The Atom

AP Physics Period 2

In the spring of 1897 J.J. Thomson demonstrated that the beam of glowing
matter in a cathode-ray tube was not made of light waves, as "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 "what ever the gas through which the discharge
passes" if gas was introduced into the tube. Since 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 success at the annual Cavendish dinner.
Armed with the electron, and knowing from other experiment that what was
left when electrons were stripped away from an atom was 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 negatively electrified corpuscles enclosed in a
sphere of uniform positive electrification" like raisins in a pudding, was a
hybrid: particulate electrons and diffuse remainder. It served the useful
purpose of demonstrating mathematically that electrons could be arranged in a
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 the electrons were responsible for chemical affinities between
elements, that chemistry was ultimately electrical.
Thomson just missed discovering X rays in 1884. 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. Thomson noticed that glass tubing held
"at a distance of some feet from the discharge-tube" 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 purse the cause. Rontgen isolated the effect
by covering his cathode-ray tube with black paper. When a nearby screen of
florescent material still glowed he realized that whatever was causing the
screen to glow was passing through the paper and intervening with the air. If
he held his hand between the covered tube and the screen, his hand slightly
reduced the glow on the screen but in the dark shadow he could see his bones.
Rontgen\'s discovery intrigued other researchers beside J.J. Thomson and
Ernest Rutherford. The Frenchman Hernri Becquerel was a third-generation
physicist who, like his father and grandfather before him, occupied the chair of
physics at the Musee Historie in Pairs; like them also he was an expert on
phosphorescence and fluorescence. In his case, particular of uranium. He heard
a report of Rontgen\'s work at the weekly meeting of the Academie des Sciences on
January 20, 1896. He learned that the X rays emerged from the fluorescence
glass, which immediately suggested to him that he should test various
fluorescence materials to see if they also emitted X rays. He worked for ten
days without success, read an article on X rays in January 30 that encouraged
him to keep working and decided to try a uranium slat, 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 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 a cathode ray releases
Rontgen\'s X rays from the glass.
The story of Becqueerel\'s subsequent serendipity is famous. When he
tried to repeat his experiment on Feb. 26 and again on February 27 Paris was
covered with clouds. He put the uncovered photographic plate away in a dark
drawer, with the uranium salt in place. On March 1 he decided to go ahead and
develop the play, "expecting to find the images very feeble. On the contrary,