160 The Poetry of Physics and The Physics of Poetry
build up more kinetic energy so that when they finally collide with a
neutral molecule, they can ionize it creating another electron-ion pair.
The new ion pair will then go on to create many more additional ion
pairs. This process snowballs and an electric discharge between the
cathode and the anode takes place, an appreciable current begins to flow,
and the tube begins to glow visibly. Not all of the electrons and ions in
the gas actually reach the anode or the cathode. Some of them recombine
on the way with new partners. Because of the kinetic energy the ion pair
has acquired from the electric field, the neutral molecule formed through
the recombination of the negative and positive ions is often in an excited
state. It rids itself of this excess energy by radiating visible light, which
partly accounts for the glow from the center of the tube.
There is also a glowing of the glass at either end of the tube, which
is due to the stream of charged particles. When this glow was first
observed, it was not understood what caused it. It was thought to be
some kind of radiation. The glow at the anode caused by the electrons,
for example, was attributed to cathode rays, which presumably emanated
from the cathode. By cutting a hole in the electrodes, William Crookes
was able to study the nature of the radiation causing the glass of the tube
to glow. William Crookes showed that the location of the glow could be
displaced by a magnet. He cited this as evidence that the so-called
radiation was actually due to a stream of charged particles.
This work was corroborated and further developed by J.J. Thomson
in 1897, who showed that the stream of particles could also be deflected
by an electric field. By comparing the deflections due to the electric
and magnetic fields, Thomson was able to determine the ratio of charge
to mass for the charged particles. His results with the positive ions were
in agreement with the results of other determinations of the charge to
mass ratio of heavy ions obtained with electrolysis studies. His results
with the negatively charged particles were extremely surprising. He
found that the ratio of charge to mass of these negatively charged
particles was thousands of times larger than the same ratio of the positive
ions observed in electrolysis. If one assumed that the magnitude of these
newly discovered particle’s charge was the same as that of the hydrogen
ion, then, its mass would be only 1/2000 the mass of the hydrogen atom.
Thomson associated this particle with the electron, named in 1891 by
George Stone, whose existence was postulated as the ultimate particle
of electricity. Thomson’s result established the electrical composition of
the atom.