remained in the universities throughout Europe continued to produce work of the highest quality. Perhaps most spectacular of all were the contributions of Einstein. As expressed by Pais,

At the same time, Einstein became an outspoken radical pacifist, which provoked a hostile reaction from the authorities in the midst of a devastating war.

By the end of the war, there had been major advances in the efforts to incorporate quantum concepts into physics at the atomic level, what is conveniently referred to as the old quantum theory. This was to continue in the years immediately following the war until the quantum revolution of 1925–27, associated with the names of Heisenberg, Born, Jordan, Dirac, Schrödinger, Pauli and many others.

The Rutherford era will always be remembered for the extraordinary achievements in radioactivity and the beginnings of the new field of nuclear physics. Jeffrey Hughes (1993) refers to the protagonists in this story as the radioactivists; they were to transform the field and open up new, and expensive, areas of basic research.

Rutherford and nuclear transformations

The α-particles remained the projectiles of choice used by Rutherford and his colleagues to study the nucleus, but the experiments were dependent upon securing sources of radioactive materials, which were generally in short supply and expensive. In 1908 it was discovered that zinc sulphide laced with 0.01% copper was extremely sensitive to α-particles and emitted most of its luminosity in the yellow-green region of the optical spectrum to which the eye is most sensitive – about a quarter of the incident energy was converted into light (Hendry, 1984). This became Rutherford's preferred method of detecting α-particles and was the technique which Geiger and Marsden used in their experimental demonstration of the validity of the Rutherford scattering law (Section 7.5.2) (Geiger and Marsden, 1913). The energies of the α-particles could be estimated from their ranges R in air at a standard temperature and pressure, which Rutherford and his colleagues took to be 15?C at normal pressure (Box 9.1). It can be seen from the abscissa of Figure 8.3 that the typical ranges of the particles were between 2 and 8 cm, corresponding to particle energies of roughly 4 to 8 MeV. The ranges of the α-particles could be measured in terms of the mass traversed by the particle per unit cross-section ζ until they were brought to rest by the process of ionisation losses. To measure ζ, the amount of absorbing material between the source of the particles and the screen was increased until there was a sudden drop in the number of scintillations counted. The range ζ could then be translated into a physical distance R in air at the standard reference conditions. The ranges of the α-particles were characteristic of each radioactive decay.

High-energy physics: the LEP era

The 1976 Report to the CERN Council recommended the construction of a Large Electron– Positron (LEP) collider capable of accelerating electrons and positrons to energies at which the W and Z bosons could be created in large numbers. The Council approved the project in 1981, the civil engineering work beginning in 1983. Excavation of the 27-kilometre circumference tunnel began in 1985 and was completed three years later – this excavation was Europe's largest civil-engineering project prior to the Channel Tunnel (see Figure 17.5). To measure the many products of electron–positron collisions, four huge detectors, ALEPH, DELPHI, L3 and OPAL, were installed at the experimental stations around the ring. As described in Section 17.3.1, CERN had already agreed a fast-track route to the discovery of theWand Z bosons and this was achieved in 1982–83. Now the task was to make precision measurements of their properties.

The LEP collider was commissioned in July 1989, its initial energy being about 91 GeV, at which energy Z bosons were created in huge numbers. LEP operated at about 100 GeV for seven years, a total of about 17 million Z particles being produced during that period. In 1995 LEP was upgraded to roughly twice that energy so that pairs of W bosons would be created as well – by 2000, the collider's maximum energy exceeded 209 GeV. During its 11 years of operation, the LEP experiments provided detailed information about the nature of the electroweak interaction as well as key tests of the standard model of particle physics. The Cavendish High Energy Physics Group became a member of the OPAL collaboration, which involved about 200 physicists from 34 institutes in Canada, Germany, Hungary, Italy, Israel, Japan, the United Kingdom and the United States – the acronym stands for Omni-Purpose Apparatus for LEP.

Janet Carter led the Cavendish participation in the OPAL experiment from about 1984. The group went through a further period of considerable crisis with the departure of Bill Neale to the Rutherford–Appleton Laboratory in 1988 and, even more seriously, the departure of the most senior scientist and head of the group, John Rushbrooke, to Australia in the same year.

Rayleigh's appointment

The original intention of the University was that the Cavendish Professorship should be a single-term appointment, but the wording of the regulations offered the possibility of extension of the position. It stated that the post was to

Again, Sir William Thomson was an obvious choice, but by now he saw his future in Glasgow. The next obvious candidate was John William Strutt, who had succeeded to the Barony as the 3rd Lord Rayleigh on the death of his father in 1873. It was not a common occurrence for a senior member of the aristocracy and major landowner to become a professional academic, but Rayleigh had already demonstrated outstanding ability in theoretical and experimental physics. He had been senior wrangler in 1865 and first Smith'sPrize winner. He had sought to improve his understanding of experimental physics, but there were limited opportunities. Fortunately, Stokes allowed Rayleigh to observe his experimental researches in his private laboratory and so he gained some familiarity with the challenges involved. By the time his name came forward as a candidate for the Cavendish Chair, he was already known for his explanation of the colour of the sky through the process of Rayleigh scattering, and he had already written profusely on a very wide range of topics in the physical sciences, including experimental researches carried out at the family home at Terling Place in Essex.

But there was more to it than simply academic prestige. The Rayleigh estates had made most of their earnings through the sale of wheat. With the opening up of the midwest American prairies for wheat production, the price of wheat had plummeted and made the Rayleigh estates unprofitable. The agricultural depression was to last for a number of years and so Rayleigh decided to change to dairy farming. In the late 1880s, ‘Lord Rayleigh'sDairies’ supplied milk for the capital (Rayleigh, 1924). Although not decisive in encouraging Rayleigh to put his name forward as a candidate for the Cavendish Chair, the appointment would certainly help the family weather the downturn in their fortunes.

Lawrence Bragg at Manchester and the National Physical Laboratory

The Vice-Chancellor of Manchester University, Sir Henry Miers, having been Professor of Crystallography, was keen to develop the discipline. In his words,

Lawrence Bragg, although only 29, was the obvious choice to succeed Rutherford as the Langworthy Professor of Physics at the University in 1919, but it was to prove a real challenge. He had never taught physics and now was in charge of a large department which had become a leader in the dynamic field of nuclear physics under Rutherford. Despite a rocky ride in getting on top of teaching and the management of the growing laboratory during his first few years, he now had his own research laboratory and concentrated upon making the discipline of crystallography an exact quantitative science. This was to be his major and distinguished contribution during his Manchester years. As expressed by David Phillips,

His research programme got off to an excellent start with his acquisition of a new Xray spectrometer and the gift of a state-of-the-art Coolidge X-ray tube from the General Electric Company at Schenectady. From his continuing studies of crystalline structures, he developed the concept of the sizes of the common ions so that interatomic distances in atomic compounds could be determined by an additive law; these dimensions agreed quite well with the measured atomic distances (Bragg, 1921).

Thomson's agenda

Prior to his appointment as Cavendish Professor, Thomson was uncertain about his ability to sustain a career in experimental physics and had the option of concentrating on theoretical studies. Once his appointment was announced, however, he put all his energies into fostering the experimental activities of all his colleagues in the Laboratory and developing his own experimental agenda through the appointment of an expert laboratory assistant, whose salary he paid from his own pocket. Ebeneezer Everett was appointed to this post in 1888 and was to remain until 1930 when he retired due to ill health. In his touching obituary of Everett, published in Nature in 1933, Thomson wrote:

For many reasons, 1895 was a turning point in the development of the Laboratory and so it is illuminating to list the titles of Thomson's papers up until that year (Table 6.1). It is apparent that his interests were wide-ranging, but increasingly there was a strong emphasis upon the conduction of electricity in gases, which dates back to his paper of 1883, the year before he was appointed to the Cavendish Chair. Before looking in more detail into these researches, let us review the role of analogy and model-building as tools for the understanding of physical phenomena.

Analogy andmodel-building

A distinctive feature of Maxwell's approach to the application of mathematics to natural philosophy was his ability to work by analogy. As early as 1856, when he was in his mid-20s, he described his approach in an essay entitled Analogies in Nature, written for the Apostles Club at Cambridge (Maxwell, 1856a). The essence of the technique can be caught in the following passage.