Edward Crisp Bullard
Bullard, Edward Crisp
BULLARD, EDWARD CRISP
(b. Norwich, England, 21 September 1907; d. La Jolla. California, 3 April 1980)
Sir Edward Bullard, known as Teddy, was the most distinguished and best-known British geophysicist of his generation; his experimental and theoretical work contributed to every aspect of the subject. He was one of the major figures in the development of the earth sciences during the twentieth century. both for his own contributions and for his influence on his colleagues and students. Bullard was the eldest of four children of Edward John Bullard, whose prosperous family produced Bullard’s Ales, and of Eleanor Howes Crisp. After an unhappy childhood he went to Cambridge in 1926 to read natural sciences. Though he obtained a first, he found the lectures in physics very disappointing. There was a lack of generality, and all but the simplest calculus was avoided.
In the summer of 1929 Bullard became a research student at the Cavendish. His supervisor was Patrick Blackett, who suggested that he might follow up the work of Carl Ramsauer, who had shown that the total scattering cross section for electrons scattered in gases decreased as the energy was reduced below about 3 eV. This was quite inexplicable by classical theories of scattering, and Blackett suggested that Bullard should study the corresponding change in angular distribution, which could be expected to be more informative than the variation in total cross section. Bullard spent the long vacation in the “attic” of the Cavendish, where beginning graduate students worked for a month or so learning experimental techniques. He worked on the vapor pressure of tap grease, and then started to build the apparatus for electron scattering, which was a bird’s nest of glass tubing for handling and circulating gases. Before he had got very far, Harrie Massey, who had started research at the same time, asked if he could join him. Bullard was delighted; the job would be much easier with two people.
After a few months Bullard and Massey had learned the tricks of vacuum electronics, and how to get reliable results. They found that there was a peak in the scattered current at an angle of about 90°, This behavior was quite inexplicable by the classical theory of collisions, but was obviously analogous to the diffraction rings around a street lamp in a fog. The explanation of Ramsauer’s findings was also apparent: they resulted from diffraction around a spherical atom. The results agreed excellently with wave mechanics, and Bullard and Massey quickly wrote them up for publication (1930, 1931). This work gave Bullard a great feeling of confidence: he had carried out some elegant experiments that agreed with the quantum mechanical calculations. Theroom in which this work was done was close to Rutherford’s, and Bullard saw more of Rutherford than he did of his supervisor, Blackett, who was on a long visit to Germany. The experience of working at the Cavendish during its most successful period made a deep impression on Bullard, and his descriptions of Rutherford’s Cavendish strongly resemble the department of geodesy and geophysics at Cambridge when Bullard was its head during the 1960’s.
Though the electron scattering work was very successful, the end of Bullard’s studentship was approaching in 1931, and there were no jobs for physicists. That year he married Margaret Ellen Thomas; during their marriage they had four daughters. They were divorced in 1974.
Academic Career . In 1931 Bullard became a demonstrator in the department of geodesy and geophysics at Cambridge, which at the time of its formation in 1921 consisted of only one person, Sir Gerald Lenox-Conyngham. By himself Lenox-Conyngham was unable to do much. By 1931 he had persuaded the university that he needed help, and had been given funds for a junior post. On the advice of Rutherford he appointed Bullard to this position. At the same time Harold Jeffreys was appointed to a readership in geophysics. In the next eight years this small group of people had a quite remarkable impact on geophysics.
When Bullard became a demonstrator in the department, the only scientific instrument it possessed was a pendulum apparatus, and he at once set to work to improve its performance. This development is described in his Ph.D. dissertation, written in 1932. He then wished to use this technology to address a major geophysical problem. The East African Rift is one of the largest geological structures on the continents, and Bullard mounted an expedition to measure the gravity field in its neighborhood throughout central Africa. Such an ambitious approach was possible only because of the improvements he had made to the pendulum. The account of the results (1936) shows the care and thoroughness with which every part of the experiment was planned. The principal conclusion of this work was that the rift was formed by compression, an idea that Bullard later recognized as completely incorrect. Nonetheless, his gravity work on the East African Rift was of great and lasting importance. It showed how simple geophysical measurements could be used to investigate the origin of major geological structures. To do so, Bullard had to overcome the difficulties of making accurate measurements under primitive conditions in the field.
The work on gravity established Bullard’s reputation, and in 1936 he was awarded the Smithson fellowship of the Royal Society. With the support of Lenox-Conyngham, he then became interested in other types of geophysical measurement of relevance to geological problems. He designed a short-period seismometer with which he and others carried out a survey of the depth to the basement beneath southeast England, using seismic refraction (1940). He also started to work on marine geophysics.
The other major project that Bullard started at this time was the measurement of heat flow from the earth’s interior. Though this problem had been of great interest fifty years earlier, and had led to the famous controversy between Lord Kelvin and the geologists, it had since then largely been neglected, though a large number of temperature measurements from boreholes had been accumulated. But the conductivity of the rocks through which the holes passed was not known. Bullard and A. E. Benfield adapted an existing technique to measure the conductivity. In the winter of 1938–1939 Bullard visited South Africa, where detailed temperature measurements had been made by L. J. Krige in several deep boreholes through hard rocks of uniform lithology. He showed that the difference of a factor of about two between the geothermal gradients in South Africa and Britain was caused by a corresponding difference in the thermal conductivity (1939).
Measurement of the heat flow on land was Bullard’s last major project before the outbreak of the war. In the eight years in which he had been working in geophysics, he had made important contributions to three of the four branches of the subject—gravity, seismology, and heat flow—principally from an experimental point of view. Bullard’s theoretical investigations during this period were experimentally motivated. As an experimentalist he was outstanding, and his work showed the strong influence of his training in the Cavendish. The international attention his experiments generated led to his election to the Royal Society in 1941.
In November 1939 Bullard became an experimental officer attached to H.M.S. Vernon, which was a laboratory of the Admiralty concerned with mine warfare. The Germans had developed a new and very effective magnetic mine that aircraft could lay in shallow water. These mines were used from the start of the war in September 1939, and by the end of December had sunk sixty ships. Bullard saw what needed to be done, and after a sharp struggle with the naval scientific establishment, was put in charge of the development of methods of protecting ships from magnetic mines. Soon afterward he became head of the group concerned with sweeping all kinds of mines. He quickly developed methods of dealing with magnetic mines, and soon had time to think about other mechanisms the Germans might develop for triggering mines, such as acoustical and pressure sensors, and to develop methods of sweeping them before they came into use. In this Bullard and his group were very effective. When the Germans deployed a new type of mine to protect the beaches during the Normandy landings, they were being swept within twelve hours of the first casualty (Hepworth, p. 72). After eighteen months the losses of ships from mines had been reduced to only 10 percent of those from submarines. Bullard then decided he should move to London to join Blackett, as assistant director of naval operational research. Here he worked on a variety of problems concerned with British mines and submarine attacks on German and Italian shipping. Like most of the scientists in their twenties and thirties who worked on wartime problems, Bullard was profoundly affected by this experience.
When Bullard returned to Cambridge after the war, he found the place in a sorry state. He began by scrubbing the floor and energetically collecting surplus equipment to provide the basic needs of the department. But both he and his wife became restless, and in 1947 he agreed to become head of the physics department in Toronto. Bullard moved to Toronto in the spring of 1948 and was a very successful head of department, starting many new projects. He encouraged the development of geochemistry, especially radiometric dating and heat production, and for this purpose installed a mass spectrometer. He also continued his work on heat flow, and became interested in computers, whose development he had followed from their beginnings at Bletchley during the war. The university had recently installed FERUT, constructed by the Farranti corporation, which Bullard used for his calculations. He worked on the generation of the earth’s magnetic field and organized a visit to Scripps Institute of Oceanography to build an instrument to measure the heat flux through the seafloor.
In the spring of 1949 Bullard was offered the directorship of the (British) National Physical Laboratory on the understanding that he would stay for a decent period, which was informally agreed to be five years, In June he accepted, and resigned his professorship from the end of the year. He then left Toronto for Scripps, where he spent the summer building a heat flow probe and analyzing the evolution of the earth’s magnetic field to determine the rate of westward drift. He had a very successful summer, so successful that President Sproul of the University of California offered him the directorship at Scripps, which he declined.
At the end of 1949 Bullard returned to Britain to be the director of the National Physical Laboratory (NPL), which had been founded in 1900 by the Royal Society and the British government. Its first director, Sir Richard Glazebrook, had seen the importance of providing a measurement service for British industry, which was still a major part of its work when Bullard became director. The laboratory was large, employing more than 1, 000 people by 1955. Bullard was a well-liked and effective director, and was knighted for his services. He particularly encouraged Louis Essen’s work on atomic frequency standards, and the group working on electronic computations on the ACE computer, which he used extensively for his dynamo calculations (1954). Bullard remained involved in military problems and served as the joint chairman of the Anglo-American Ballistic Missile Commission. The most surprising feature of this period of Bullard’s life was its scientific productivity; he wrote several of his most important and influential papers (“The Flow of Heat Through the Floor of the Atlantic Ocean,” “Homogeneous Dynamos and Terrestrial Magnetism,” “Heat Flow Through the Deep Sea Floor”) at the NPL. Indeed, it was probably the most effective period of his whole career. Bullard made use of the workshops to construct his marine heat-flow equipment. He also used the laboratory to determine the thermal conductivity of rocks. He later remarked that it was less time-consuming to administer the NPL than the department at Cambridge, because the NPL provided effective assistance.
When he was appointed, Bullard had originally agreed to stay at the NPL for five years; in fact he stayed for six. In 1954 he began to sound out his friends at Cambridge about the possibility of his returning. James Chadwick, now master of Caius College, arranged for him to be appointed to a Bye fellowship at the college in the summer of 1955. This appointment was for three years and was not a university post. On the strength of this agreement. Bullard resigned as director of NPL. But before he could take up the Caius post, Keith Runcorn accepted the professorship of physics at Newcastle. Runcorn’s departure left a vacancy as an assistant director of research, and Ben Browne, who was then reader and head of the department of geodesy and geophysics, immediately appointed Bullard to this post.
Bullard made the department into one of the best places to study geophysics in the world. He attracted excellent students and made sure they had the facilities they needed. He set them off in directions where their abilities would be best displayed. He was generous to colleagues who would otherwise have been overshadowed by his great distinction and fame. He used his contacts in the universities, industry, and government in the United Kingdom and the United States to make sure that people did excellent work. Most of the successes of the department were a direct consequence of his insight and encouragement.
The largest group that remained at the department was led by Maurice Hill. It was concerned with marine geophysics and continued the work Bullard had started before the war. Hill had developed seismic refraction at sea in order to understand the structure of the ocean basins. Bullard broadened this effort and encouraged students to build new oceangoing instruments, They constructed a proton precession magnetometer that was towed behind the ship, and the heat flow through the seafloor was measured with Bullard’s heat-flow instrument. Bullard also encouraged the development of land seismology, particularly because of his interest in the detection of underground nuclear explosions. Several of his students worked on the dynamo problem, using the excellent computer facilities available in the mathematics laboratory. He himself was one of the few people in the university who was allowed to use the machine at night, when the operators were not present; he used it to develop methods of automatic data collection and reduction. Bullard also took an interest in geochemical instrumentation and in the design of sensitive mass spectrometers that Jack Miller was developing.
During this period the problem of the evolution of the ocean basins slowly came to dominate work in the department. The observations of Runcorn and his co-workers, especially that of Edward Irving on the magnetization of Australian rocks, could be understood only if large relative movements between continents occurred. But none of the marine geophysicists, including Hill, could understand how such displacements were taken up by the structures in the ocean basins. The key suggestion was made by Harry Hess of Princeton (1962), who proposed that new seafloor was created only on ridge axes. In making this suggestion Hess was strongly influenced by the discovery of heat flow anomalies associated with ridges, which had been made by Bullard, A. E. Maxwell, and R. R. D. Revelle (1956). Bullard invited Hess to talk about his ideas at a conference in Cambridge in January 1962, and many people who later worked on this problem were in the audience. When Fred Vine became a research student at the department, and found a reversely magnetized seamount in the Indian Ocean. Bullard was well aware of the reality of reversals because of his interest in the dynamo problem. He encouraged Vine and his supervisor, Drummond Matthews, to publish their ideas on how seafloor spreading and reversals could together account for the oceanic magnetic lineations. In 1965 Hess and J. Tuzo Wilson of Toronto spent their sabbaticals at the department. During this period Wilson wrote a number of papers that proposed many of the concepts of what is now called plate tectonics. Hess persuaded Vine to go to Princeton, where he used his ideas to interpret magnetic profiles from all parts of the world. Under Bullard the department at Cambridge played a major role in the construction of the new theory. His personal contribution was the use of Euler’s theorem to describe the motions of continents on the earth’s surface, but his influence on others involved in the work was very extensive.
Bullard’s period as head of department was his most successful as a scientific administrator. He remained at Cambridge for eighteen years, during fourteen of which he was head of department. This was a much longer period than he spent at Toronto or the NPL. Because the organization was small, with a rapid turnover of students, Bullard influenced a large number of people who now occupy senior positions in the United Kingdom and North America. He also was head at a particularly important period in the development of the earth sciences. He gave considerable thought to what would happen when he retired, and was involved in discussions whose aim was the formation of a single department of earth sciences. Though Bullard strongly supported this idea, his buccaneering style frightened the other departments, and amalgamation became possible only in 1980, after he had retired. The department of geodesy and geophysics was then renamed the Bullard Laboratories in his honor.
Though most of Bullard’s time was spent on the affairs of the department, he remained involved in government matters, though to a lesser degree than when he was at NPL. He attended the Geneva discussions on nuclear disarmament in 1958 as an adviser to the British government. He also became chairman of the committee in charge of British space research. Bullard enjoyed his connections with industry. He took an active interest in the family brewery, especially after his uncle died, until it was taken over by Watneys. He became a director of IBM U.K., and helped to persuade the university to buy its first IBM computer. He made a large collection of scientific books, and was especially interested in Halley, whose ability to analyze data of variable reliability he found particularly impressive.
When Bullard retired from Cambridge, his health was failing, but he was determined to make the most of his remaining time and started a new life with great bravery. In 1974 he married Ursula Cooke Curnow and moved to Scripps. He also became a geophysical consultant to the University of Alaska. Only months before he died, a group of his former students and colleagues organized a meeting in his honor at Scripps that he bravely attended. Though he was physically frail and in great pain, his comments and questions to the speakers showed his usual grasp of the essentials. Bullard died in his sleep the night after finishing his last paper.
Besides being a fellow of the Royal Society, Bullard was foreign honorary member of the American Academy of Arts and Sciences (1953), foreign associate of the U.S. National Academy of Sciences (1959), Bakerian Lecturer of the Royal Society (1967), and foreign member of the American Philosophical Society (1969). Among his awards were the Hughes Medal of the Royal Society (1953), the Chree Medal of the Physics Society (1956), the Day Medal of the Geological Society of America (1959), the gold medal of the Royal Astronomical Society (1965), the Agassiz Medal of the U.S. National Academy of Sciences (1965), the Wollaston Medal of the Geological Society of London (1967), the Vetlesen Prize of Columbia University (1968), the Bowie Medal of the American Geophysical Union (1975), the Royal Medal of the Royal Society (1975), and the Ewing Medal of the American Geophysical Union (1978).
Major Scientific Work . Heat Flow on Land and at Sea. The methods now used to measure the heat flow through the earth’s surface both on land and at sea were devised by Bullard before and shortly after the war. On land a number of accurate measurements of temperatures in boreholes were available before he started work, but no systematic measurements of the thermal conductivity of rocks had been carried out. Furthermore, the heat flow was obtained by determining the temperature gradient from temperature differences, which Bullard (1939) showed was not the most accurate method of estimating the heat flux. But the principal error was caused by the uncertainty in the thermal conductivity of the rocks through which the boreholes passed. Bullard systematically measured its variation with rock type, and showed that the low temperature gradient in the South African gold mines compared with British coal mines resulted from the high thermal conductivity of the quartzite compared with shales. His work in this area established standards and techniques that have remained largely unchanged. His borehole observations established a reliable average value for the heat flux through the continents, but he did not discover any of the major regional variations in heat flux that are now known.
Bullard thought about the problems of measuring heat flow at sea when he first became interested in marine geology before the war. He thought the best way to measure it was to drive a spike containing the thermometers into the sediment on the seafloor and measure the temperature gradient. He built such an instrument at Scripps in 1949, using thermojunctions to measure the temperature, with a galvanometer and a camera as the recording system. The thermal conductivity of the sediments had to be measured later in the laboratory, using samples obtained from a core in the vicinity. The interior of the instrument contained air at atmospheric pressure, and the case was sealed with O-rings. Bullard knew about this method of sealing from its use in airplane hydraulic systems during the war, but this was the first time O-rings had been used in marine geophysics. They are now universally used in all deep-sea instrumentation. Bullard wrote an extensive account of the instrument in 1954, and in 1956 reviewed the results from both the Atlantic and the Pacific with Revelle and Maxwell. The observations in the Pacific showed the band of elevated heat flow that is now known to be a universal feature of spreading ridges. This paper contains a discussion of mantle rheology and of how the heat flow anomaly could be maintained by mantle convection.
As so often has happened in marine geophysics, most of the major results were obtained with the first instrument. The only major discovery that has since been made is the importance of heat transfer by movement of seawater into and out of the ocean crust. Water at temperatures as great as 350° C emerges through vents in the seafloor and transport heat by advection. Bullard’s work established that the conductive heat transport through oceans and continents was the same. When the advected heat is included, the heat loss through the ocean floor considerably exceeds that through the continents.
The Earth’s Magnetic Field. Bullard first became interested in the main magnetic field of the earth during the early part of the war. In connection with his work on magnetic mines he read Sydney Chapman and Julius Bartels’ book on geomagnetism (1940). After the war he started a general investigation of how motions in the core might maintain the magnetic field. His first concern was with the energetics of the core motions (“The Magnetic Field Within the Earth”) and the observed secular changes of the magnetic field (“Electromagnetic Induction in a Rotating Sphere,” “The Westward Drift of the Earth’s Magnetic Field”). He showed that the various astronomical effects, such as tidal retardation and changes in the length of day, could not generate enough energy to maintain the field against ohmic dissipation, and that the most likely energy source was the release of gravitational potential energy through some form of convection in the core. This view is now widely accepted, though little has been added to Bullard’s order-of-magnitude arguments. Meanwhile, Walter Elsasser had investigated the kinematic problem of how a uniform conducting sphere could act as a self-exciting dynamo. His discussion was concerned with the general problem of how a complicated velocity field interacted with both poloidal and toroidal fields to produce a dynamo. Starting at Toronto, Bullard attempted to construct a model for a real homogeneous dynamo using Elsasser’s approach. To do so he employed the digital computers at Toronto and later at NPL. This work is one of the first attempts to obtain numerical solutions to fluid mechanical problems that are analytically intractable. Such numerical experiments are now a powerful and widely used method of investigating complicated nonlinear problems. Bullard’s work on the dynamo problem was one of the first uses of this approach for nonmilitary purposes. However, the model he proposed is now known not to maintain a dynamo. Nonetheless, the approach he pioneered has been widely adopted, and selfexciting kinematic dynamos similar to the one he proposed with H. Gellman have since been discovered.
Continental Fits. In the late 1950’s Bullard became convinced that large horizontal displacements of parts of the earth’s surface occurred. The two observations (1964) that particularly affected his views were the demonstration of offsets in the magnetic anomaly pattern of more than 1, 000 kilometers off the west coast of North America and the paleomagnetic observations from Australia. which were not compatible with polar wandering and required relative motion between Australia and Europe. Bullard decided to fit the continents round the Atlantic together geometrically. He wished to demonstrate how excellent the fits were, and to separate this question from the problem of the mechanism that had caused the displacements. The confusion between these two separate problems had bedeviled the hypothesis of continental drift since A. L. Wegener’s influential work. Bullard decided to produce the fits by minimizing the misfit between the continental margins rather than that of the coastlines. Both Wegener and A. L. Du Toit had argued that this was the correct procedure. S. W. Carey had previously attempted to make geometrically correct reconstructions in the same way, but he had used them to support his physically implausible idea of an expanding Earth, Bullard and his colleagues were principally interested in producing convincing fits, and the maps they generated (1965) have been widely reproduced in modern textbooks. Bullard needed a convenient description of the movement of a rigid continent on the surface of the earth. He was the first to use Euler’s theorem for the purpose, and obtained the pole and rotation angle required by minimizing the misfit. Though he regarded this description simply as a convenient way of describing the motions, this theorem later became the cornerstone of plate tectonics, and was not widely known to geophysicists before his work.
I. Original Works. “Remarks on the Scattering of Electrons by Atomic fields,” in Proceedings of the Cambridge Philosophical Society, 26 (1930), 556–563, with H. S. W. Massey; “The Elastic Scattering of Slow Electrons in Argon,” in Proceedings of the Royal Society, A130 (1931), 579–590, with H. S. W. Massey: “The Elastic Scattering of Slow Electrons in Gases, II ,” ibid., A133 (1931), 637–651, with H. S. W. Massey: “Gravity Measurements in East Africa,” in Philosophical Transactions of the Royal Society, A235 (1936), 445–531; “Heat flow in South Africa,” in Proceedings of the Royal Society, A173 (1939), 474–502; “Seismic Investigations on the Paleozoic floor of East England,” in Philosophical Transactions of the Royal Society, A239 (1940), 29–94, with T. F. Gaskell, W. B. Harland, and C. Kerr-Grant: “Submarine Seismic Investigations,” in Proceedings of the Royal Society, A177 (1941), 476–499, with T. F. Gaskell; “The Protection of Ships from Magnetic Mines,” in Proceedings of the Royal Institution, 33 (1946), 554–566; “The Time Necessary for a Bore-hole to Attain Temperature Equilibrium,” in Monthly Notices of the Royal Astronomical Society, geophysical supp., 5 (1947), 127–130; “Electromagnetic Induction in a Rotating Sphere,” in Proceedings of the Royal Society, A199 (1949), 413–433; “The Magnetic Field Within the Earth,” ibid., A197 (1949), 433–453.
“The Westward Drift of the Earth“s Magnetic Field.” in Philosophical Transactions of the Royal Society, A243 (1950), 67–92, with C. Freedman, H. Gellman, and J. Nixon; “The Flow of Heat Through the Floor of the Atlantic Ocean,” in Proceedings of the Royal Society, A222 (1954), 408–429; “Homogeneous Dynamos and Terrestrial Magnetism,” in Philosophical Transactions of the Royal Society, A247 (1954), 213–278, with H. Gellman; “The Stability of a Homopolar Dynamo,” in Proceedings of the Cambridge Philosophical Society, 51 (1955), 744–760; “Heat Flow Through the Deep Sea Floor,” in Advances in Geophysics, 3 (1956), 153–181 with A. E. Maxwell and R. Revelle; “Continental Drift,” in Quarterly Journal of the Geological Society of London, 120 (1964), 1–34; “The Fit of the Continents Around the Atlantic,” in Philosophical Transactions of the Royal Society, A258 (1965), 41–51, with J. E. Everett and A. C. Smith; “Reversals of the Earth“s Magnetic field,” ibid., A263 (1968), 481–524: “The Origin of the Oceans,” in Scientific American. 221 (September 1969), 66–75; and “Electromagnetic Induction in the Oceans,” in A. Maxwell. ed., The Sea, IV, pt. 1 (New York, 1970). 695–730, with R. L. Parker.
II. Secondary Literature. T. Hepworth, “On the Atlantic Shelf: A Trawlers cruise with a Purpose,” 2 pts. in Yachting Monthly, 81 (1946), 70–75 and 148–151; and D. P. McKenzie, “Edward Crisp Bullard,” in Biographical Memoirs of Fellows of the Royal Society, 33 (1987), 67–97.
D. P. McKenzie