Richard Rhodes - The Making of the Atomic Bomb

It seems probable that J. J. Thomson sat eager young Ernest Rutherford down in the darkly paneled rooms of the Gothic Revival Cavendish Laboratory that Clerk Maxwell had founded, at the university where Newton wrote his great Principia, and kindly told him he could not serve God and Mammon at the same time. It seems probable that the news that the distinguished director of the Cavendish had written the Olympian Lord Kelvin about the commercial ambitions of a brash New Zealander chagrined Rutherford to the bone and that he went away from the encounter feeling grotesquely like a parvenu. He would never make the same mistake again, even if it meant strapping his laboratories for funds, even if it meant driving away the best of his protégés, as eventually it did. Even if it meant that energy from his cherished atom could be nothing more than moonshine. But if Rutherford gave up commercial wealth for holy science, he won the atom in exchange. He found its constituent parts and named them. With string and sealing wax he made the atom real.

The sealing wax was blood red and it was the Bank of England’s most visible contribution to science. British experimenters used Bank of England sealing wax to make glass tubes airtight. 127Rutherford’s earliest work on the atom, like J. J. Thomson’s work with cathode rays, grew out of nineteenthcentury examination of the fascinating effects produced by evacuating the air from a glass tube that had metal plates sealed into its ends and then connecting the metal plates to a battery or an induction coil. Thus charged with electricity, the emptiness inside the sealed tube glowed. The glow emerged from the negative plate—the cathode—and disappeared into the positive plate—the anode. If you made the anode into a cylinder and sealed the cylinder into the middle of the tube you could project a beam of glow—of cathode rays—through the cylinder and on into the end of the tube opposite the cathode. If the beam was energetic enough to hit the glass it would make the glass fluoresce. The cathode-ray tube, suitably modified, its all-glass end flattened and covered with phosphors to increase the fluorescence, is the television tube of today.

In the spring of 1897 Thomson demonstrated that the beam of glowing matter in a cathode-ray tube was not made up of light waves, as (he wrote drily) “the almost unanimous opinion of German physicists” held. Rather, cathode rays were negatively charged particles boiling off the negative cathode and attracted to the positive anode. These particles could be deflected by an electric field and bent into curved paths by a magnetic field. They were much lighter than hydrogen atoms and were identical “whatever the gas through which the discharge passes” if gas was introduced into the tube. 128Since they were lighter than the lightest known kind of matter and identical regardless of the kind of matter they were born from, it followed that they must be some basic constituent part of matter, and if they were a part, then there must be a whole. The real, physical electron implied a real, physical atom: the particulate theory of matter was therefore justified for the first time convincingly by physical experiment. They sang J. J.’s success at the annual Cavendish dinner:

Armed with the electron, and knowing from other experiments that what was left when electrons were stripped away from an atom was a much more massive remainder that was positively charged, Thomson went on in the next decade to develop a model of the atom that came to be called the “plum pudding” model. The Thomson atom, “a number of negativelyelectrified corpuscles enclosed in a sphere of uniform positive electrification” like raisins in a pudding, was a hybrid: particulate electrons and diffuse remainder. 130It served the useful purpose of demonstrating mathematically that electrons could be arranged in stable configurations within an atom and that the mathematically stable arrangements could account for the similarities and regularities among chemical elements that the periodic table of the elements displays. It was becoming clear that electrons were responsible for chemical affinities between elements, that chemistry was ultimately electrical.

Thomson just missed discovering X rays in 1894. He was not so unlucky in legend as the Oxford physicist Frederick Smith, who found that photographic plates kept near a cathode-ray tube were liable to be fogged and merely told his assistant to move them to another place. 131, 132Thomson noticed that glass tubing held “at a distance of some feet from the dischargetube” fluoresced just as the wall of the tube itself did when bombarded with cathode rays, but he was too intent on studying the rays themselves to pursue the cause. 133Röntgen isolated the effect by covering his cathode-ray tube with black paper. When a nearby screen of fluorescent material still glowed he realized that whatever was causing the screen to glow was passing through the paper and the intervening air. 134If he held his hand between the covered tube and the screen, his hand slightly reduced the glow on the screen but in dark shadow he could see its bones.

Röntgen’s discovery intrigued other researchers besides J. J. Thomson and Ernest Rutherford. The Frenchman Henri Becquerel was a third-generation physicist who, like his father and grandfather before him, occupied the chair of physics at the Musée d’Histoire Naturelle in Paris; like them also he was an expert on phosphorescence and fluorescence—in his case, particularly of uranium. He heard a report of Röntgen’s work at the weekly meeting of the Académie des Sciences on January 20, 1896. He learned that the X rays emerged from the fluorescing glass, which immediately suggested to him that he should test various fluorescing materials to see if they also emitted X rays. He worked for ten days without success, read an article on X rays on January 30 that encouraged him to keep working and decided to try a uranium salt, uranyl potassium sulfate.

His first experiment succeeded—he found that the uranium salt emitted radiation—but misled him. He had sealed a photographic plate in black paper, sprinkled a layer of the uranium salt onto the paper and “exposed the whole thing to the sun for several hours.” When he developed the photographic plate “I saw the silhouette of the phosphorescent substance in black on the negative.” He mistakenly thought sunlight activated the effect, much as cathode rays released Röntgen’s X rays from the glass. 135

The story of Becquerel’s subsequent serendipity is famous. When he tried to repeat his experiment on February 26 and again on February 27 Paris was gray. He put the covered photographic plate away in a dark drawer, uranium salt in place. On March 1 he decided to go ahead and develop the plate, “expecting to find the images very feeble. On the contrary, the silhouettes appeared with great intensity. I thought at once that the action might be able to go on in the dark.” Energetic, penetrating radiation from inert matter unstimulated by rays or light: now Rutherford had his subject, as Marie and Pierre Curie, looking for the pure element that radiated, had their backbreaking work. 136

Between 1898, when Rutherford first turned his attention to the phenomenon Henri Becquerel found and which Marie Curie named radioactivity, and 1911, when he made the most important discovery of his life, the young New Zealand physicist systematically dissected the atom.

He studied the radiations emitted by uranium and thorium and named two of them: “There are present at least two distinct types of radiation—one that is very readily absorbed, which will be termed for convenience the α [alpha] radiation, and the other of a more penetrative character, which will be termed the β [beta] radiation.” 137(A Frenchman, P. V. Villard, later discovered the third distinct type, a form of high-energy X rays that was named gamma radiation in keeping with Rutherford’s scheme. 138) The work was done at the Cavendish, but by the time he published it, in 1899, when he was twenty-seven, Rutherford had moved to Montreal to become professor of physics at McGill University. A Canadian tobacco merchant had given money there to build a physics laboratory and to endow a number of professorships, including Rutherford’s. “The McGill University has a good name,” Rutherford wrote his mother. 139“£500 is not so bad [a salary] and as the physical laboratory is the best of its kind in the world, I cannot complain.”