him at dinner parties.
But Thomson never got around to reading his dissertation. The first meeting had not, in fact, gone so well. The new student from Denmark had done more than explain his ideas; he had shown Thomson the errors he found in Thomson's electron-theory work. “I wonder,” Bohr wrote Mar-grethe soon after, “what he will say to my disagreement with his ideas.” And a little later: “I'm longing to hear what Thomson will say. He's a great man. I hope he will not get angry with my silly talk.”
Thomson may or may not have been angry. He was not much interested in electrons anymore. He had turned his attention to positive rays — the experiment he assigned Bohr concerned such rays and Bohr found it distinctly unpromising — and in any case had very little patience with theoretical discussions. “It takes half a year to get to know an Englishman,” Bohr said in his last interview. “… It was the custom in England that they would be polite and so on, but they wouldn't be interested to see anybody… I went Sundays to the dinner in Trinity College… I was sitting there, and nobody spoke to me ever in many Sundays. But then they understood that I was not more eager to speak to them than they were to speak to me. And then we were friends, you see, and then the whole thing was different.” The insight is generalized; Thomson's indifference was perhaps its first specific instance.
Then Rutherford turned up at Cambridge.
He “came down from Manchester to speak at the annual Cavendish Dinner,” says Bohr. “Although on this occasion I did not come into personal contact with [him], I received a deep impression of the charm and power of his personality by which he had been able to achieve almost the incredible wherever he worked. The dinner” — in December — “took place in a most humorous atmosphere and gave the opportunity for several of Rutherford's colleagues to recall some of the many anecdotes which already then were attached to his name.” Rutherford spoke warmly of the recent work of the physicist C. T. R. Wilson, the inventor of the cloud chamber (which made the paths of charged particles visible as lines of water droplets hovering in supersaturated fog) and a friend from Cambridge student days. Wilson had “just then,” says Bohr, photographed alpha particles in his cloud chamber scattering from interactions with nuclei, “the phenomenon which only a few months before had led [Rutherford] to his epoch-making discovery of the atomic nucleus.”
Bohr had matters on his mind that he would soon relate to the problem of the nucleus and its theoretically unstable electrons, but it was Rutherford's enthusiastic informality that most impressed him at the annual dinner. Remembering this period of his life long afterward, he would single out for special praise among Rutherford's qualities “the patience to listen to every young man when he felt he had any idea, however modest, on his mind.” In contrast, presumably, to J. J. Thomson, whatever Thomson's other virtues.
Soon after the dinner Bohr went up to Manchester to visit “one of my recently deceased father's colleagues who was also a close friend of Rutherford,” whom Bohr wanted to meet. The close friend brought them together. Rutherford looked over the young Dane and liked what he saw despite his prejudice against theoreticians. Someone asked him later about the discrepancy. “Bohr's different,” Rutherford roared, disguising affection with bluster. “He's a football player!” Bohr was different in another regard as well; he was easily the most talented of all Rutherford's many students — and Rutherford trained no fewer than eleven Nobel Prize winners during his life, an unsurpassed record.
Bohr held up his decision between Cambridge and Manchester until he could go over everything with Harald, who visited him in Cambridge in January 1912 for the purpose. Then Bohr eagerly wrote Rutherford for permission to study at Manchester, as they had discussed in December. Rutherford had advised him then not to give up on Cambridge too quickly — Manchester is always here, he told him, it won't run away — and so Bohr proposed to arrive for spring term, which began in late March. Rutherford gladly agreed. Bohr felt he was being wasted at Cambridge. He wanted substantial work.
His first six weeks in Manchester he spent following “an introductory course on the experimental methods of radioactive research,” with Geiger and Marsden among the instructors. He continued pursuing his independent studies in electron theory. He began a lifelong friendship with a young Hungarian aristocrat, George de Hevesy, a radiochemist with a long, sensitive face dominated by a towering nose. De Hevesy's father was a court councillor, his mother a baroness; as a child he had hunted partridge in the private game park of the Austro-Hungarian emperor Franz Josef next to his grandfather's estate. Now he was working to meet a challenge Rutherford had thrown at him one day to separate radioactive decay products from their parent substances. Out of that work he developed over the next several decades the science of using radioactive tracers in medical and biological research, one more useful offspring of Rutherford's casual but fecund paternity.
Bohr learned about radiochemistry from de Hevesy. He began to see connections with his electron-theory work. His sudden burst of intuitions then was spectacular. He realized in the space of a few weeks that radioactive properties originated in the atomic nucleus but chemical properties depended primarily on the number and distribution of electrons. He realized — the idea was wild but happened to be true — that since the electrons determined the chemistry and the total positive charge of the nucleus determined the number of electrons, an element's position on the periodic table of the elements was exactly the nuclear charge (or “atomic number”): hydrogen first with a nuclear charge of 1, then helium with a nuclear charge of 2 and so on up to uranium at 92.
De Hevesy remarked to him that the number of known radioelements already far outnumbered the available spaces on the periodic table and Bohr made more intuitive connections. Soddy had pointed out that the radioelements were generally not new elements, only variant physical forms of the natural elements (he would soon give them their modern name, isotopes). Bohr realized that the radioelements must have the same atomic number as the natural elements with which they were chemically identical. That enabled him to rough out what came to be called the radioactive displacement law: that when an element transmutes itself through radioactive decay it shifts its position on the periodic table two places to the left if it emits an alpha particle (a helium nucleus, atomic number 2), one place to the right if it emits a beta ray (an energetic electron, which leaves behind in the nucleus an extra positive charge).
All these first rough insights would be the work of other men's years to anchor soundly in theory and experiment. Bohr ran them in to Rutherford. To his surprise, he found the discoverer of the nucleus cautious about his own discovery. “Rutherford… thought that the meagre evidence [so far obtained] about the nuclear atom was not certain enough to draw such consequences,” Bohr recalled. “And I said to him that I was sure that it would be the final proof of his atom.” If not convinced, Rutherford was at least impressed; when de Hevesy asked him a question about radiation one day Rutherford responded cheerfully, “Ask Bohr!”
Rutherford was well prepared for surprises, then, when Bohr came to see him again in mid-June. Bohr told Harald what he was on to in a letter on June 19, after the meeting:
It could be that I've perhaps found out a little bit about the structure of atoms. You must not tell anyone anything about it, otherwise I certainly could not write you this soon. If I'm right, it would not be an indication of the nature of a possibility… but perhaps a little piece of reality… You understand that I may yet be wrong, for it hasn't been worked out fully yet (but I don't think so); nor do I believe that Rutherford thinks it's completely wild; he is the right kind of man and would never say that he was convinced of something that was not entirely worked out. You can imagine how anxious I am to finish quickly.
Bohr had caught a first glimpse of how to stabilize the electrons that orbited with such theoretical instability around Rutherford's nucleus. Rutherford sent him off to his rooms to work it out. Time was running short; he planned to marry Margrethe N0rland in Copenhagen on August 1. He wrote Harald on July 17 that he was “getting along fairly well; I believe I have found out a few things; but it is certainly taking more time to work them out than I was foolish enough to believe at first. I hope to have a little paper ready to show to Rutherford before I leave, so I'm busy, so busy; but the unbelieveable heat here in Manchester doesn't exactly help my diligence. How I look forward to talking to you!” By the following Wednesday, July 22, he had seen Rutherford, won further encouragement, and was making plans to meet Harald on the way home.
Bohr married, a serene marriage with a strong, intelligent and beautiful woman that lasted a lifetime. He
