FREE ESSAY ON J.J THOMSON

J.J THOMSON

Joseph John Thomson was born on December 18, 1856 near Manchester, England. His father
died when
J.J.. was only sixteen. The young Thomson attended Owens College in Manchester, where his
professor of
mathematics encouraged him to apply for a scholarship at Trinity College, one of the most
prestigious of the
colleges at Cambridge University. Thomson won the scholarship, and in 1880 finished
second in his class in
the grueling graduation examination in mathematics. Trinity gave him a fellowship and he
stayed on there,
trying to craft mathematical models that would reveal the nature of atoms and
electromagnetic forces. 
One hundred years ago, amidst glowing glass tubes and the hum of electricity, the British
physicist
J.J.. Thomson went venturing into the interior of the atom. At the Cavendish Laboratory
at Cambridge
University, Thomson was experimenting with currents of electricity inside empty glass
tubes. He was
investigating a long-standing puzzle known as cathode rays. His experiments prompted him
to make a bold
proposal: these mysterious rays are streams of particles much smaller than atoms, they
are in fact minuscule
pieces of atoms. He called these particles corpuscles, and suggested that they might make
up all of the
matter in atoms. It was startling to imagine a particle residing inside the atom--most
people thought that the
atom was indivisible, the most fundamental unit of matter.
Thomson's speculation was not explicitly supported by his experiments. It took more
experimental
work by Thomson and others to sort out the confusion. The atom is now known to contain
other particles as
well. Yet Thomson's bold suggestion that cathode rays were material constituents of atoms
turned out to be
correct. The rays are made up of electrons: very small, negatively charged particles that
are indeed
fundamental parts of every atom.
Modern ideas and technologies based on the electron, leading to television and the
computer and
much else, evolved through many difficult steps. Thomson's careful experiments and
adventurous hypotheses
were followed by crucial experimental and theoretical work by many others in the United
Kingdom,
Germany, France and elsewhere. These physicists opened for us a new perspective--a view
from inside the
atom.
First, in a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode ray
tube ending
in a pair of metal cylinders with a slit in them. These cylinders were in turn connected
to an electrometer, a
device for catching and measuring electrical charge. Perrin had found that cathode rays
deposited an electric
charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate
the charge from the
rays. He found that when the rays entered the slit in the cylinders, the electrometer
measured a large amount
of negative charge. The electrometer did not register much electric charge if the rays
were bent so they
would not enter the slit. As Thomson saw it, the negative charge and the cathode rays
must somehow be
stuck together: you cannot separate the charge from the rays.
All attempts had failed when physicists tried to bend cathode rays with an electric
field. Now
Thomson thought of a new approach. A charged particle will normally curve as it moves
through an electric
field, but not if it is surrounded by a conductor (a sheath of copper, for example).
Thomson suspected that
the traces of gas remaining in the tube were being turned into an electrical conductor by
the cathode rays
themselves. To test this idea, he took great pains to extract nearly all of the gas from
a tube, and found that
now the cathode rays did bend in an electric field after all.
Thomson concluded from these two experiments, I can see no escape from the conclusion
that
[cathode rays] are charges of negative electricity carried by particles of matter. But,
he continued, What
are these particles? are they atoms, or molecules, or matter in a still finer state of
subdivision?
Thomson's third experiment sought to determine the basic properties of the particles.
Although he couldn't
measure directly the mass or the electric charge of such a particle, he could measure how
much the rays
were bent by a magnetic field, and how much energy they carried. From this data he could
calculate the ratio
of the mass of a particle to its electric charge (m/e). He collected data using a variety
of tubes and using
different gases.
Theories about the atom proliferated in the wake of Thomson's 1897 work. If Thomson had
found
the single building block of all atoms, how could atoms be built up out of these
corpuscles? Thomson
proposed a model, sometimes called the plum pudding or raisin cake model, in which
thousands of tiny,
negatively charged corpuscles swarm inside a sort of cloud of massless positive charge.
This theory was
struck down by Thomson's own former student, Ernest Rutherford. Using a different kind of
particle beam,
Rutherford found evidence that the atom has a small core, a nucleus. Rutherford suggested
that the atom
might resemble a tiny solar system, with a massive, positively charged center circled by
only a few electrons.
Later this nucleus was found to be built of new kinds of particles (protons and
neutrons), much heavier than
electrons.
The results were astounding. Just as Emil Wiechert had reported earlier that year, the
mass-to-charge ratio for cathode rays turned out to be over one thousand times smaller
than that of a
charged hydrogen atom. Either the cathode rays carried an enormous charge (as compared
with a charged
atom), or else they were amazingly light relative to their charge. 
The choice between these possibilities was settled by Philipp Lenard. Experimenting on
how
cathode rays penetrate gases, he showed that if cathode rays were particles they had to
have a very small
mass--far smaller than the mass of any atom. The proof was far from conclusive. But
experiments by others
in the next two years yielded an independent measurement of the value of the charge (e)
and confirmed this
remarkable conclusion.
Thomson boldly announced the hypothesis that we have in the cathode rays matter in a new
state,
a state in which the subdivision of matter is carried very much further than in the
ordinary gaseous state: a
state in which all matter... is of one and the same kind; this matter being the substance
from which all the
chemical elements are built up. Thomson presented three hypotheses about cathode rays
based on his 1897
experiments: Cathode rays are charged particles (which he called corpuscles), these
corpuscles are
constituents of the atom, and the corpuscles are the only constituents of the atom. 
Thomson's speculations met with some skepticism. The second and third hypotheses were
especially controversial (the third hypothesis indeed turned out to be false). Years
later he recalled, At first
there were very few who believed in the existence of these bodies smaller than atoms. I
was even told long
afterwards by a distinguished physicist who had been present at my lecture at the Royal
Institution that he
thought I had been 'pulling their legs.'
On January 2, 1890, J.J. married Rose Paget. They had 2 kids. His son, George Thomson
also
went into the field of atomics. Throughout the marriage, the word electron, coined by G.
Johnstone
Stoney in 1891, had been used to denote the unit of charge found in experiments that
passed electric current
through chemicals. In this sense the term was used by Joseph Larmor, J.J.. Thomson's
Cambridge classmate.
Larmor devised a theory of the electron that described it as a structure in the ether.
But Larmor's theory did
not describe the electron as a part of the atom. When it was discovered in 1897 that
Thomson's corpuscles
were really free electrons, he was actually disagreeing with Thomson's hypotheses.
FitzGerald had in mind
the kind of electron described by Larmor's theory. 
Gradually scientists accepted Thomson's first and second hypotheses, although with some
subtle
changes in their meaning. Experiments by Thomson, Lenard, and others through the crucial
year of 1897
were not enough to settle the uncertainties. In 1906, Thomson won the Nobel Peace Prize
for his work and
in 1918 he became the master of his college. J.J. deceased on August 30th, 1940. Real
understanding
required many more experiments over later years