The Neutron and the Bomb, page 10
(3) Provision of three additional lecturers of high standing, competent to direct advanced study in research in the new departments mentioned above.
(4) The endowment of another Chair of Physics in the University.
... It is estimated that the cost of the new buildings would not be more than £75,000 and that an additional sum of £125,000 will be necessary as an adequate endowment...
Sir Joseph Larmor, the Lucasian Professor of Mathematics at Cambridge, was an elector for the Cavendish Chair in 1919. He had first come to prominence in 1880 as Senior Wrangler in the Mathematical Tripos, when he beat J.J. Thomson into second place. Perhaps Rutherford’s strongest supporter, Larmor had written to him at a time when he was experiencing mixed feelings about leaving Manchester:
The fame of the Cavendish ought to make it easy to beg for funds for an extension. Shipley [the Vice-Chancellor] and our other public men are past masters at importunate mendicancy, and would be available... I now see the possibilities of expansion that are practical if people here had a strong lead. They complain of J.J. that he not only did not give a lead but poured cold water over projects.
Rutherford certainly offered a strong lead, but there was no largesse forthcoming. His own attitude to money was ambivalent and he does not seem to have followed through after his initial demand — a lack of persistence which meant that he would never join the past masters of begging. Indeed, with one or two notable exceptions, during his tenure as director, any expansion of the Cavendish was piecemeal, and frugality was the watchword of the laboratory’s economic management. In many ways, the consequences of this approach would be felt most acutely by Chadwick, whose responsibility it gradually became to keep the Cavendish functioning from day to day and its research workers tolerably content.
Whatever his frailties as a department manager, Rutherford had none as a leader. He and Chadwick1 agreed early on that the main thrust of research in the Cavendish should be aimed at understanding the structure of the atomic nucleus, and this policy endured for the period of Rutherford’s directorship. Rutherford was mindful of the disruption that the war had brought to the international world of physics, and immediately set about restoring his close ties with former colleagues in Europe and further afield. He was a tireless correspondent and traveller, who did much to get cooperation and research moving again by his selfless advice and practical help. In part he was able to do this because Chadwick proved so effective as an organizer, releasing him from many mundane and time-consuming duties. With his reserved, unobtrusive personality, Chadwick1 made a quiet start as Rutherford’s research assistant. In his words,
Everything had been diverted to the war effort. There was in fact very little equipment... So, gradually, part of my duties in helping Rutherford — it was a matter between ourselves, it wasn’t coupled to my studentship at all, there were no conditions of that kind attached to it, but it was understood between us that I was to do anything that I could to help him on the trivial matters. And I began to help in saying, well, we need this piece of apparatus, and ordering them.
Another reason for Chadwick to feel his way cautiously was that he was still relatively junior in scientific terms, and in 1919 the Cavendish contained some famous men other than its newly appointed director. Besides J.J. Thomson, who still came to the laboratory regularly with Rutherford’s blessing, there was C.T.R. Wilson,21 whose inspirational invention, the cloud chamber, had already proved to be one of the most useful pieces of apparatus ever devised in experimental physics. At the time of Chadwick’s arrival, Wilson was 50 years old and would continue to work alone, ‘a shy but enduring genius’18 — he received the Nobel Prize for Physics in 1927. Edward Appleton22 had started his research career at the Cavendish in 1914, but left to join the Army on the outbreak of war. He was recruited by the Army Signals Service and this shaped his future work on the thermionic valve and long-distance radio propagation. His subsequent discoveries represent a scientific hybrid, perpetuating both the Maxwell-J.J. Thomson tradition of electromagnetism and C.T.R. Wilson’s interest in atmospheric research. Rutherford encouraged his investigations of radio transmission and appointed him an assistant demonstrator.
Francis William Aston,23 the mechanically gifted son of a wealthy Birmingham metal merchant, was, like Wilson, a scientific loner. In 1919, he was busy perfecting his mass spectrograph with which he would soon show that neon, chlorine and other elements were mixtures of isotopes.10 Aston’s work was quickly recognized with the Nobel Prize for Chemistry in 1922: Chadwick24 later described him as one of the best experimenters he had known. Another great individualist appointed by J.J. was G.I. Taylor,14 who became the country’s leading expert on fluid mechanics. He had the misfortune to occupy the room next to Rutherford and Chadwick’s laboratory, and all visitors to the director had to pass through Taylor’s room. Rutherford unfairly compounded the inconvenience by treating Taylor as an unofficial guide, but he was of equable temperament and did not complain. Indeed there seems to have been no resentment towards the new professor and his assistant from any of these men; Aston and Taylor became two of Rutherford’s regular golf partners in a Trinity College foursome.
Less well-known to the outside world and slightly more prickly was Dr. G.F.C. Searle.14 He was a popular and revered figure in the Cavendish Laboratory, who had begun his fifty-five year career as an experimental demonstrator there before Chadwick was born. Searle excelled as a practical teacher and generations of undergraduates benefited from his wisdom and gentle humour. He openly opposed Rutherford’s suggestion to the University that the intermediate degree of an M.Sc. should be introduced for those students who could not meet the time or academic requirements of the three year Ph.D.; he20 foresaw ‘overcrowding of laboratories by students of little ability and resource’ and feared that any further increase in numbers would lead to the ‘discouragement of ordinary university teaching’.
While it is sometimes stated that Chadwick came to the Cavendish as Rutherford’s research lieutenant, he was in reality the senior research student, given the extra duty of supervising the Radium Room. In Chadwick’s1 words, he had ‘no authority or responsibility except to Rutherford.’ Rutherford, in the early days, kept a fatherly eye on him and was instrumental in strengthening his scientific credentials. In May 1920, Chadwick25 wrote to Rutherford applying for the vacant Clerk Maxwell Studentship.14 This was a University award, first made in 1890, after a bequest by Mrs Clerk Maxwell; previous holders had included C.T.R. Wilson and Aston. Mrs Maxwell’s original intention had been to enable students not endowed with private means to continue in research. One of the conditions of the studentship was that it should not depend on the result of an examination, but on promise in research. In his application, Chadwick listed seven papers previously published in the scientific journals; in other times, this might have excluded him for being overqualified, but he was given the prestigious award. At about this time, Chadwick also registered for the new Ph.D. degree.
The first part of his thesis26 was the important work already described, The atomic nucleus and the law of force. The second part explored more deeply the question of forces between nuclei at very close proximity, and the size and shape of the helium nucleus or α-particle. For this second experiment he was joined in the autumn of 1920 by Etienne Bieler,27 an 1851 Exhibition Scholar visiting from McGill University, Montreal. Bieler11 had no previous experience in radioactivity, but came to the Cavendish with a strong recommendation from A.S. Eve, Rutherford’s friend and successor as Professor of Physics in Montreal. Under Chadwick’s tutelage, Bieler learned the experimental techniques quickly, and Chadwick28 found him ‘a delightful companion both in and out of the laboratory’.
Part II of Chadwick’s Ph.D. thesis began with the following words:
When α-particles pass through hydrogen gas or a substance containing hydrogen, close collisions between an α-particle and a hydrogen nucleus occasionally take place. As a result of such a close collision, the hydrogen nucleus is set in swift motion and can be detected by the scintillation it produces on a zinc sulphide screen. Assuming that both the α-particle and the hydrogen nucleus can be regarded as points, and that the forces between them arise from their charges, C.G. Darwin [in 1914] calculated the number [of hydrogen nuclei] within any given angle to the path of the α-particle. Rutherford, [in 1919] however, found that the number and angular distribution of the projected H-particles did not agree with the simple theory, and he attributed the divergence to the complex structure of the α-particle.
In making his original conclusion, Rutherford had based his argument on the then accepted theory that the α-particle, or helium nucleus, was composed of four hydrogen nuclei and two electrons. Chadwick pointed out that Rutherford’s experiment was ‘of a preliminary nature, and... not carried out to any high degree of accuracy. Recently, the optical condition of counting scintillations have been so greatly improved, that a more direct method of attacking the problem was possible.’ What Chadwick had done was to study the catalogues of lens makers to find the microscope objective with the biggest aperture. He had selected ‘a Watson holoscopic objective of 16 mm focal length and .45 numerical aperture combined with a low-power eyepiece. Compared with the old system, this increased greatly the brightness of the scintillations and gave at the same time a larger field of view, i.e. a larger number of particles, other conditions remaining constant.’29 He also adapted the optically symmetrical apparatus that he had devised in the first experiment on the charge on the atomic nucleus. In order to employ this arrangement, the hydrogen target had to be held in the ring as the metal foils had been previously, and he decided to use paraffin wax, which is a combination of hydrogen and carbon atoms. Chadwick made the following, apparently sweeping, assumption:
In these collisions it is immaterial whether the hydrogen is present in the form of hydrogen gas or in the combined state as paraffin wax.
Although he offered no evidence in support of this statement, Chadwick had begun to assist Rutherford in experiments to disintegrate light elements by α-particle bombardment, and they had found no evidence for the splintering of carbon nuclei. On this basis, the presence of carbon in the paraffin wax target should not have any influence on the observed results — although a few years later the behaviour of carbon would become the central issue in a celebrated scientific controversy. What Chadwick and Bieler found was that for α-particles of low velocity (which had insufficient energy to approach the hydrogen nucleus very closely) the inverse square law, as between two point charges, was obeyed. For the high-velocity α-particles the numbers of projected hydrogen nuclei were much greater, in some cases more than a hundred times as great, than would be expected according to the inverse square law. This was a startling finding and clearly perplexed Chadwick for sometime. He had a deadline to meet for the submission of his thesis, and the final Discussion section covered one side of paper! Its opening sentence was a masterpiece of scientific conservatism, and much more in keeping with his innate caution than the assumption, quoted earlier, that opened the Method section. The Discussion began:
Until the fullest information which such experiments can yield has been obtained, it does not seem advisable to discuss in any detail the structure of the α-particle, or Helium nucleus, and the field of force around it.
The Cambridge University Reporter recorded that James Chadwick of Gonville and Caius College received his Ph.D. on 21 June 1921: he was one of the first to be conferred the degree. Five years later, Dirac30 was awarded his Ph.D. for a thesis entitled ‘Quantum Mechanics’, but few of the hundreds of dissertations submitted to Cambridge University down the years can have contained such profoundly important work as Chadwick’s.
Chadwick and Bieler29 soon arrived at more definite conclusions about their results and published them in a paper, The Collisions of Particles with Hydrogen Nuclei. This added some further results and a full Discussion, but was otherwise identical to the Ph.D. dissertation. The penultimate paragraph of the Discussion is reproduced below:
As regards the structure of the α particle, it will be apparent at once that no system of four H nuclei and two electrons united by inverse square law forces could give a field of force of such intensity over so large an extent. We must conclude either that the α particle is not made up of four H nuclei and two electrons, or that the law of force is not the inverse square in the immediate neighbourhood of an electric charge. It is simpler to choose the latter alternative, particularly as other experimental, as well as theoretical, considerations point in this direction. The present experiments do not seem to throw any light on the nature or the law of variation of the forces at the seat of an electric charge, but merely show that the forces are of very great intensity.
It is now known that the α-particle does not consist of four protons and two electrons, and that the inverse square law does not account for the interactions within the atomic nucleus. Since the mid-1930s, physicists have believed the nuclear constituents are held together by the strong interaction, a novel kind of force different from gravity or electromagnetic attraction. The 1921 paper of Chadwick and Bieler has been cited ‘as marking the birth of the strong interactions’.31 When asked, years later, about his earliest notions of the strong forces, Chadwick’s1 answer was revealing:
That was clear about 1921 or ‘22. But we had no explanation of it. I put it this way: Any idea one might have about the structure of the nucleus — particles had to be held together somewhere. So that in addition to the repulsive force between the positively charged particles, there had to be an attractive force somewhere. And I played around with various forms of the force with an attraction varying as the inverse fourth power of the distance. And I remember I had to brush up my mathematics a bit in order to solve some of the equations. And it was also clear, I think in Bieler’s mind, too. I remember our making lantern slides showing the potential well and the peak and so forth, and they were used in lectures.
During the first two decades of the century, experimental physicists like Chadwick and Rutherford were attempting to unlock the innermost secrets of the atom by observing phenomena that they contrived to create in a controlled way. Their observations were subjective to a degree: the information did not present itself in a digital or mathematical form, it depended on the vagaries of human perception. As Hughes32 has recently pointed out, disagreements between physicists during the 1920s often ‘were not over matters of interpretation of facts; they were disputes about what the facts were.’ The lack of any comprehensive theory of atomic structure beyond the Rutherford-Bohr model hampered the elucidation of experimental results; as we have already seen in Chadwick’s work, it was frequently necessary to fly in the face of accepted wisdom in order to present new findings. Atomic physics, or radioactivity as its Cambridge practitioners would have termed it, was still an empirical subject, where experimental evidence was gathered and used to shape theoretical developments; in a later phase, the new quantum mechanics became such a powerful tool that specific predictions could be made and then tested experimentally.
In June 1920 Sir Ernest Rutherford33 was accorded the unusual honour by the Royal Society of being invited to deliver his second Bakerian Lecture. Entitled ‘Nuclear constitution of atoms’, it encapsulated much of the state of knowledge on the subject, including a brief review of work completed during the previous decade and important speculations about the future. Early in his talk Rutherford made the following observation: ‘The question whether the atomic number of an element is the actual measure of its nuclear charge is a matter of such fundamental importance that all methods of attack should be followed up’. He then stated that ‘the results so far obtained by Mr Chadwick strongly support the identity of the atomic number with the nuclear charge...’ He also referred to his own work on α-particle scattering by hydrogen atoms in a section headed ‘Dimensions of Nuclei’, but did not mention Chadwick and Bieler in this context, suggesting that their experiment had not yet begun. Rutherford deduced that ‘the law of inverse squares no longer holds when the nuclei approach to within a distance of 3 × 10-13 cm of each other... in such close encounters there were enormous forces between the nuclei, and probably the structure of the nuclei was much deformed during the collision’. This suggests that Rutherford was already postulating a new force field in the immediate vicinity of the nucleus.
In contrast to the above passage, which can be construed as anticipating scientific discoveries of the 1930s, as Rutherford got to the heart of his lecture he stated his thoughts on the constitution of the nucleus in the following words:
From a study of radioactivity we know that the nuclei of the radio-active elements consist in part of helium nuclei of charge 2e. We also have strong reason for believing that the nuclei of atoms contain electrons as well as positively charged bodies, and that the positive charge on the nucleus represents the excess positive charge.
These views were very much of their time and had held sway in Rutherford’s mind for some years. He had been the first to recognize the emission of α- and β-particles from radioactive substances, and later correctly interpreted their production as nuclear events. Taking the argument a simple but misleading step further, he34 had written in a 1914 paper: ‘The helium nucleus is a very stable configuration which survives the intense disturbances resulting in its expulsion with high velocity from the radioactive atom, and is one of the units of which possibly the great majority of the atoms are composed’. The second part of the hypothesis, as presented by Rutherford35 in a discussion at the Royal Society in March 1914, seems utterly convincing on its face: ‘The general evidence indicates that the primary β-particles arise from a disturbance of the nucleus. The latter must consequently be considered as a very complex structure consisting of positive particles and electrons...’ But, for once, Rutherford’s celebrated powers of physical intuition, which allowed him to visualize events happening at a subatomic level, were leading him astray. Although he was correct that radioactivity is a nuclear phenomenon, it does not follow that α- and β-particles are stable constituents of the nucleus. During the 1920s, even he would find that the paradoxes of physics were no longer susceptible to intuition alone, and mathematical abstraction rather than descriptions based on mechanistic images would become the subject’s leading language. The exploration of the atom could not be accomplished by referring to mental charts that had been drawn up from the study of more commonplace physical surroundings and events.
